Structure/function relationships responsible for coenzyme specificity and the isomerase activity of human type 1 3 beta-hydroxysteroid dehydrogenase/isomerase

Double-duty isomerases: a case study of isomerization-coupled enzymatic catalysis

Enzymes can usually be unambiguously assigned to one of seven classes specifying the basic chemistry of their catalyzed reactions. Less frequently, two or more reaction classes are catalyzed by a single enzyme within one active site. Two examples are an isomerohydrolase and an isomero-oxygenase that catalyze isomerization-coupled reactions crucial for production of vision-supporting 11-cis-retinoids. In these enzymes, isomerization is obligately paired and mechanistically intertwined with a second reaction class. A handful of other enzymes carrying out similarly coupled isomerization reactions have been described, some of which have been subjected to detailed structure-function analyses. Herein we review these rarefied enzymes, focusing on the mechanistic and structural basis of their reaction coupling with the goal of revealing catalytic commonalities.

Proteo-transcriptomic profiles reveal key regulatory pathways and functions of LDHA in the ovulation of domestic chickens (Gallus gallus)

BACKGROUND

In poultry, the smooth transition of follicles from the preovulatory-to-postovulatory phase impacts egg production in hens and can benefit the poultry industry. However, the regulatory mechanism underlying follicular ovulation in avians is a complex biological process that remains unclear.

RESULTS

Critical biochemical events involved in ovulation in domestic chickens (Gallus gallus) were evaluated by transcriptomics, proteomics, and in vitro assays. Comparative transcriptome analyses of the largest preovulatory follicle (F1) and postovulatory follicle (POF1) in continuous laying (CL) and intermittent laying (IL) chickens indicated the greatest difference between CL_F1 and IL_F1, with 950 differentially expressed genes (DEGs), and the smallest difference between CL_POF1 and IL_POF1, with 14 DEGs. Additionally, data-independent acquisition proteomics revealed 252 differentially abundant proteins between CL_F1 and IL_F1. Perivitelline membrane synthesis, steroid biosynthesis, lysosomes, and oxidative phosphorylation were identified as pivotal pathways contributing to ovulation regulation. In particular, the regulation of zona pellucida sperm-binding protein 3, plasminogen activator, cathepsin A, and lactate dehydrogenase A (LDHA) was shown to be essential for ovulation. Furthermore, the inhibition of LDHA decreased cell viability and promoted apoptosis of ovarian follicles in vitro.

CONCLUSIONS

This study reveals several important biochemical events involved in the process of ovulation, as well as crucial role of LDHA. These findings improve our understanding of ovulation and its regulatory mechanisms in avian species.

Regulation of proliferation, apoptosis, hormone secretion and gene expression by acetyl-L-carnitine in yak (Bos grunniens) granulosa cells

Supplementation with acetyl-l-carnitine (ALC) during in vitro maturation significantly improves the rates of oocyte cleavage and morula and blastocyst formation in sheep and buffalo; however, the mode of action of ALC in improving oocyte competence is not completely understood. Therefore, the aim of this study was to investigate the effects of ALC on proliferation, antioxidant properties, lipid droplet accumulation and steroid hormone secretion in yak (Bos grunniens) granulosa cells (GCs). Yak GCs were identified using FSHR immunofluorescence. The cells were treated with different concentrations of ALC, cell proliferation was detected by cell counting kit-8, and the optimal concentration and treatment time were determined for subsequent experiments. Then, reactive oxygen species (ROS) were detected by a DCFH-DA probe, and lipid droplet accumulation was observed by oil red O staining. Estradiol (E2) and progesterone (P4) in the medium were detected by ELISA, and the expression of genes related to cell proliferation, apoptosis, the cell cycle, antioxidants and steroid synthesis was determined by RT‒qPCR. The results showed that 1 mM ALC treatment for 48 h was the optimum treatment. It significantly increased cell viability (P < 0.05), significantly decreased the amount of ROS and lipid droplet content, and promoted P4 and E2 secretion (P < 0.05) of yak GCs. RT‒qPCR results verified that GCs treated with 1 mM ALC for 48 h significantly increased the expression of genes related to anti-apoptosis and the cell cycle (BCL-2, PCNA, CCND1 and CCNB1), antioxidants (CAT, SOD2 and GPX1), and E2 and P4 secretion (StAR, CYP19A1 and HSD3B1) (P < 0.05), but it significantly decreased the expression of apoptosis genes (BAX and P53) (P < 0.05). In conclusion, ALC increased the viability of yak GCs, reduced the amount of ROS and lipid droplets, increased P4 and E2 synthesis and affected the expression of related genes in yak GCs.

Effects of phenolic compounds on 3β-hydroxysteroid dehydrogenase activity in human and rat placenta: Screening, mode of action, and docking analysis

Human 3β-hydroxysteroid dehydrogenase type I (HSD3B1) and rat type IV (HSD3B4) in placentas catalyze the conversion of pregnenolone to progesterone, which plays a key role in maintaining pregnancy. Many phenolic compounds potentially inhibit HSD3B in placentas as endocrine disruptors. In this study, the effects of 16 phenolic compounds on the activity of human HSD3B1 and rat HSD3B4 were determined and the structure-activity relationship was compared. HSD3B1 in human placental microsomes and HSD3B4 in rat placental microsomes were used to measure their activities and pregnenolone and NAD⁺ were used as substrates. Of the 16 phenolic compounds, 4-nonylphenol, pentabromophenol, and 2-bromophenol resulted in residual human HSD3B1 activity lower than 50 % and 4-nonylphenol and pentabromophenol resulted in residual rat HSD3B4 activity lower than 50 %. 4-Nonylphenol, pentabromophenol, and 2-bromophenol were mixed inhibitors of human HSD3B1, with Ki values of 2.31, 3.58 and 4.86 µM, respectively, while 4-nonylphenol and pentabromophenol were noncompetitive inhibitors of rat HSD3B4 with Ki values of 20.86 and 141.8 µM. Molecular docking showed that 4-nonylphenol, pentabromophenol, and 2-bromophenol docked to the active sites of human HSD3B1 and rat HSD3B4, and the shift of residue S125 in human HSD3B1 to T125 in rat HSD3B4 could explain the species-dependent difference in their inhibitory potency and mode of action. This study demonstrates that 4-nonylphenol, pentabromophenol, and 2-bromophenol are mixed inhibitors of human placental HSD3B1, while 4-nonylphenol and pentabromophenol are noncompetitive inhibitors of rat HSD3B4, possibly blocking the placental steroidogenesis.

Structure-activity relationship analysis of perfluoroalkyl carbonic acids on human and rat placental 3β-hydroxysteroid dehydrogenase activity

Placenta contains 3β-hydroxysteroid dehydrogenase/steroid Δ^5,4-isomerase (HSD3B), which catalyzes pregnenolone to progesterone for maintaining pregnancy. Perfluoroalkyl carbonic acids (PFC) are subclass of perfluoroalkyl substances containing 4-14 carbons (C4-C14) in the carbon backbone and are potential endocrine disruptors. Whether PFC inhibit HSD3B and structure-activity relationship (SAR) remains unclear. Herein, we screened 11 PFC for inhibiting human type I HSD3B (HSD3B1) and rat type IV HSD3B (HSD3B4) activities and determined SAR and mode of inhibition. HSD3B was measured by converting pregnenolone to progesterone assisted by NAD⁺ in placental microsomes. Of the 11 PFC, C9-C14 significantly inhibited human HSD3B1 activity at 100 μM. Half-maximal inhibitory concentration (IC₅₀) values of C9-C14 compounds were 363.56 ± 12.14, 12.78 ± 0.69, 6.54 ± 0.65, 20.88 ± 0.41, 118.35 ± 0.16, and 149.26 ± 21.67 μM, respectively. We determined Ki values and mode of inhibition of three most potent PFC (C10-C12), and found that they were mixed inhibitors against pregnenolone, with Ki values of 5.57 ± 4.37, 2.04 ± 2.26, and 9.93 ± 7.71, respectively. Docking analysis showed that they bound steroid-binding site. Effects of PFC on rat placental HSD3B4 were performed. Of the 11 PFC, C10-C12 significantly inhibited rat HSD3B4 activity at 100 μM. IC₅₀ values of C10-C12 compounds were 45.85 ± 1.49, 36.08 ± 1.50, and 88.74 ± 1.99 µM, respectively. Ki values and inhibition modes of the three most potent PFC (C10-C12) were studied. It was found that they were mixed inhibitors against pregnenolone, with Ki values of 48.16 ± 20.44, 36.28 ± 53.07, and 91.79 ± 21.75 μM, respectively. Docking analysis showed that they bound steroid-binding site of rat HSD3B4. In conclusion, PFC showed significant SAR differences. The potency of inhibiting HSD3B activity increased from C9 to C11, and then declined. Human HSD3B1 was more sensitive to the inhibition of rat HSD3B4.

