Proteomics, modeling, and fluorescence assays delineate cytochrome b5 residues involved in binding and stimulation of cytochrome P450 17A1 17,20-lyase

Cytochrome b5 (b5) is known to stimulate some catalytic activities of cytochrome P450 (P450, CYP) enzymes, although mechanisms still need to be defined. The reactions most strongly enhanced by b5 are the 17,20-lyase reactions of P450 17A1 involved in steroid biosynthesis. We had previously used a fluorescently labeled human b5 variant (Alexa 488-T70C-b5) to characterize human P450 17A1-b5 interactions, but subsequent proteomic analyses indicated that lysines in b5 were also modified with Alexa 488 maleimide in addition to Cys-70, due to disulfide dimerization of the T70C mutant. A series of b5 variants were constructed with Cys replacements for the identified lysine residues and labeled with the dye. Fluorescence attenuation and the function of b5 in the steroid lyase reaction depended on the modified position. Apo-b5 (devoid of heme group) studies revealed the lack of involvement of the b5 heme in the fluorescence attenuation. A structural model of b5 with P450 17A1 was predicted using AlphaFold-Multimer algorithms/Rosetta docking, based upon the individual structures, which predicted several new contacts not previously reported, that is, interactions of b5 Glu-48:17A1 Arg-347, b5 Glu-49:17A1 Arg-449, b5 Asp-65:17A1 Arg-126, b5 Asp-65:17A1 Arg-125, and b5 Glu-61:17A1 Lys-91. Fluorescence polarization assays with two modified b5 variants yielded Kd values (for b5-P450 17A1) of 120 to 380 nM, the best estimate of binding affinity. We conclude that both monomeric and dimeric b5 can bind to P450 17A1 and stimulate activity. Results with the mutants indicate that several Lys residues in b5 are sensitive to the interaction with P450 17A1, including Lys-88 and Lys-91.

The association between CYB5A gene rs1790834 polymorphism and clinical improvement after 6 months of leflunomide treatment in women with rheumatoid arthritis

INTRODUCTION/OBJECTIVES

Rheumatoid arthritis (RA) is a disease affecting around 1% of the population in developed countries and can be treated with leflunomide. The higher prevalence of RA among women and numerous previous studies suggested the crucial role of sex hormones. Cytochrome CYB5A regulates the synthesis of androgens. Therefore, the aim of this study was to determine the association between common CYB5A gene polymorphism and the response to leflunomide in women with RA.

METHODS

This study included 111 patients. All of them received oral leflunomide monotherapy at a dose of 20 mg daily. Women were genotyped for the presence of CYB5A rs1790834 polymorphism and evaluated monthly for 6 months following the initiation of treatment.

RESULTS

After 6 months of therapy, patients with the GG genotype had higher DAS28 values and less improvement in DAS28 compared to patients with the GA and AA genotypes (p = 0.04). No statistically significant differences were found in relation to other disease activity parameters.

CONCLUSIONS

The results of the current study suggest a possible association of the CYB5A rs1790834 polymorphism with some disease activity parameters in RA patients treated with leflunomide during the initial therapy period. However, confirmation of the effect of this polymorphism on the efficacy of leflunomide treatment requires further studies. Key Points • Leflunomide is the synthetic disease-modifying anti-rheumatic drug used in the therapy of rheumatoid arthritis. • CYB5A rs1790834 gene polymorphism may influence the clinical improvement after 6 months of leflunomide treatment in women with rheumatoid arthritis.

Function and regulation of a steroidogenic CYP450 enzyme in the mitochondrion of Toxoplasma gondii