Hypoxia-Reoxygenation Couples 3βHSD1 Enzyme and Cofactor Upregulation to Facilitate Androgen Biosynthesis and Hormone Therapy Resistance in Prostate Cancer

Androgen deprivation therapy suppresses tumor androgen receptor (AR) signaling by depleting circulating testosterone and is a mainstay treatment for advanced prostate cancer. Despite initial treatment response, castration-resistant prostate cancer nearly always develops and remains driven primarily by the androgen axis. Here we investigated how changes in oxygenation affect androgen synthesis. In prostate cancer cells, chronic hypoxia coupled to reoxygenation resulted in efficient metabolism of androgen precursors to produce androgens and activate AR. Hypoxia induced 3βHSD1, the rate-limiting androgen synthesis regulator, and reoxygenation replenished necessary cofactors, suggesting that hypoxia and reoxygenation both facilitate potent androgen synthesis. The EGLN1/VHL/HIF2α pathway induced 3βHSD1 expression through direct binding of HIF2α to the 5' regulatory region of HSD3B1 to promote transcription. Overexpression of HIF2α facilitated prostate cancer progression, which largely depended on 3βHSD1. Inhibition of HIF2α with the small-molecule PT2399 prevented prostate cancer cell proliferation. These results thus identify HIF2α as a regulator of androgen synthesis and potential therapeutic target in prostate cancer.

SIGNIFICANCE

Hypoxia followed by reoxygenation in prostate cancer drives androgen deprivation therapy resistance via increasing the rate-limiting enzyme and cofactors for androgen synthesis, revealing HIF2α as a therapeutic target to subvert resistance.

Steroid disulfates - Sulfation double trouble

Sulfation pathways have recently come into the focus of biomedical research. For steroid hormones and related compounds, sulfation represents an additional layer of regulation as sulfated steroids are more water-soluble and tend to be biologically less active. For steroid diols, an additional sulfation is possible, carried out by the same sulfotransferases that catalyze the first sulfation step. The steroid disulfates that are formed are the focus of this review. We discuss both their biochemical production as well as their putative biological function. Steroid disulfates have also been linked to various clinical conditions in numerous untargeted metabolomics studies. New analytical techniques exploring the biosynthetic routes of steroid disulfates have led to novel insights, changing our understanding of sulfation in human biology. They promise a bright future for research into sulfation pathways, hopefully too for the diagnosis and treatment of several associated diseases.

Human placental 3β-hydroxysteroid dehydrogenase/steroid Δ5,4-isomerase 1: Identity, regulation and environmental inhibitors

Human placental 3β-hydroxysteroid dehydrogenase/steroid Δ5, 4-isomerase 1 (HSD3B1), a high-affinity type I enzyme, uses pregnenolone to make progesterone, which is critical for maintenance of pregnancy. HSD3B1 is located in the mitochondrion and the smooth endoplasmic reticulum of placental cells and is encoded by HSD3B1 gene. HSD3B1 contains GATA and TEF-5 regulatory elements. Many endocrine disruptors, including phthalates, methoxychlor and its metabolite, organotins, and gossypol directly inhibit placental HSD3B1 thus blocking progesterone production. In this review, we discuss the placental HSD3B1, its gene regulation, biochemistry, subcellular location, and inhibitors from the environment.

Combined microRNAome and transcriptome analysis of follicular phase and luteal phase in porcine ovaries

The aim of this study was to explore the expression difference of miRNAs and mRNAs between the follicular phase (FP) and luteal phase (LP) in porcine ovaries and provide a theoretical basis for the research on mammalian reproductive regulation. RNA-Seq and miRNA-Seq were used to identify differentially expressed genes (DEGs) and miRNAs (DEMs) between the FP and LP in ovaries of six sows (3-year-old Yorkshire pigs with similar weights and same parities). Bioinformatic analysis was used to screen potential genes and miRNAs related to porcine ovarian function. Real-time qualitative PCR was used to validate the sequencing results. RNA-Seq results showed that 3,078 genes were up-regulated, and 1,444 genes were down-regulated in the LP compared with the FP, and DEGs were significantly enriched in 242 Gene Ontology (GO) terms and 33 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. miRNA-Seq identified 112 DEMs, of which 25 were up-regulated and 87 were down-regulated in the LP compared with the FP. We obtained 186 intersection genes (IGs) between the 4,522 DEGs and 2,444 target genes predicted from the 112 DEMs. After constructing a miRNA-gene-pathway network, we identified key miRNAs and genes including miR-17-3p, miR-214, miR-221-5p, miR-125b, FGF1, YWHAG, YWHAZ, FDFT1 and DHCR24, which are enriched in Hippo and PI3K-Akt signalling pathways, and various metabolic pathways. These results indicate that these key genes and miRNAs may play important roles in the developmental transition from FP to LP in porcine ovaries and represent candidate targets for further study.

Cloning, prokaryotic expression and function of the Bufo bufo gargarizans 3β-hydroxysteroid dehydrogenase (3βHSD) gene

Bufadienolides, one kind of steroids, are the major active component secreted by ear-side gland of Bufo species. Preliminary studies on high-throughput transcriptome sequencing about B. bufo gargarizans showed that the expression of 3β-Hydroxysteroid dehydrogenase (3βHSD) in ear-side gland was nearly 20 times higher than that in liver. The enzyme 3βHSD is an essential step in the biosynthesis of steroid such as progesterone, estrogens and androgens in steroidogenic tissues. Accordingly, 3βHSD is probably an important enzyme involved in the biosynthesis of bufadienolides. In this study, Bbg-3βHSD cDNA was cloned from the ear-side gland of B. bufo gargarizans. Genetic engineering techniques were used to construct a recombinant prokaryotic fusion expression plasmid pCOLD-Bbg3βHSD which was introduced into E. coli BL21 for prokaryotic expression. Bbg-3βHSD has an open reading frame (ORF) of 1134 bp and encodes 377 amino acid residues. The speculated protein molecular weight is 42.8 kDa and its theoretical isoelectric point is 8.68. Amino acid sequence homologous analysis showed that Bbg-3βHSD was highly homologous to the 3βHSD protein of other species. Phylogenetic tree showed the highest similarity between Bbg-3βHSD and 3βHSD from Rana rugosa. The optimized expression of recombinant Bbg-3βHSD were achieved by inducing with 0.1 mmol L^-1 IPTG at 15 °C for 20 h. Enzymatic activity in vitro shows that pregnenolone and dehydroepiandroesterone could be 3β-oxidized by Bbg-3βHSD when NAD⁺ was used as the coenzyme. Enzymatic properties showed that the optimum reaction temperature of recombinant Bbg-3βHSD was 40 °C, the optimum pH was 8.5, and the optimum coenzyme concentration was 1.5 mmol L^-1.

Third DWF1 paralog in Solanaceae, sterol Δ24-isomerase, branches withanolide biosynthesis from the general phytosterol pathway

A large part of chemodiversity of plant triterpenes is due to the modification of their side chains. Reduction or isomerization of double bonds in the side chains is often an important step for the diversification of triterpenes, although the enzymes involved are not fully understood. Withanolides are a large group of structurally diverse C28 steroidal lactones derived from 24-methylenecholesterol. These compounds are found in the Indian medicinal plant Withania somnifera, also known as ashwagandha, and other members of the Solanaceae. The pathway for withanolide biosynthesis is unknown, preventing sustainable production via white biotechnology and downstream pharmaceutical usages. In the present study, based on genome and transcriptome data we have identified a key enzyme in the biosynthesis of withanolides: a DWF1 paralog encoding a sterol Δ24-isomerase (24ISO). 24ISO originated from DWF1 after two subsequent duplication events in Solanoideae plants. Withanolides and 24ISO appear only in the medicinal plants in the Solanoideae, not in crop plants such as potato and tomato, indicating negative selection during domestication. 24ISO is a unique isomerase enzyme evolved from a reductase and as such has maintained the FAD-binding oxidoreductase structure and requirement for NADPH. Using phylogenetic, metabolomic, and gene expression analysis in combination with heterologous expression and virus-induced gene silencing, we showed that 24ISO catalyzes the conversion of 24-methylenecholesterol to 24-methyldesmosterol. We propose that this catalytic step is the committing step in withanolide biosynthesis, opening up elucidation of the whole pathway and future larger-scale sustainable production of withanolides and related compounds with pharmacological properties.

Structure-function relationships for the selective inhibition of human 3β-hydroxysteroid dehydrogenase type 1 by a novel androgen analog

3β-Hydroxysteroid dehydrogenase type 1 (3β-HSD1) is selectively expressed in human placenta, mammary glands and breast tumors in women. Human 3β-HSD2 is selectively expressed in adrenal glands and ovaries. Based on AutoDock 3 and 4 results, we have exploited key differences in the amino acid sequences of 3β-HSD1 (Ser194, Arg195) and 3β-HSD2 (Gly194, Pro195) by designing a selective inhibitor of 3β-HSD1. 2,16-Dicyano-4,5-epoxy-androstane-3,17-dione (16-cyano-17-keto-trilostane or DiCN-AND) was synthesized in a 4-step procedure from androstenedione. In purified 3β-HSD inhibition studies, DiCN-AND competitively inhibited 3β- HSD1 with K_i=4.7μM and noncompetitively inhibited 3β-HSD2 with a 6.5-fold higher K_i=30.7μM. We previously reported similar isoenzyme-specific inhibition profiles for trilostane. Based on our docking results, we created, expressed and purified the chimeric S194G-1 mutant of 3β-HSD1. Trilostane inhibited S194G-1 (K_i=0.67μM) with a noncompetitive mode compared to its 6.7-fold higher affinity, competitive inhibition of 3β-HSD1 (K_i=0.10μM). DiCN-AND inhibited S194G-1 with a 6.3-fold higher K_i (29.5μM) than measured for 3β-HSD1 (K_i=4.7μM) but with the same competitive mode for both enzyme species. Since DiCN-AND noncompetitively inhibits 3β-HSD2, which has the Gly194 and Pro195 of 3β-HSD2 in place of the Ser194 and Arg195 in 3β-HSD1, this suggests that Arg195 alone in 3β-HSD1 or S194G-1 is required to bind DiCN-AND in the substrate binding site (competitive inhibition). However, both Ser194 and Arg195 are required to bind trilostane in the 3β-HSD1 substrate site based on its noncompetitive inhibition of S194G-1 and 3β-HSD2. In support of this hypothesis, DiCN-AND inhibited our chimeric R195P-1 mutant noncompetitively with a K_i=41.3μM (similar to the 3β-HSD2 inhibition profile). Since DiCN-AND competitively inhibited S194G-1 that still contains R195 but noncompetitively inhibited R195P-1 that still contains S194, our data provides strong evidence that the Arg195 being mutated to Pro195 (as present in 3β-HSD2) shifts the inhibition mode from competitive to noncompetitive in 3β-HSD1. This supports the key role of Arg195 in 3β-HSD1 for the high affinity, competitive binding of the trilostane analogs. Our new structure/function information for the design of targeted 3β-HSD1 inhibitors may lead to important new treatments for the prevention of spontaneous premature birth.