As an obligate intracellular parasite, Toxoplasma gondii must import essential nutrients from the host cell into the parasitophorous vacuole. We previously reported that the parasite scavenges cholesterol from host endocytic organelles for incorporation into membranes and storage as cholesteryl esters in lipid droplets. In this study, we have investigated whether Toxoplasma utilizes cholesterol as a precursor for the synthesis of metabolites, such as steroids. In mammalian cells, steroidogenesis occurs in mitochondria and involves membrane-bound type I cytochrome P450 oxidases that are activated through interaction with heme-binding proteins containing a cytochrome b5 domain, such as members of the membrane-associated progesterone receptor (MAPR) family. Our LC-MS targeted lipidomics detect selective classes of hormone steroids in Toxoplasma, with a predominance for anti-inflammatory hydroxypregnenolone species, deoxycorticosterone and dehydroepiandrosterone. The genome of Toxoplasma contains homologs encoding a single type I CYP450 enzyme (we named TgCYP450mt) and a single MAPR (we named TgMAPR). We showed that TgMAPR is a hemoprotein with conserved residues in a heme-binding cytochrome b5 domain. Both TgCYP450 and TgMAPR localize to the mitochondrion and show interactions in in situ proximity ligation assays. Genetic ablation of cyp450mt is not tolerated by Toxoplasma; we therefore engineered a conditional knockout strain and showed that iΔTgCYP450mt parasites exhibit growth impairment in cultured cells. Parasite strains deficient for mapr could be generated; however, ΔTgMAPR parasites suffer from poor global fitness, loss of plasma membrane integrity, aberrant mitochondrial cristae, and an abnormally long S-phase in their cell cycle. Compared to wild-type parasites, iΔTgCYP450mt and ΔTgMAPR lost virulence in mice and metabolomics studies reveal that both mutants have reduced levels of steroids. These observations point to a steroidogenic pathway operational in the mitochondrion of a protozoan that involves an evolutionary conserved TgCYP450mt enzyme and its binding partner TgMAPR.

Regulating Effect of Cytochrome b5 Overexpression on Human Breast Cancer Cells

Imbalance in the cellular redox system is thought to be associated with the induction and progression of breast cancers, and heme proteins may regulate the redox balance. Cytochrome b₅ (Cyt b₅) is a small mitochondrial heme protein. Its function and regulating mechanism in breast cancer remain unknown. In this study, we elucidated the level of endogenous oxidative stress in breast cancer cells, MCF-7 cells (hormone receptor-positive cells) and MDA-MB-231 cells (triple-negative cells), and investigated the difference in Cyt b₅ content. Based on the low content of Cyt b₅ in MDA-MB-231 cells, the overexpression of Cyt b₅ was found to regulate the oxidative stress and apoptosis cascades, including ERK1/2 and Akt signaling pathways. The overexpressed Cyt b₅ MDA-MB-231 cells were shown to exhibit decreased oxidative stress, less phosphorylation of ERK1/2 and Akt, and less cleavage of caspases 3 and 9 upon treatment with H₂O₂, as compared to those of normal MDA-MB-231 cells. Moreover, the overexpressed Cyt b₅ most likely functioned by interacting with its protein partner, Cyt c, as suggested by co-immunoprecipitation studies. These results indicated that Cyt b₅ has different effects on breast cancer cells of different phenotypes, which provides useful information for understanding the multiple roles of Cyt b₅ and provides clues for clinical treatment.

Functional variants in cytochrome b5 type A (CYB5A) are enriched in Southwest American Indian individuals and associate with obesity

OBJECTIVE

This study aimed to identify genetic variants enriched in Southwest American Indian (SWAI) individuals that associate with BMI.

METHODS

Whole genome sequencing data (n = 296) were used to identify potentially functional variants that are common in SWAI individuals (minor allele frequency ≥10%) but rare in other ethnic groups (minor allele frequency < 0.1%). Enriched variants were tested for association with BMI in 5,870 SWAI individuals. One variant was studied using a luciferase reporter, and haplotypes that included this variant were analyzed for association with various measures of obesity (n = 917-5,870), 24-hour energy expenditure (24-h EE; n = 419), and skeletal muscle biopsy expression data (n = 207).

RESULTS

A 5' untranslated region variant in cytochrome b5 type A (CYB5A), rs548402150, met the enrichment criteria and associated with increased BMI (β = 2%, p = 0.004). Functionally, rs548402150 decreased luciferase expression by 30% (p = 0.003) and correlated with decreased skeletal muscle CYB5A expression (β = -0.5 SD, p = 0.0008). Combining rs548402150 with two splicing quantitative trait loci in CYB5A identified a haplotype carried almost exclusively in SWAI individuals that associated with increased BMI (β = 3%, p = 0.0003) and decreased CYB5A expression, whereas the most common haplotype in all ethnic groups associated with lower BMI and percentage of body fatness, increased 24-h EE, and increased CYB5A expression.

CONCLUSIONS

Further studies on the effects of CYB5A on 24-h EE and BMI may provide insights into obesity-related physiology.