Δ4-3-ketosteroids as a new class of substrates for the cytosolic sulfotransferases

Cytosolic sulfotransferase (SULT)-mediated sulfation is generally known to involve the transfer of a sulfonate group from the active sulfate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS), to a hydroxyl group or an amino group of a substrate compound. We report here that human SULT2A1, in addition to being able to sulfate dehydroepiandrosterone (DHEA) and other hydroxysteroids, could also catalyze the sulfation of Δ⁴-3-ketosteroids, which carry no hydroxyl groups in their chemical structure. Among a panel of Δ⁴-3-ketosteroids tested as substrates, 4-androstene-3,17-dione and progesterone were found to be sulfated by SULT2A1. Mass spectrometry analysis and structural modeling supported a reaction mechanism which involves the isomerization of Δ⁴-3-ketosteroids from the keto form to an enol form, prior to being subjected to sulfation. Results derived from this study suggested a potential role of SULT2A1 as a Δ⁴-3-ketosteroid sulfotransferase in steroid metabolism.

Structural modeling and in silico analysis of non-synonymous single nucleotide polymorphisms of human 3β-hydroxysteroid dehydrogenase type 2

Single-nucleotide polymorphisms (SNPs), a most common type of genetic mutations, result from single base pair alterations. Non-synonymous SNPs (nsSNP) occur in the coding regions of a gene and result in single amino acid substitution which might have the potential to affect the function as well as structure of the corresponding protein. In human the 3β-hydroxysteroid dehydrogenases/Δ^4,5-isomerase type 2 (HSD3B2) is an important membrane-bound enzyme involved in the dehydrogenation and Δ^4,5-isomerization of the Δ⁵-steroid precursors into their respective Δ⁴-ketosteroids in the biosynthesis of steroid hormones such as glucocorticoids, mineralocorticoids, progesterone, androgens, and estrogens in tissues such as adrenal gland, ovary, and testis. Most of the nsSNPs of HSD3B2 are still uncharacterized in terms of their disease causing potential. So, this study has been undertaken to explore and extend the knowledge related to the effect of nsSNPs on the stability and function of the HSD3B2. In this study sixteen nsSNP of HSD3B2 were subjected to in silico analysis using nine different algorithms: SIFT, PROVEAN, PolyPhen, MutPred, SNPeffect, nsSNP Analyzer, PhD SNP, stSNP, and I Mutant 2.0. The results obtained from the analysis revealed that the prioritization of diseases associated amino acid substitution as evident from possible alteration in structure–function relationship. Structural phylogenetic analysis using ConSurf revealed that the functional residues are highly conserved in human HSD3B2; and most of the disease associated nsSNPs are within these conserved residues. Structural theoritical models of HSD3B2 were created using HHPred, Phyre2 and RaptorX server. The predicted models were evaluated to get the best one for structural understanding of amino acid substitutions in three dimensional spaces.

Cytochrome b5 modulates multiple reactions in steroidogenesis by diverse mechanisms

Cytochrome b5 (cyt-b5) is a relatively small haemoprotein which plays an important role in the regulation of mammalian steroidogenesis. This unique protein has the ability to modulate the activity of key steroidogenic enzymes via a number of diverse reaction mechanisms. Cyt-b5 can augment the 17,20-lyase activity of CYP17A1 by promoting the interaction of CYP17A1 and POR; enhance the 16-ene-synthase activity of CYP17A1 by acting as an electron donor; and enhance the activity of 3βHSD by increasing the affinity of 3βHSD for its cofactor NAD(+). We review the modulation of CYP17A1 and 3βHSD activity by cyt-b5 and discuss the reaction mechanisms associated with each activity. The physiological importance of cyt-b5 in regulating mammalian steroidogenesis is presented and the impact of inactivating cyt-b5 mutations are reviewed. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Regulation of human 3-beta-hydroxysteroid dehydrogenase type-2 (3βHSD2) by molecular chaperones and the mitochondrial environment affects steroidogenesis

Human 3-β-hydroxysteroid dehydrogenase/isomerase types 1 and 2 (3βHSD1 and 3βHSD2, respectively) are expressed in a tissue-specific pattern by different genes. Site-directed mutagenesis studies have confirmed the function of the catalytic amino acids (Tyr154, Lys 158, Ser124 in both isoenzymes), substrate/inhibitor isoform-specific residues (His156 and Arg195 in 3βHSD1) and cofactor binding residues (Asp36 provides NAD(+) specificity in both isoenzymes). However, detailed analysis of isoform-specific organelle localization and characterization is difficult due to the 93% amino acid identity between the two isoforms. With recent advances in the knowledge of mitochondrial architecture and localization of the various translocases, our laboratory has studied the mechanisms regulating mitochondrial 3βHSD2 localization. The mitochondrial N-terminal leader sequence of 3βHSD2 directs its entry into the mitochondria where it is localized to the intermembrane space (IMS). Unlike other mitochondrial proteins, the N-terminal signal sequence of 3βHSD2 is not cleaved upon mitochondrial import. 3βHSD2 interacts with the mitochondrial translocase, Tim50, to regulate progesterone and androstenedione formation. Our studies suggest that its activity at the IMS is facilitated in a partially unfolded "molten globule" conformation by the proton pump between the matrix and IMS. The unfolded protein is refolded by the mitochondrial chaperones. The protons at the IMS are absorbed by the lipid vesicles, to maintain the proton pump and recycle 3βHSD2. As a result, one molecule of 3βHSD2 may participate in multiple catalytic reactions. In summary, the steroidogenic cell recycles 3βHSD2 to catalyze the reactions needed to produce androstenedione, progesterone and 17α-hydroxyprogesterone on demand in coordination with the mitochondrial translocase, Tim50. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Regioselective hydroxylation of steroid hormones by human cytochromes P450

This article reviews in vitro metabolic activities [including Michaelis constants (Km), maximal velocities (Vmax) and Vmax/Km] and drug-steroid interactions [such as induction and cooperativity (activation)] of cytochromes P450 (P450 or CYP) in human tissues, including liver and adrenal gland, for 14 kinds of endogenous steroid compounds, including allopregnanolone, cholesterol, cortisol, cortisone, dehydroepiandrosterone, estradiol, estrone, pregnenolone, progesterone, testosterone and bile acids (cholic acid). First, we considered the drug-metabolizing P450s. 6β-Hydroxylation of many steroids, including cortisol, cortisone, progesterone and testosterone, was catalyzed primarily by CYP3A4. CYP1A2 and CYP3A4, respectively, are likely the major hepatic enzymes responsible for 2-/4-hydroxylation and 16α-hydroxylation of estradiol and estrone, steroids that can contribute to breast cancer risk. In contrast, CYP1A1 and CYP1B1 predominantly metabolized estrone and estradiol to 2- and 4-catechol estrogens, which are endogenous ultimate carcinogens if formed in the breast. Some metabolic activities of CYP3A4, including dehydroepiandrosterone 7β-/16α-hydroxylation, estrone 2-hydroxylation and testosterone 6β-hydroxylation, were higher than those for polymorphically expressed CYP3A5. Next, we considered typical steroidogenic P450s. CYP17A1, CYP19A1 and CYP27A1 catalyzed steroid synthesis, including hydroxylation at 17α, 19 and 27 positions, respectively. However, it was difficult to predict which hepatic drug-metabolizing P450 or steroidogenic P450 will be mainly responsible for metabolizing each steroid hormone in vivo based on these results. Further research is required on the metabolism of steroid hormones by various P450s and on prediction of their relative contributions to in vivo metabolism. The findings collected here provide fundamental and useful information on the metabolism of steroid compounds.