A genome-wide association study reveals that the cytochrome b5 involved in seed reserve mobilization during seed germination in rice

A candidate gene cytochrome b5 for the major QTL qSRMP9 for rice seed reserve mobilization was validated during seed germination using a genome-wide association study approach. Seed reserve mobilization plays important roles in the early seedling growth in rice. However, the genetic basis underlying this process is poorly understood. In this study, the genetic architecture of variation in seed reserve mobilization during seed germination was studied using a genome-wide association study approach in rice. Three quantitative trait loci (QTL) including qSRMP6, qSRMP9, and qSRMP12 for seed reserve mobilization percentage were identified. In which, the candidate gene cytochrome b5 (OsCyb5) for the major QTL qSRMP9 was validated. Disruption of this gene in Oscyb5 mutants reduced the seed reserve mobilization and seedling growth compared with wild-type (WT) in rice. There were no significant differences of grain size, starch, protein and total soluble sugar content in the mature grains between Oscyb5 mutants and WT. However, the α-amylase activity in the germinating seeds of Oscyb5 mutants was significantly decreased compared to that of WT, and then, the starch and sugar mobilization and the glucose accumulation during seed germination were significantly decreased in Oscyb5 mutants. Two elite haplotypes of OsCyb5 associated with the higher seed reserve mobilization percentage and its elite single nucleotide polymorphism variations were mainly existed in the INDICA and AUS accessions. The natural variation of OsCyb5 contributing to seed reserve mobilization might be useful for the future rice breeding.

Morphologic and Molecular Characterization of Adrenals and Adrenal Rest Affected by Congenital Adrenal Hyperplasia

Introduction

Adrenocortical hyperplasia and adrenal rest tumor (ART) formation are common in congenital adrenal hyperplasia (CAH). Although driven by excessive corticotropin, much is unknown regarding the morphology and transformation of these tissues. Our study objective was to characterize CAH-affected adrenals and ART and compare with control adrenal and gonadal tissues.

Patients/Methods

CAH adrenals, ART and control tissues were analyzed by histology, immunohistochemistry, and transcriptome sequencing. We investigated protein expression of the ACTH receptor (MC2R), steroidogenic (CYP11B2, CYP11B1, CYB5A) and immune (CD20, CD3, CD68) biomarkers, and delta-like 1 homolog (DLK1), a membrane bound protein broadly expressed in fetal and many endocrine cells. RNA was isolated and gene expression was analyzed by RNA sequencing (RNA-seq) followed by principle component, and unsupervised clustering analyses.

Results

Based on immunohistochemistry, CAH adrenals and ART demonstrated increased zona reticularis (ZR)-like CYB5A expression, compared to CYP11B1, and CYP11B2, markers of zona fasciculata and zona glomerulosa respectively. CYP11B2 was mostly absent in CAH adrenals and absent in ART. DLK1 was present in CAH adrenal, ART, and also control adrenal and testis, but was absent in control ovary. Increased expression of adrenocortical marker MC2R, was observed in CAH adrenals compared to control adrenal. Unlike control tissues, significant nodular lymphocytic infiltration was observed in CAH adrenals and ART, with CD20 (B-cell), CD3 (T-cell) and CD68 (macrophage/monocyte) markers of inflammation. RNA-seq data revealed co-expression of adrenal MC2R, and testis-specific INSL3, HSD17B3 in testicular ART indicating the presence of both gonadal and adrenal features, and high expression of DLK1 in ART, CAH adrenals and control adrenal. Principal component analysis indicated that the ART transcriptome was more similar to CAH adrenals and least similar to control testis tissue.

Conclusions

CAH-affected adrenal glands and ART have similar expression profiles and morphology, demonstrating increased CYB5A with ZR characteristics and lymphocytic infiltration, suggesting a common origin that is similarly affected by the abnormal hormonal milieu. Immune system modulators may play a role in tumor formation of CAH.