Binding domain-driven intracellular trafficking of sterols for synthesis of steroid hormones, bile acids and oxysterols

Steroid hormones, bioactive oxysterols and bile acids are all derived from the biological metabolism of lipid cholesterol. The enzymatic pathways generating these compounds have been an area of intense research for almost a century, as cholesterol and its metabolites have substantial impacts on human health. Owing to its high degree of hydrophobicity and the chemical properties that it confers to biological membranes, the distribution of cholesterol in cells is tightly controlled, with subcellular organelles exhibiting highly divergent levels of cholesterol. The manners in which cells maintain such sterol distributions are of great interest in the study of steroid and bile acid synthesis, as limiting cholesterol substrate to the enzymatic pathways is the principal mechanism by which production of steroids and bile acids is regulated. The mechanisms by which cholesterol moves within cells, however, remain poorly understood. In this review, we examine the subcellular machinery involved in cholesterol metabolism to steroid hormones and bile acid, relating it to both lipid- and protein-based mechanisms facilitating intracellular and intraorganellar cholesterol movement and delivery to these pathways. In particular, we examine evidence for the involvement of specific protein domains involved in cholesterol binding, which impact cholesterol movement and metabolism in steroidogenesis and bile acid synthesis. A better understanding of the physical mechanisms by which these protein- and lipid-based systems function is of fundamental importance to understanding physiological homeostasis and its perturbation.

Environmental pollutants and hydroxysteroid dehydrogenases

Hydroxysteroid dehydrogenases (HSD) are a group of steroidogenic enzymes that are involved in the steroid biosynthesis and metabolism. Four classes of HSDs, namely, 3β-, 11β-, 17β-, and 20α-HSDs, are discussed. 3β-HSDs catalyze the conversion of pregnenolone, 17α-hydroxypregnenolone, and dehydroepiandrosterone to progesterone, 17α-hydroxyprogesterone, and androstenedione, respectively. 11β-HSDs catalyze the interconversion between active cortisol and inactive cortisone. 17β-HSDs catalyze the interconversion between 17β-hydroxyl steroids and 17-ketoandrogens and estrogens. 20α-HSDs catalyze the conversion of progesterone into 20α-hydroxyprogesterone. Many environmental pollutants directly inhibit one or more enzymes of these HSDs, thus interfering with endogenous active steroid hormone levels. These chemicals include industrial materials (perfluoroalkyl compounds, phthalates, bisphenol A, and benzophenone), pesticides/biocides (methoxychlor, organotins, 1,2-dibromo-3-chloropropane, and prochloraz), and plant constituents (genistein, gossypol, and licorice). This chapter reviews these inhibitors targeting on HSDs.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Transcriptional profiling of swine lung tissue after experimental infection with Actinobacillus pleuropneumoniae

Porcine pleuropneumonia is a highly contagious respiratory disease that causes great economic losses worldwide. In this study, we aimed to explore the underlying relationship between infection and injury by investigation of the whole porcine genome expression profiles of swine lung tissues post-inoculated with experimentally Actinobacillus pleuropneumoniae. Expression profiling experiments of the control group and the treatment group were conducted using a commercially available Agilent Porcine Genechip including 43,603 probe sets. Microarray analysis was conducted on profiles of lung from challenged versus non-challenged swine. We found 11,929 transcripts, identified as differentially expressed at the p ≤0.01 level. There were 1188 genes annotated as swine genes in the GenBank Data Base. GO term analysis identified a total of 89 biological process categories, 82 cellular components and 182 molecular functions that were significantly affected, and at least 27 biological process categories that were related to the host immune response. Gene set enrichment analysis identified 13 pathways that were significantly associated with host response. Many proinflammatory-inflammatory cytokines were activated and involved in the regulation of the host defense response at the site of inflammation; while the cytokines involved in regulation of the host immune response were suppressed. All changes of genes and pathways of induced or repressed expression not only led to a decrease in antigenic peptides presented to T lymphocytes by APCs via the MHC and alleviated immune response injury induced by infection, but also stimulated stem cells to produce granulocytes (neutrophils, eosinophils, and basophils) and monocyte, and promote neutrophils and macrophages to phagocytose bacterial and foreign antigen at the site of inflammation. The defense function of swine infection with Actinobacillus pleuropneumoniae was improved, while its immune function was decreased.

Allosteric interaction between 3β-hydroxysteroid dehydrogenase/Δ⁵-Δ⁴ isomerase and cytochrome b5 influences cofactor binding

The biosynthesis of steroid hormones, essential to the survival of all mammals, is dependent on the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD). 3βHSD activity is, in turn, influenced by cytochrome-b(5) (Cyt-b(5)). However, the mechanism through which this occurs is unknown. In this study, we investigated this mechanism by evaluating the influence of Cyt-b(5) on the dehydrogenase and isomerase activities of 3βHSD. Capra hircus 3βHSD was overexpressed in SF-9 cells, using a baculovirus expression system, and purified. Substrate and cofactor kinetics were determined spectrophotometrically in the presence and absence of purified Ovis aries liver Cyt-b(5). Nonspecific enzyme activity was evaluated by zero-enzyme, -substrate, and -cofactor blanks. Fusion proteins, 3βHSD-eCFP, and Cyt-b(5)-eYFP were subsequently coexpressed in COS-1 cells and analyzed for FRET. A CFP-YFP fusion protein served as positive control, while coexpression of 3βHSD-eCFP and cytochrome P450 17α-hydroxylase/17,20 lyase-eYFP (CYP17A1-eYFP) served as negative control. Results showed Cyt-b(5) to decrease the K(m,)(NAD(+)) value of 3βHSD ≈3.5-fold while increasing the V(max,app) of the dehydrogenase reaction ≈17%. FRET analysis showed COS-1 cells coexpressing 3βHSD-eCFP and Cyt-b(5)-eYFP to exhibit a FRET signal ≈9-fold greater than that of the negative control. These results indicate that Cyt-b(5) augments 3βHSD activity via an allosteric mechanism by increasing the affinity of the enzyme toward NAD(+).

In silico structural, functional and pathogenicity evaluation of a novel mutation: an overview of HSD3B2 gene mutations

Mutations of 3 beta hydroxysteroid dehydrogenase type II (HSD3B2) gene result in different clinical consequences. We explain a patient who demonstrated a salt wasting form of 3βHSD deficiency in infancy. Signs of hyponatremia and hyperkalemia were recognized in the infant with ambiguous genitalia and perineal hypospadias. The 46,XY male was genotyped by direct sequencing of HSD3B2 gene. Steroid profiles showed elevated concentration of 17 hydroxyprogesterone, and decrease in concentration of cortisol, and testosterone. Dehydroepiandrotone (DHEA) to androstenedione ratio had 6 fold increases. Direct sequencing of the patient revealed homozygous missense A82P mutation in exon 3. This mutation was confirmed by segregation analysis of the parents. Bioinformatic tools were used for in silico structural and functional analyses. Also, the pathological effect of the mutation was validated by different software. Alanine is a conserved amino acid in the membrane binding domain of the enzyme and proline substitution was predicted to destabilize the protein. This report may highlight the importance of the screening programs of the disorder in Iran.

Mitochondrial 3β-hydroxysteroid dehydrogenase enzyme activity requires reversible pH-dependent conformational change at the intermembrane space

The inner mitochondrial membrane protein 3β-hydroxysteroid dehydrogenase 2 (3βHSD2) synthesizes progesterone and androstenedione through its dehydrogenase and isomerase activities. This bifunctionality requires 3βHSD2 to undergo a conformational change. Given its proximity to the proton pump, we hypothesized that pH influences 3βHSD2 conformation and thus activity. Circular dichroism (CD) showed that between pH 7.4 and 4.5, 3βHSD2 retained its primarily α-helical character with a decrease in α-helical content at lower pH values, whereas the β-sheet content remained unchanged throughout. Titrating the pH back to 7.4 restored the original conformation within 25 min. Metabolic conversion assays indicated peak 3βHSD2 activity at pH 4.5 with ~2-fold more progesterone synthesized at pH 4.5 than at pH 3.5 and 7.4. Increasing the 3βHSD2 concentration from 1 to 40 μg resulted in a 7-fold increase in progesterone at pH 4.5, but no change at pH 7.4. Incubation with guanidinum hydrochloride (GdmHCl) showed a three-step cooperative unfolding of 3βHSD2 from pH 7.4 to 4.5, possibly due to the native state unfolding to the intermediate ion core state. With further decreases in pH, increasing concentrations of GdmHCl led to rapid two-step unfolding that may represent complete loss of structure. Between pH 4 and 5, the two intermediate states appeared stable. Stopped-flow kinetics showed slower unfolding at around pH 4, where the protein is in a pseudostable state. Based on our data, we conclude that at pH 4-5, 3βHSD2 takes on a molten globule conformation that promotes the dual functionality of the enzyme.

Steroidogenic enzyme histochemistry in the testis of Sprague Dawley rats following the administration the water extracts from Carica papaya seed

Water extracts from pawpaw seed have been reported to reversibly decrease the testicular weight and to suppress spermatogenesis, and fertility of Wistar rats. The reversible changes become evident, 30 - 45 days after the withdrawal of the extract. The possible effect of this extract on the activities of steroidogenic enzymes of the testis has not been investigated. Water extract of papaya seeds was administered to male Sprague Dawley rats ad libitum for 84 days. Following the discontinuation of the extracts, ten rats each were sacrificed on days 0, 10, 20 and 30 after the withdrawal. Their testes were quickly dissected out and frozen. Cryostat sections, 10µm thick were cut. These sections were used for immunohistochemical stains for side chain cleavage enzyme and aromatase, and for histochemical stains for 17-β Hydroxysteroid dehydrogenase, 3-β Hydroxysteroid dehydrogenase. We conclude that the water extract of papaya seed suppresses the activities of steroidogenic enzymes in the testis of Sprague Dawley rats, and that this may contribute to reversible suppression of spermatogenesis, a property that gives a possible male contraceptive potential.