Effect of mutations in porcine CYB5A and CYP17A1 on the metabolism of pregnenolone

Cytochrome P450 17A1 (CYP17A1) catalyses the 17α-hydroxylation and 17,20 lyase reactions to convert pregnenolone to 17α-hydroxypregnenolone (17OHP) and subsequently the androgen dehydroepiandrosterone (DHEA). In pigs and humans, CYP17A1 also catalyses the delta-16-synthase reaction to produce the 16-androstene steroid 5,16-androstadien-3β-ol (16A), which is then further metabolised to the sex pheromone androstenone. Cytochrome b5A (CYB5A) stimulates the 17,20 lyase reaction and is required for the delta 16-synthase reaction. We have identified and mutated residues in porcine CYP17A1 and CYB5A that may alter the synthesis of DHEA and 16A. This included residues in the steroid binding pocket of CYP17A1 and residues on the surface of CYP17A1 and CYB5A that are involved in binding of CYP17A1 to CYB5A. We then expressed the various mutations of CYB5A and CYP17A1 along with porcine cytochrome P450 oxidoreductase (POR) and cytochrome b5 reductase (CYB5R3) in HEK293 cells and measured the formation of metabolites 16A, 17OHP and DHEA from radiolabelled pregnenolone by high performance liquid chromatography (HPLC). Mutations were identified in both CYP17A1 and CYB5A that affected the production of the different metabolites and also affected the overall production of metabolites. Several combinations of mutations decreased the production of both 16A and DHEA and increased production of 17OHP, while the N62S mutation of CYB5A with wild type CYP17A1 increased production of both 16A and DHEA. The best combination of mutations to reduce the production of 16A, while maintaining the production of DHEA and the overall conversion similar to wild type are the N21K, L28V, N21K/L28V and the R52 M/N62S mutations of CYB5A with the D103S mutation of CYP17A1.

Soybean Cytochrome b5 Is a Restriction Factor for Soybean Mosaic Virus

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybeans (Glycine max). In this study, an interaction between the SMV P3 protein and cytochrome b5 was detected by yeast two-hybrid assay, and bimolecular fluorescence complementation assay showed that the interaction took place at the cell periphery. Further, the interaction was confirmed by co-immunoprecipitation analysis. Quantitative real-time polymerase chain reaction analysis revealed that GmCYB5 gene was differentially expressed in resistant and susceptible soybean plants after inoculation with SMV-SC15 strain. To test the involvement of this gene in SMV resistance, the GmCYB5 was silenced using a bean pod mottle virus (BPMV)-based vector construct. Results showed that GmCYB5-1 was 83% and 99% downregulated in susceptible (NN1138-2) and resistant (RN-9) cultivars, respectively, compared to the empty vector-treated plants. Silencing of GmCYB5 gene promotes SMV replication in soybean plants. Our results suggest that during SMV infection, the host CYB5 protein targets P3 protein to inhibit its proliferation. Taken together, these results suggest that CYB5 is an important factor in SMV infection and replication in soybeans, which could help soybean breeders develop SMV resistant soybean cultivars.

Application of hepatic cytochrome b5/P450 reductase null (HBRN) mice to study the role of cytochrome b5 in the cytochrome P450-mediated bioactivation of the anticancer drug ellipticine

The anticancer drug ellipticine exerts its genotoxic effects after metabolic activation by cytochrome P450 (CYP) enzymes. The present study has examined the role of cytochrome P450 oxidoreductase (POR) and cytochrome b5 (Cyb5), electron donors to P450 enzymes, in the CYP-mediated metabolism and disposition of ellipticine in vivo. We used Hepatic Reductase Null (HRN) and Hepatic Cytochrome b5/P450 Reductase Null (HBRN) mice. HRN mice have POR deleted specifically in hepatocytes; HBRN mice also have Cyb5 deleted in the liver. Mice were treated once with 10 mg/kg body weight ellipticine (n = 4/group) for 24 h. Ellipticine-DNA adduct levels measured by 32P-postlabelling were significantly lower in HRN and HBRN livers than in wild-type (WT) livers; however no significant difference was observed between HRN and HBRN livers. Ellipticine-DNA adduct formation in WT, HRN and HBRN livers correlated with Cyp1a and Cyp3a enzyme activities measured in hepatic microsomes in the presence of NADPH confirming the importance of P450 enzymes in the bioactivation of ellipticine in vivo. Hepatic microsomal fractions were also utilised in incubations with ellipticine and DNA in the presence of NADPH, cofactor for POR, and NADH, cofactor for Cyb5 reductase (Cyb5R), to examine ellipticine-DNA adduct formation. With NADPH adduct formation decreased as electron donors were lost which correlated with the formation of the reactive metabolites 12- and 13-hydroxy-ellipticine in hepatic microsomes. No difference in adduct formation was observed in the presence of NADH. Our study demonstrates that Cyb5 contributes to the P450-mediated bioactivation of ellipticine in vitro, but not in vivo.