Variation in 3β-hydroxysteroid dehydrogenase activity and in pregnenolone supply rate can paradoxically alter androstenedione synthesis

The 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3β-HSD) and 17α-hydroxylase/17,20-lyase cytochrome P450 (P450c17) enzymes are important in determining the balance of the synthesis of different steroids such as progesterone (P4), glucocorticoids, androgens, and estrogens. How this is achieved is not a simple matter because each of the two enzymes utilizes more than one substrate and some substrates are shared in common between the two enzymes. The two synthetic pathways, Δ(4) and Δ(5), are interlinked such that it is difficult to predict how the synthesis of each steroid changes when any of the enzyme activities is varied. In addition, the P450c17 enzyme exhibits different substrate specificities among species, particularly with respect to the 17,20-lyase activity. The mathematical model developed in this study simulates the network of reactions catalyzed by 3β-HSD and P450c17 that characterizes steroid synthesis in human, non-human primate, ovine, and bovine species. In these species, P450c17 has negligible 17,20-lyase activity with the Δ(4)-steroid 17α-hydroxy-progesterone (17OH-P4); therefore androstenedione (A4) is synthesized efficiently only from dehydroepiandrosterone (DHEA) through the Δ(5) pathway. The model helps to understand the interplay between fluxes through the Δ(4) and Δ(5) pathways in this network, and how this determines the response of steroid synthesis to the variation in 3β-HSD activity or in the supply of the precursor substrate, pregnenolone (P5). The model simulations show that A4 synthesis can change paradoxically when 3β-HSD activity is varied. A decrease in 3β-HSD activity to a certain point can increase A4 synthesis by favouring metabolism through the Δ(5) pathway, though further decrease in 3β-HSD activity beyond that point eventually limits A4 synthesis. The model also showed that due to the competitive inhibition of the enzymes' activities by substrates and products, increasing the rate of P5 supply above a certain point can suppress the synthesis of A4, DHEA, and 17OH-P4, and consequently drive more P5 towards P4 synthesis.

Lipid-mediated unfolding of 3β-hydroxysteroid dehydrogenase 2 is essential for steroidogenic activity

For inner mitochondrial membrane (IMM) proteins that do not undergo N-terminal cleavage, the activity may occur in the absence of a receptor present in the mitochondrial membrane. One such protein is human 3β-hydroxysteroid dehydrogenase 2 (3βHSD2), the IMM resident protein responsible for catalyzing two key steps in steroid metabolism: the conversion of pregnenolone to progesterone and dehydroepiandrosterone to androstenedione. Conversion requires that 3βHSD2 serve as both a dehydrogenase and an isomerase. The dual functionality of 3βHSD2 results from a conformational change, but the trigger for this change remains unknown. Using fluorescence resonance energy transfer, we found that 3βHSD2 interacted strongly with a mixture of dipalmitoylphosphatidylglycerol (DPPG) and dipalmitoylphosphatidylcholine (DPPC). 3βHSD2 became less stable when incubated with the individual lipids, as indicated by the decrease in thermal denaturation (T(m)) from 42 to 37 °C. DPPG, alone or in combination with DPPC, led to a decrease in α-helical content without an effect on the β-sheet conformation. With the exception of the 20 N-terminal amino acids, mixed vesicles protected 3βHSD2 from trypsin digestion. However, protein incubated with DPPC was only partially protected. The lipid-mediated unfolding completely supports the model in which a cavity forms between the α-helix and β-sheet. As 3βHSD2 lacks a receptor, opening the conformation may activate the protein.

Cytochrome b5 augments 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase activity

During adrenal steroidogenesis the competition between 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD) and cytochrome P450 17α-hydroxylase/17,20 lyase (CYP17A1) for Δ(5) steroid intermediates greatly influences steroidogenic output. Cytochrome-b(5) (Cyt-b(5)), a small electron transfer hemoprotein, known to augment the lyase activity of CYP17A1, has been shown to alter the steroidogenic outcome of this competition. In this study, the influence of Cyt-b(5) on 3βHSD activity was investigated. In COS-1 cells, Cyt-b(5) was shown to significantly increase the activity of both caprine and ovine 3βHSD towards pregnenolone, 17-OH pregnenolone and dehydroepiandrosterone in a substrate and species specific manner. Furthermore, kinetic studies revealed Cyt-b(5) to have no influence on the K(m) values while significantly increasing the V(max) values of ovine 3βHSD for all its respective substrates. In addition, the activity of ovine 3βHSD in microsomal preparations was significantly influenced by the addition of either purified Cyt-b(5) or anti-Cyt-b(5) IgG. The results presented in this study indicate that Cyt-b(5) augments 3βHSD activity and represents the first documentation of such augmentation in any species.

Inner mitochondrial translocase Tim50 interacts with 3β-hydroxysteroid dehydrogenase type 2 to regulate adrenal and gonadal steroidogenesis

In the adrenals, testes, and ovaries, 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2) catalyzes the conversion of pregnenolone to progesterone and dehydroepiandrostenedione to androstenedione. Alterations in this pathway can have deleterious effects, including sexual development impairment, spontaneous abortion, and breast cancer. 3βHSD2, synthesized in the cytosol, is imported into the inner mitochondrial membrane (IMM) by translocases. Steroidogenesis requires that 3βHSD2 acts as both a dehydrogenase and isomerase. To achieve this dual functionality, 3βHSD2 must undergo a conformational change; however, what triggers that change remains unknown. We propose that 3βHSD2 associates with IMM or outer mitochondrial membrane translocases facing the intermembrane space (IMS) and that this interaction promotes the conformational change needed for full activity. Fractionation assays demonstrate that 3βHSD2 associated with the IMM but did not integrate into the membrane. Through mass spectrometry and Western blotting of mitochondrial complexes and density gradient ultracentrifugation, we show that that 3βHSD2 formed a transient association with the translocases Tim50 and Tom22 and with Tim23. This association occurred primarily through the interaction of Tim50 with the N terminus of 3βHSD2 and contributed to enzymatic activity. Tim50 knockdown inhibited catalysis of dehydroepiandrostenedione to androstenedione and pregnenolone to progesterone. Although Tim50 knockdown decreased 3βHSD2 expression, restoration of expression via proteasome and protease inhibition did not rescue activity. In addition, protein fingerprinting and CD spectroscopy reveal the flexibility of 3βHSD2, a necessary characteristic for forming multiple associations. In summary, Tim50 regulates 3βHSD2 expression and activity, representing a new role for translocases in steroidogenesis.

Selective inhibition of human 3β-hydroxysteroid dehydrogenase type 1 as a potential treatment for breast cancer

Human 3β-hydroxysteroid dehydrogenase/isomerase type 1 (3β-HSD1) is a critical enzyme in the conversion of DHEA to estradiol in breast tumors and may be a target enzyme for inhibition in the treatment of breast cancer in postmenopausal women. Human 3β-HSD2 participates in the production of cortisol and aldosterone in the human adrenal gland in this population. In our recombinant human breast tumor MCF-7 Tet-off cells that express either 3β-HSD1 or 3β-HSD2, trilostane and epostane inhibit the DHEA-induced proliferation of MCF-7 3β-HSD1 cells with 12-16-fold lower IC(50) values compared to the MCF-7 3β-HSD2 cells. Trilostane and epostane also competitively inhibit purified human 3β-HSD1 with 12-16-fold lower K(i) values compared to the noncompetitive K(i) values measured for human 3β-HSD2. Using our structural model of 3β-HSD1, trilostane was docked in the active site of 3β-HSD1, and Arg195 in 3β-HSD1 or Pro195 in 3β-HSD2 was identified as a potentially critical residue. The R195P-1 mutant of 3β-HSD1 and the P195R-2 mutant of 3β-HSD2 were created, expressed and purified. Kinetic analyses of enzyme inhibition suggest that the high-affinity, competitive inhibition of 3β-HSD1 by trilostane may be related to the presence of Arg195 in 3β-HSD1 versus Pro195 in 3β-HSD2. In addition, His156 in 3β-HSD1 may play a role in the higher affinity of 3β-HSD1 for substrates and inhibitors compared to 3β-HSD2 containing Try156. Structural modeling of the 3β-HSD1 dimer identified a possible interaction between His156 on one subunit and Gln105 on the other. Kinetic analyses of the H156Y-1, Q105M-1 and Q105M-2 support subunit interactions that contribute to the higher affinity of 3β-HSD1 for the inhibitor, epostane, compared to 3β-HSD2. Article from the Special issue on Targeted Inhibitors.

Homology modeling and molecular dynamics simulation studies of human type 1 3β-hydroxysteroid dehydrogenase: Toward the understanding of cofactor specificity

The human type 1 isoform of 3β-hydroxysteroid dehydrogenase is a member of short-chain oxidoreductase family that catalyzes the conversion of dehydroepiandrosterone to androstenedione. To compare the molecular events underlying cofactor specificity in the wild-type and D35A/K36R mutant enzymes, molecular dynamics (MD) simulations of fully solvated cofactor-3β-HSD1 (wild-type and mutant) complex are performed. Molecular modeling methods are applied to construct three-dimensional models of cofactor-3β-HSD1 complexes based on Uridine diphosphate (UDP)-galactose 4-epimerase crystal structure from Escherichia coli. The binding mode and binding energy analysis between four different complexes indicate that Asp35 and Lys36 are key residues for the cofactor specificity of 3β-HSD1, which is in agreement with mutagenesis studies results obtained by Thomas et al.8 The MD results also display that the residue Glu41 may be another important residue except Asp35 and Lys36 for the cofactor specificity and that this result needs further mutational experiment for validation.