The use of tail-anchored protein chimeras to enhance liposomal cargo delivery

Background

Liposomes are employed as drug delivery vehicles offering a beneficial pharmacokinetic/distribution mechanism for in vivo therapeutics. Therapeutic liposomes can be designed to target specific cell types through the display of epitope-specific targeting peptides on their surface. The majority of peptides are currently attached by chemical modification of lipid constituents. Here we investigate an alternative and novel method of decorating liposomes with targeting ligand, using remotely and spontaneously inserting chimeric tail-anchored membrane (TA) proteins to drug loaded liposomes.

Methods and results

An artificial TA protein chimera containing the transmembrane domain from the spontaneously inserting TA protein cytochrome b5 (Cytb5) provided a robust membrane tether for the incorporation of three different targeting moieties into preformed liposomes. The moieties investigated were the transactivator of transcription (TAT) peptide, the EGF-receptor binding sequence GE11 and the placental and tumour homing ligand CCGKRK. In all cases, TA protein insertion neither significantly altered the size of the liposomes nor reduced drug loading. The efficacy of this novel targeted delivery system was investigated using two human cell lines, HeLa M and BeWo. Short term incubation with one ligand-modified TA chimera, incorporating the TAT peptide, significantly enhanced liposomal delivery of the encapsulated carboxyfluorescein reporter.

Conclusion

The Cytb5 TA was successfully employed as a membrane anchor for the incorporation of the desired peptide ligands into a liposomal drug delivery system, with minimal loss of cargo during insertion. This approach therefore provides a viable alternative to chemical conjugation and its potential to accommodate a wider range of targeting ligands may provide an opportunity for enhancing drug delivery.

Efficiency of the sulfate pathway in comparison to the Δ4- and Δ5-pathway of steroidogenesis in the porcine testis

Sulfonated steroids are increasingly recognized as a circulating reservoir of precursors for the local production of active steroids in certain target tissues. As an alternative to sulfonation of unconjugated steroids by cytosolic sulfotransferases, their direct formation from sulfonated precursors has been described. However, productivity and physiological relevance of this sulfate pathway of steroidogenesis are still widely unclear. Applying the porcine testis as a model, conversion of pregnenolone sulfate (P5S, sulfate pathway) by CYP17A1 was assessed in comparison to the parallel conversions of pregnenolone (P5, Δ5-pathway) and progesterone (P4, Δ4-pathway). To characterize conversions in the virtual absence of competing enzyme activities, in a first series of experiments porcine recombinant CYP17A1 was incubated with the respective substrate in the presence of bovine recombinant cytochrome P450 oxidoreductase (CPR) and cytochrome b5 (b5). Moreover, porcine testicular microsomal fractions were used as a source of homologous CYP17A1, CPR and b5. Invariably 17α-hydroxylation of P5S was, if at all, only minimal and no formation of dehydroepiandrosterone sulfate from P5S was detectable. Consistent with earlier studies porcine CYP17A1 efficiently metabolized P4 and P5 in both assay systems. Metabolism of P4 and P5 by testicular microsomal protein varied substantially between the five animals tested. In conclusion, a physiologically relevant sulfate pathway for the production of C19-steroids from P5S via CYP17A1 is very unlikely in the porcine testis.

Steroid 17-hydroxylase and 17,20-lyase deficiencies, genetic and pharmacologic

Steroid 17-hydroxylase 17,20-lyase (cytochrome P450c17, P450 17A1, CYP17A1) catalyzes two major reactions: steroid 17-hydroxylation followed by the 17,20-lyase reactions. The most severe mutations in the cognate CYP17A1 gene abrogate all activities and cause combined 17-hydroxylase/17,20-lyase deficiency (17OHD), a biochemical phenotype that is replicated by treatment with the potent CYP17A1 inhibitor abiraterone acetate. The adrenals of patients with 17OHD synthesize 11-deoxycorticosterone (DOC) and corticosterone but no 19-carbon steroids, similar to the rodent adrenal, and DOC causes hypertension and hypokalemia. Loss of 17,20-lyase activity precludes sex steroid synthesis and leads to sexual infantilism. Rare missense CYP17A1 mutations minimally disrupt 17-hydroxylase activity but cause isolated 17,20-lyase deficiency (ILD), Mutations in the POR gene encoding the required cofactor protein cytochrome P450-oxidoreductase causes a spectrum of disease from ILD to 17OHD combined with 21-hydroxylase and aromatase deficiencies, sometimes including skeletal malformations. Mutations in the CYB5A gene encoding a second cofactor protein cytochrome b₅ also selectively disrupt 17,20-lyase activity and cause the purest form of ILD. The clinical manifestations of these conditions are best understood in the context of the biochemistry of CYP17A1.

Using Photoconvertible and Extractable Fluorescent Proteins to Study Autophagy in Plants

Several methodologies have been employed to understand the kinetics of induced autophagic degradation in plants, but most of them are not capable of distinguishing the autophagic cargo proteins before and after induction of autophagy in cells. Here, we designed a mass photoconverter that allowed us to simultaneously monitor protein synthesis and degradation in tobacco BY-2 cells using a photoconvertible fluorescence marker protein, Kikume Green Red (KikGR). An example of a new protocol for the analysis of autophagy progression using a fusion protein of cytochrome b5 and KikGR under phosphate starvation is described. The other example described is the analysis of the proliferation of Golgi apparatus in tobacco BY-2 cells using the fusion protein of a prolyl 4-hydroxylase NtP4H1.1 and monomeric KikGR. A detailed protocol on key analysis, as well as tips and notes for experiments using KikGR proteins, are described.

CB5C affects the glucosinolate profile in Arabidopsis thaliana

Cytochrome b5 (CB5) proteins are small heme-binding proteins, that influence cytochrome P450 activity. While only one CB5 isoform is found in mammals, higher plants have several isoforms of these proteins. The roles of the many CB5 isoforms in plants remain unknown. We hypothesized that CB5 proteins support the cytochrome P450 enzymes of plant specialized metabolism and found CB5C from Arabidopsis thaliana to co-express with glucosinolate biosynthetic genes. We characterized the glucosinolate profiles of 2 T-DNA insertion mutants of CB5C, and found that long-chained aliphatic glucosinolates were reduced in one of the mutant lines - a phenotype that was exaggerated upon methyl-jasmonate treatment. These results support the hypothesis, that CB5C influences glucosinolate biosynthesis, however, the mode of action remains unknown. Furthermore, the mutants differed in their biomass response to methyl jasmonate treatment. Thereby, our results highlight the varying effects of T-DNA insertion sites, as the 2 analyzed alleles show different phenotypes.

Monooxygenase system in Guerin’s carcinoma of rats under conditions of ω-3 polyunsaturated fatty acids administration

The aim of the study was to determine the variations of function in components of monooxygenase system (MOS) of rat Guerin’s carcinoma under ω-3 polyunsaturated fatty acids (PUFAs) administration. The activity of Guerin’s carcinoma microsomal NADH-cytochrome b5 reductase, the content and the rate of cytochrome b5 oxidation-reduction, the content and the rate of cytochrome Р450 oxidation-reduction have been investigated in rats with tumor under conditions of ω-3 PUFAs administration. ω-3 PUFAs supplementation before and after transplantation of Guerin’s carcinoma resulted in the increase of NADH-cytochrome b5 reductase activity and decrease of cytochrome b5 level in the Guerin’s carcinoma microsomal fraction in the logarithmic phases of carcinogenesis as compared to the tumor-bearing rats. Increased activity of NADH-cytochrome b5 reductase facilitates higher electron flow in redox-chain of MOS. Under decreased cytochrome b5 levels the electrons are transferred to oxygen, which leads to heightened generation of superoxide (O2•-) in comparison to control. It was shown, that the decrease of cytochrome P450 level in the Guerin’s carcinoma microsomal fraction in the logarithmic phases of oncogenesis under ω-3 PUFAs administration may be associated with its transition into an inactive form – cytochrome P420. This decrease in cytochrome P450 coincides with increased generation of superoxide by MOS oxygenase chain.