Structural aspects of the p.P222Q homozygous mutation of HSD3B2 gene in a patient with congenital adrenal hyperplasia

Type II 3β-hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase (3β-HSD2), encoded by the HSD3B2 gene, is a key enzyme involved in the biosynthesis of all the classes of steroid hormones. Deleterious mutations in the HSD3B2 gene cause the classical deficiency of 3β-HSD2, which is a rare autosomal recessive disease that leads to congenital adrenal hyperplasia (CAH). CAH is the most frequent cause of ambiguous genitalia and adrenal insufficiency in newborn infants with variable degrees of salt losing. Here we report the molecular and structural analysis of the HSD3B2 gene in a 46,XY child, who was born from consanguineous parents, and presented with ambiguous genitalia and salt losing. The patient carries a homozygous nucleotide c.665C>A change in exon 4 that putatively substitutes the proline at codon 222 for glutamine. Molecular homology modeling of normal and mutant 3β-HSD2 enzymes emphasizes codon 222 as an important residue for the folding pattern of the enzyme and validates a suitable model for analysis of new mutations.

The functions of key residues in the inhibitor, substrate and cofactor sites of human 3beta-hydroxysteroid dehydrogenase type 1 are validated by mutagenesis

In postmenopausal women, human 3beta-hydroxysteroid dehydrogenase type 1 (3beta-HSD1) is a critical enzyme in the conversion of DHEA to estradiol in breast tumors, while 3beta-HSD2 participates in the production of cortisol and aldosterone in the human adrenal gland. The goals of this project are to determine if Arg195 in 3beta-HSD1 vs. Pro195 in 3beta-HSD2 in the substrate/inhibitor binding site is a critical structural difference responsible for the higher affinity of 3beta-HSD1 for inhibitor and substrate steroids compared to 3beta-HSD2 and whether Asp61, Glu192 and Thr8 are fingerprint residues for cofactor and substrate binding using site-directed mutagenesis. The R195P-1 mutant of 3beta-HSD1 and the P195R-2 mutant of 3beta-HSD2 have been created, expressed, purified and characterized kinetically. Dixon analyses of the inhibition of the R195P-1 mutant, P195R-2 mutant, wild-type 3beta-HSD1 and wild-type 3beta-HSD2 by trilostane has produced kinetic profiles that show inhibition of 3beta-HSD1 by trilostane (K(i)=0.10microM, competitive) with a 16-fold lower K(i) and different mode than measured for 3beta-HSD2 (K(i)=1.60microM, noncompetitive). The R195P-1 mutation shifts the high-affinity, competitive inhibition profile of 3beta-HSD1 to a low-affinity (trilostane K(i)=2.56microM), noncompetitive inhibition profile similar to that of 3beta-HSD2 containing Pro195. The P195R-2 mutation shifts the low-affinity, noncompetitive inhibition profile of 3beta-HSD2 to a high-affinity (trilostane K(i)=0.19microM), competitive inhibition profile similar to that of 3beta-HSD1 containing Arg195. Michaelis-Menten kinetics for DHEA, 16beta-hydroxy-DHEA and 16alpha-hydroxy-DHEA substrate utilization by the R195P-1 and P195R-2 enzymes provide further validation for higher affinity binding due to Arg195 in 3beta-HSD1. Comparisons of the Michaelis-Menten values of cofactor and substrate for the targeted mutants of 3beta-HSD1 (D61N, D61V, E192A, T8A) clarify the functions of these residues as well.

Biochemical and physiological aspects of endogenous androgens

This review attempts to give a synopsis of the major aspects concerning the biochemistry of endogenous androgens, supplemented with several facets of physiology, particularly with respect to testosterone. Testosterone continues to be the most common adverse finding declared by World Anti-Doping Agency accredited laboratories, such samples having an augmented testosterone to epitestosterone ratio. Knowledge regarding the precursors and metabolism of endogenous testosterone is therefore fundamental to understanding many of the issues concerning doping with testosterone and its prohormones, including the detection of their administration. Further, adverse findings for nandrolone are frequent, but this steroid and 19-norandrostenedione are also produced endogenously, an appealing hypothesis being that they are minor by-products of the aromatization of androgens. At sports tribunals pertaining to adverse analytical findings of natural androgen administration, experts often raise issues that concern some aspect of steroid biochemistry and physiology. Salient topics included within this review are the origins and interconversion of endogenous androgens, the biosynthesis of testosterone and epitestosterone, the mechanism of aromatization, the molecular biology of the androgen receptor, the hypothalamic-pituitary-testicular axis, disturbances to this axis by anabolic steroid administration, the transport (binding) of androgens in blood, and briefly the metabolism and excretion of androgens.

Hypocortisolism in the South African angora goat: the role of 3betaHSD

South African Angora goats are susceptible to cold stress, due to their inability to produce sufficient levels of cortisol. During adrenal steroidogenesis the production of cortisol relies on the activity of two key enzymes, namely cytochrome P450 17alpha-hydroxylase and 3beta-hydroxysteroid dehydrogenase. Cytochrome P450 17alpha-hydroxylase has previously been identified as a factor contributing to hypocortisolism in the South African Angora goat. In this comparative study, the catalytic activity of Angora and ovine 3beta-hydroxysteroid dehydrogenase, which differ by five amino acid residues, was characterized. The conversion of 17-hydroxypregnenolone and dehydroepiandosterone to their corresponding products, 17-hydroxyprogesterone and androstenedione, by the two enzymes differed significantly. The enzymes were subsequently co-expressed with Angora P450 17alpha-hydroxylase. Major differences were observed in pregnenolone metabolism with a significant reduction in the formation of the cortisol precursor, 17-hydroxyprogesterone, by cells expressing Angora 3beta-hydroxysteroid dehydrogenase, implicating 3beta-hydroxysteroid dehydrogenase as an additional factor contributing to hypocortisolism in the South African Angora goat.

Studies on Baeyer-Villiger oxidation of steroids: DHEA and pregnenolone D-lactonization pathways in Penicillium camemberti AM83

Penicillium camemberti AM83 strain is able to carry out effective Baeyer-Villiger type oxidation of DHEA, pregnenolone, androstenedione and progesterone to testololactone. Pregnenolone and DHEA underwent oxidation to testololactone via two routes: through 4-en-3-ketones (progesterone and/or androstenedione respectively) or through 3beta-hydroxy-17a-oxa-d-homo-androst-5-en-17-one. Analysis of transformation progress of studied substrates as function of time indicates that the 17beta-side chain cleavage and oxidation of 17-ketones to d-lactones are catalyzed by two different, substrate-induced, BVMOs. In the presence of a C-21 substrate (pregnenolone or progesterone) induction of the enzyme catalyzing cleavage at 17beta-acetyl chain was observed, whereas DHEA and androstenedione induced activity of the BVMO responsible for the ring-D oxidation; 5-en-3beta-alcohol was a more effective inducer that the respective 4-en-3-ketone.

Leydig cell aging and the mechanisms of reduced testosterone synthesis

In males, serum testosterone levels decline with advancing age. Though part of a complex process, this age-related decline in testosterone appears to occur, in part, due to a significant decline in the ability of aged Leydig cells to produce testosterone maximally in response to luteinizing hormone (LH). The structure of the molecular machinery responsible for the synthesis of testosterone is described, and placed in the context of Leydig cell biology. Multiple parameters related to the synthesis of testosterone by the Leydig cell have been observed to change with age. Relationships among these changes are reviewed. A discussion of potential causes of the age-related decline in Leydig cell steroidogenic capacity presents a model in which the inability of aged cells to adequately respond to hormonal stimulation results in cellular regression with concomitant decline in maximal testosterone output.

Transformation of 5-ene steroids by the fungus Aspergillus tamarii KITA: mixed molecular fate in lactonization and hydroxylation pathways with identification of a putative 3beta-hydroxy-steroid dehydrogenase/Delta5-Delta4 isomerase pathway

The fungus Aspergillus tamarii metabolizes progesterone to testololactone in high yield through a sequential four step enzymatic pathway which, has demonstrated flexibility in handling a range of steroidal probes. These substrates have revealed that subtle changes in the molecular structure of the steroid lead to significant changes in route of metabolism. It was therefore of interest to determine the metabolism of a range of 5-ene containing steroidal substrates. Remarkably the primary route of 5-ene steroid metabolism involved a 3beta-hydroxy-steroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD/isomerase) enzyme(s), generating 3-one-4-ene functionality and identified for the first time in a fungus with the ability to handle both dehydroepiansdrosterone (DHEA) as well as C-17 side-chain containing compounds such as pregnenolone and 3beta-hydroxy-16alpha,17alpha-epoxypregn-5-en-20-one. Uniquely in all the steroids tested, 3beta-HSD/isomerase activity only occurred following lactonization of the steroidal ring-D. Presence of C-7 allylic hydroxylation, in either epimeric form, inhibited 3beta-HSD/isomerase activity and of the substrates tested, was only observed with DHEA and its 13alpha-methyl analogue. In contrast to previous studies of fungi with 3beta-HSD/isomerase activity DHEA could also enter a minor hydroxylation pathway. Pregnenolone and 3beta-hydroxy-16alpha,17alpha-epoxypregn-5-en-20-one were metabolized solely through the putative 3beta-HSD/isomerase pathway, indicating that a 17beta-methyl ketone functionality inhibits allylic oxidation at C-7. The presence of the 3beta-HSD/isomerase in A. tamarii and the transformation results obtained in this study highlight an important potential role that fungi may have in the generation of environmental androgens.

Structure/function of the inhibition of human 3beta-hydroxysteroid dehydrogenase type 1 and type 2 by trilostane

The human type 1 (placenta, breast tumors) and type 2 (gonads, adrenals) isoforms of 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) are key enzymes in biosynthesis of all active steroid hormones. Human 3beta-HSD1 is a critical enzyme in the conversion of DHEA to estradiol in breast tumors and may be a major target enzyme for the treatment of breast cancer. 3beta-HSD2 participates in the production of cortisol and aldosterone in the human adrenal gland. The goals of this project are to evaluate the role of the 2alpha-cyano group on trilostane (2alpha-cyano-4alpha,5alpha-epoxy-17beta-ol-androstane-3-one) and determine which amino acids may be critical for 3beta-HSD1 specificity. Trilostane without the 2alpha-cyano group, 4alpha,5alpha-epoxy-testosterone, was synthesized. Using our structural model of 3beta-HSD1, trilostane or 4alpha,5alpha-epoxy-testosterone was docked in the active site using Autodock 3.0, and the potentially critical residues (Met187 and Ser124) were identified. The M187T and S124T mutants of 3beta-HSD1 were created, expressed and purified. Dixon analyses of the inhibition of wild-type 3beta-HSD1, 3beta-HSD2, M187T and S124T by trilostane and 4alpha,5alpha-epoxy-testosterone suggest that the 2alpha-cyano group of trilostane is anchored by Ser124 in both isoenzymes. Kinetic analyses of cofactor and substrate utilization as well as the inhibition kinetics of M187T and the wild-type enzymes suggest that the 16-fold higher-affinity inhibition of 3beta-HSD1 by trilostane may be related to the presence of Met187 in 3beta-HSD1 and Thr187 in 3beta-HSD2. This structure/function information may lead to the production of more highly specific inhibitors of 3beta-HSD1 to block the hormone-dependent growth of breast tumors.

Genetic identification of HSD-1, a conserved steroidogenic enzyme that directs larval development inCaenorhabditis elegans

In C. elegans, steroid hormones function in conjunction with insulin/IGF-1-like signaling in promoting reproductive development over entry into the diapausal dauer stage. The NCR-1 and -2 (NPC1-related) intracellular cholesterol transporters function redundantly in preventing dauer arrest,presumably by regulating the availability of substrates for steroid hormone synthesis. We have identified hsd-1 as a new component of this cholesterol trafficking/processing pathway, using an ncr-1 enhancer screen. HSD-1 is orthologous to 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerases(3β-HSDs), which are key steroidogenic enzymes in vertebrates, and is exclusively expressed in two neuron-like XXX cells that are crucial in preventing dauer arrest, suggesting that it is involved in biosynthesis of dauer-preventing steroid hormones. The hsd-1 null mutant displays defects in inhibiting dauer arrest: it forms dauers in the deletion mutant backgrounds of ncr-1 or daf-28/insulin; as a single mutant,it is hypersensitive to dauer pheromone. We found that hsd-1 defects can be rescued by feeding mutant animals with several steroid intermediates that are either downstream of or in parallel to the 3β-HSD function in the dafachronic acid biosynthetic pathway, suggesting that HSD-1 functions as a 3β-HSD. Interestingly, sterols that rescued hsd-1 defects also bypassed the need for the NCR-1 and/or -2 functions, suggesting that HSD-1-mediated steroid hormone production is an important functional output of the NCR transporters. Finally, we found that the HSD-1-mediated signal activates insulin/IGF-I signaling in a cell non-autonomous fashion, suggesting a novel mechanism for how these two endocrine pathways intersect in directing development.

Structure/function of human type 1 3beta-hydroxysteroid dehydrogenase: An intrasubunit disulfide bond in the Rossmann-fold domain and a Cys residue in the active site are critical for substrate and coenzyme utilization

The human type 1 (placenta, breast tumors) and type 2 (gonads, adrenals) isoforms of 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) are key enzymes in steroidogenic pathways leading to the production of all active steroid hormones. Kinetic analyses of purified 3beta-HSD1 show that the Michaelis-Menten constants (Km) for substrates and cofactor are decreased dramatically (three- to eight-fold) by the addition of beta-mercaptoethanol (BME), which suggest that a disulfide bond may be critical to ligand utilization. Western immunoblots and SDS-PAGE of purified 3beta-HSD1 in the presence or absence of BME showed a lack of intersubunit disulfide bonds in the dimeric enzyme. The Rossmann-fold domain of 3beta-HSD1 contains two Cys residues, Cys72 and Cys111, which are capable of forming an intrasubunit disulfide bond based on their proximity in our structural model. Our structural model also predicts that Cys83 may affect the orientation of substrate and cofactor. To test these predictions, the C72S, C72F, C111S, C111A, C83S and C83A mutants of 3beta-HSD1 were produced, expressed, and purified. BME failed to diminish the Km values of substrate and cofactor for C72S, C72F, C111S and C111A but produced a 2.5 decrease in Km values for C83A ligands similar to wild-type 3beta-HSD. Thus, our results support the presence of an intrasubunit disulfide bond between Cys72 and Cys111 that participates in the tertiary structure of the Rossmann-fold domain. Although C83S had no enzyme activity, the C83A mutant enzyme exhibited two- to five-fold higher Km values for substrate and cofactor but had similar K(cat) values compared to wild-type 3beta-HSD. These data characterize the roles of Cys residues in 3beta-HSD and validate the predictions of our structural model.

Rv1106c from Mycobacterium tuberculosis is a 3beta-hydroxysteroid dehydrogenase

New approaches are required to combat Mycobacterium tuberculosis (Mtb), especially the multi-drug resistant and extremely drug resistant organisms (MDR-TB and XDR-TB). There are many reports that mycobacteria oxidize 3beta-hydroxysterols to 3-ketosteroids, but the enzymes responsible for this activity have not been identified in mycobacterial species. In this work, the Rv1106c gene that is annotated as a 3beta-hydroxysteroid dehydrogenase in Mtb has been cloned and heterologously expressed. The purified enzyme was kinetically characterized and found to have a pH optimum between 8.5 and 9.5. The enzyme, which is a member of the short chain dehydrogenase superfamily, uses NAD+ as a cofactor and oxidizes cholesterol, pregnenolone, and dehydroepiandrosterone to their respective 3-keto-4-ene products. The enzyme forms a ternary complex with NAD+ binding before the sterol. The enzyme shows no substrate preference for dehydroepiandrosterone versus pregnenolone with second-order rate constants (kcat/Km) of 3.2 +/- 0.4 and 3.9 +/- 0.9 microM-1 min-1, respectively, at pH 8.5, 150 mM NaCl, 30 mM MgCl2, and saturating NAD+. Trilostane is a competitive inhibitor of dehydroepiandrosterone with a Ki of 197 +/- 8 microM. The expression of the 3beta-hydroxysteroid dehydrogenase in Mtb is intracellular. Disruption of the 3beta-hydroxysteroid dehydrogenase gene in Mtb abrogates mycobacterial cholesterol oxidation activity. These data are consistent with the Rv1106c gene being the one responsible for 3beta-hydroxysterol oxidation in Mtb.

Cofactors, redox state, and directional preferences of hydroxysteroid dehydrogenases

The hydroxysteroid dehydrogenases (HSDs) interconvert pairs of weak and potent steroids, thus serving as key enzymes in the regulation of intracellular hormone potency. These enzymes may appear to drive unidirectional steroid flux in intact cells but actually catalyze bi-directional metabolism that achieve pseudo-equilibria with strong directional preferences. Even small shifts in the magnitude of these pseudo-equilibria can profoundly change steroid potency and thus contribute to disease. Consequently, we are studying the structural and biochemical principles that govern these directional preferences and the resilience of these pseudo-equilibria in intact cells. HSD directional preferences in intact cells are governed largely by relative affinities for nicotinamide cofactors [NAD(P)(H)] and existing cofactor gradients. We can attenuate the directional preferences for human 17betaHSD type 1 and rat AKR1C9 in intact cells by either diminishing the NADPH/NADP(+) gradient or by mutating the arginine residues that form salt bridges with the 2'-phosphate of NADP(H) (R38 and R276, respectively).

Rational proteomics V: structure-based mutagenesis has revealed key residues responsible for substrate recognition and catalysis by the dehydrogenase and isomerase activities in human 3beta-hydroxysteroid dehydrogenase/isomerase type 1

Mammalian 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) is a member of the short chain dehydrogenase/reductase. It is a key steroidogenic enzyme that catalyzes the first step of the multienzyme pathway conversion of circulating dehydroepiandrosterone and pregnenolone to active steroid hormones. A three dimensional model of a ternary complex of human 3beta-HSD type 1 (3beta-HSD_1) with an NAD cofactor and androstenedione product has been developed based upon X-ray structures of the ternary complex of E. coli UDP-galactose 4-epimerase (UDPGE) with an NAD cofactor and substrate (PDB_AC: 1NAH) and the ternary complex of human type 1 17beta-hydroxysteroid dehydrogenase (17beta-HSD_1) with an NADP cofactor and androstenedione (PDB_AC: 1QYX). The dimeric structure of the enzyme was built from two monomer models of 3beta-HSD_1 by respective 3D superposition with A and B subunits of the dimeric structure of Streptococcus suis DTDP-D-glucose 4,6-dehydratase (PDB_AC: 1KEP). The 3D model structure of 3beta-HSD_1 has been successfully used for the rational design of mutagenic experiments to further elucidate the key substrate binding residues in the active site as well as the basis for dual function of the 3beta-HSD_1 enzyme. The structure based mutant enzymes, Asn100Ser, Asn100Ala, Glu126Leu, His232Ala, Ser322Ala and Asn323Leu, have been constructed and functionally characterized. The mutagenic experiments have confirmed the predicted roles of the His232 and Asn323 residues in recognition of the 17-keto group of the substrate and identified Asn100 and Glu126 residues as key residues that participate for the dehydrogenase and isomerization reactions, respectively.

Structure-guided engineering of xylitol dehydrogenase cosubstrate specificity

Xylitol dehydrogenase (XDH) is one of several enzymes responsible for assimilating xylose into eukaryotic metabolism and is useful for fermentation of xylose contained in agricultural byproducts to produce ethanol. For efficient xylose utilization at high flux rates, cosubstrates should be recycled between the NAD+-specific XDH and the NADPH-preferring xylose reductase, another enzyme in the pathway. To understand and alter the cosubstrate specificity of XDH, we determined the crystal structure of the Gluconobacter oxydans holoenzyme to 1.9 angstroms resolution. The structure reveals that NAD+ specificity is largely conferred by Asp38, which interacts with the hydroxyls of the adenosine ribose. Met39 stacked under the purine ring and was also located near the 2' hydroxyl. Based on the location of these residues and on sequence alignments with related enzymes of various cosubstrate specificities, we constructed a double mutant (D38S/M39R) that was able to exclusively use NADP+, with no loss of activity.

Characterization of cDNAs encoding cholesterol side chain cleavage and 3beta-hydroxysteroid dehydrogenase in the freshwater stingray Potamotrygon motoro

The interrenal gland (adrenocortical homolog) of elasmobranchs produces a unique steroid, 1alpha-hydroxycorticosterone (1alpha-B). The synthesis of this and most other steroids requires both cholesterol side chain cleavage (CYP11A) and 3beta-hydroxysteroid dehydrogenase (HSD3). To facilitate the study of elasmobranch steroidogenesis, we isolated complementary DNAs encoding CYP11A and HSD3 from the freshwater stingray Potamotrygon motoro. The P. motoro CYP11A (2182 bp total length) and HSD3 (2248 bp total length) cDNAs harbor open reading frames that encode proteins of 542 and 376 amino acids (respectively) that are similar (CYP11A: 39-61% identical; HSD3: 36-53% identical) to their homologs from other vertebrates. In molecular phylogenetic analysis, P. motoro CYP11A segregates with CYP11A proteins (and not with related CYP11B proteins) and P. motoro HSD3 segregates with steroidogenic HSD3 proteins from other fishes. CYP11A and HSD3 mRNA is found only in interrenal and gonadal tissues, indicating de novo steroidogenesis is restricted to these tissues. Because 1alpha-B is thought to act in the elasmobranch response to hydromineral disturbances, we examined the effect of adapting P. motoro to 10 ppt seawater on mRNAs encoding steroidogenic genes. The P. motoro response to this salinity challenge does not include interrenal hypertrophy or an increase in the levels of interrenal CYP11A, HSD3 or steroidogenic acute regulatory protein (StAR) mRNA. This study is the first to isolate full length cDNAs encoding elasmobranch CYP11A and HSD3 and the first to examine the regulation of steroidogenic genes in elasmobranch interrenal cells.

Determining structure and function of steroid dehydrogenase enzymes by sequence analysis, homology modeling, and rational mutational analysis

The short-chain oxidoreductase (SCOR) family of enzymes includes over 6,000 members identified in sequenced genomes. Of these enzymes, approximately 300 have been characterized functionally, and the three-dimensional crystal structures of approximately 40 have been reported. Since some SCOR enzymes are steroid dehydrogenases involved in hypertension, diabetes, breast cancer, and polycystic kidney disease, it is important to characterize the other members of the family for which the biological functions are currently unknown and to determine their three-dimensional structure and mechanism of action. Although the SCOR family appears to have only a single fully conserved residue, it was possible, using bioinformatics methods, to determine characteristic fingerprints composed of 30-40 residues that are conserved at the 70% or greater level in SCOR subgroups. These fingerprints permit reliable prediction of several important structure-function features including cofactor preference, catalytic residues, and substrate specificity. Human type 1 3beta-hydroxysteroid dehydrogenase isomerase (3beta-HSDI) has 30% sequence identity with a human UDP galactose 4-epimerase (UDPGE), a SCOR family enzyme for which an X-ray structure has been reported. Both UDPGE and 3-HSDI appear to trace their origins back to bacterial 3alpha,20beta-HSD. Combining three-dimensional structural information and sequence data on the 3alpha,20beta-HSD, UDPGE, and 3beta-HSDI subfamilies with mutational analysis, we were able to identify the residues critical to the dehydrogenase function of 3-HSDI. We also identified the residues most probably responsible for the isomerase activity of 3beta-HSDI. We test our predictions by specific mutations based on sequence analysis and our structure-based model.

Molecular biology of the 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase gene family

The 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) isoenzymes are responsible for the oxidation and isomerization of Delta(5)-3beta-hydroxysteroid precursors into Delta(4)-ketosteroids, thus catalyzing an essential step in the formation of all classes of active steroid hormones. In humans, expression of the type I isoenzyme accounts for the 3beta-HSD activity found in placenta and peripheral tissues, whereas the type II 3beta-HSD isoenzyme is predominantly expressed in the adrenal gland, ovary, and testis, and its deficiency is responsible for a rare form of congenital adrenal hyperplasia. Phylogeny analyses of the 3beta-HSD gene family strongly suggest that the need for different 3beta-HSD genes occurred very late in mammals, with subsequent evolution in a similar manner in other lineages. Therefore, to a large extent, the 3beta-HSD gene family should have evolved to facilitate differential patterns of tissue- and cell-specific expression and regulation involving multiple signal transduction pathways, which are activated by several growth factors, steroids, and cytokines. Recent studies indicate that HSD3B2 gene regulation involves the orphan nuclear receptors steroidogenic factor-1 and dosage-sensitive sex reversal adrenal hypoplasia congenita critical region on the X chromosome gene 1 (DAX-1). Other findings suggest a potential regulatory role for STAT5 and STAT6 in transcriptional activation of HSD3B2 promoter. It was shown that epidermal growth factor (EGF) requires intact STAT5; on the other hand IL-4 induces HSD3B1 gene expression, along with IL-13, through STAT 6 activation. However, evidence suggests that multiple signal transduction pathways are involved in IL-4 mediated HSD3B1 gene expression. Indeed, a better understanding of the transcriptional factors responsible for the fine control of 3beta-HSD gene expression may provide insight into mechanisms involved in the functional cooperation between STATs and nuclear receptors as well as their potential interaction with other signaling transduction pathways such as GATA proteins. Finally, the elucidation of the molecular basis of 3beta-HSD deficiency has highlighted the fact that mutations in the HSD3B2 gene can result in a wide spectrum of molecular repercussions, which are associated with the different phenotypic manifestations of classical 3beta-HSD deficiency and also provide valuable information concerning the structure-function relationships of the 3beta-HSD superfamily. Furthermore, several recent studies using type I and type II purified enzymes have elegantly further characterized structure-function relationships responsible for kinetic differences and coenzyme specificity.

Minireview: cellular redox state regulates hydroxysteroid dehydrogenase activity and intracellular hormone potency

Hydroxysteroid dehydrogenases (HSDs) interconvert potent and relatively inactive forms of individual steroid hormones using nicotinamide cofactors NADPH/NADP(+) and NADH/NAD(+) [nicotinamide adenine dinucleotide (phosphate), reduced/oxidized forms]. Although reactions with purified enzymes in vitro may be driven in either direction depending on the assay conditions, HSD enzymes appear to function in one direction or the other in intact cells. At least for some of these enzymes, however, the apparent unidirectional metabolism actually reflects bidirectional catalysis that reaches a pseudoequilibrium state with a strong directional preference. This directional preference, in turn, derives from intracellular concentration gradients for the nicotinamide cofactors and the relative affinities of each HSD for these cofactors. Because the concentrations of free cofactor exceed those of steroids by many orders of magnitude, the activities of these enzymes are predominantly driven by cofactor abundance, which is linked to intermediary metabolism. Consequently, the amount of active steroids in cells containing HSDs may be modulated by cofactor abundance and, hence, intracellular redox state. We will review the evidence linking cofactor handling and HSD activity, speculate on additional ways that intracellular metabolism can alter HSD activity and, thus, hormone potency, and discuss fruitful avenues of further investigation.