Mechanistic Scrutiny Identifies a Kinetic Role for Cytochrome b5 Regulation of Human Cytochrome P450c17 (CYP17A1, P450 17A1)

Cytochrome P450c17 (P450 17A1, CYP17A1) is a critical enzyme in the synthesis of androgens and is now a target enzyme for the treatment of prostate cancer. Cytochrome P450c17 can exhibit either one or two physiological enzymatic activities differentially regulated by cytochrome b5. How this is achieved remains unknown. Here, comprehensive in silico, in vivo and in vitro analyses were undertaken. Fluorescence Resonance Energy Transfer analysis showed close interactions within living cells between cytochrome P450c17 and cytochrome b5. In silico modeling identified the sites of interaction and confirmed that E48 and E49 residues in cytochrome b5 are essential for activity. Quartz crystal microbalance studies identified specific protein-protein interactions in a lipid membrane. Voltammetric analysis revealed that the wild type cytochrome b5, but not a mutated, E48G/E49G cyt b5, altered the kinetics of electron transfer between the electrode and the P450c17. We conclude that cytochrome b5 can influence the electronic conductivity of cytochrome P450c17 via allosteric, protein-protein interactions.

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'.

Effect of Cytochrome b5 Content on the Activity of Polymorphic CYP1A2, 2B6, and 2E1 in Human Liver Microsomes

Human cytochrome b5 (Cyt b5) plays important roles in cytochrome P450 (CYP)-mediated drug metabolism. However, the expression level of Cyt b5 in normal human liver remains largely unknown. The effect of Cyt b5 on overall CYP activity in human liver microsomes (HLM) has rarely been reported and the relationship between Cyt b5 and the activity of polymorphic CYP has not been systematically investigated. In this study, we found that the median value of Cyt b5 protein was 270.01 pmol/mg from 123 HLM samples, and 12- and 19-fold individual variation was observed in Cyt b5 mRNA and protein levels, respectively. Gender and smoking clearly influenced Cyt b5 content. In addition, we found that Cyt b5 protein levels significantly correlated with the overall activity of CYP1A2, 2B6, and 2E1 in HLM. However, when the CYP activities were sorted by single nucleotide polymorphisms (SNP), the effect of Cyt b5 protein on the kinetic parameters varied greatly. There were significant correlations between Cyt b5 content and V_max and CL_int of CYP1A2 wild-types (3860GG, 2159GG, and 5347CC) as well as homozygous mutants (163AA and 3113GG). In contrast to V_max and CL_int, the K_m of CYP2B6 516GG and 785AA genotypes was inversely associated with Cyt b5 content. Correlations between Cyt b5 content and V_max and CL_int of CYP2E1 -1293GG, -1293GC, 7632TT, 7632TA, -333TT, and -352AA genotypes were also observed. In conclusion, Cyt b5 expression levels varied considerably in the Chinese cohort from this study. Cyt b5 had significant impact on the overall activity of CYP1A2, 2B6, and 2E1 in HLM and the effects of Cyt b5 protein on polymorphic CYP1A2, 2B6, and 2E1 activity were SNP-dependent. These findings suggest that Cyt b5 plays an important role in CYP-mediated activities in HLM and may possibly be a contributing factor for the individual variation observed in CYP enzyme activities.

A steroidogenic pathway for sulfonated steroids: the metabolism of pregnenolone sulfate

In many tissues sulfonated steroids exceed the concentration of free steroids and recently they were also shown to fulfill important physiological functions. While it was previously demonstrated that cholesterol sulfate (CS) is converted by CYP11A1 to pregnenolone sulfate (PregS), further conversion of PregS has not been studied in detail. To investigate whether a steroidogenic pathway for sulfonated steroids exists similar to the one described for free steroids, we examined the interaction of PregS with CYP17A1 in a reconstituted in-vitro system. Difference spectroscopy revealed a Kd-value of 74.8±4.2μM for the CYP17A1-PregS complex, which is 2.5-fold higher compared to the CYP17A1-pregnenolone (Preg) complex. Mass spectrometry experiments proved for the first time that PregS is hydroxylated by CYP17A1 at position C17, identically to pregnenolone. A higher Km- and a lower kcat-value for CYP17A1 using PregS compared with Preg were observed, indicating a 40% reduced catalytic efficiency when using the sulfonated steroid. Furthermore, we analyzed whether the presence of cytochrome b5 (b5) has an influence on the CYP17A1 dependent conversion of PregS, as was demonstrated for Preg. Interestingly, with 17OH-PregS no scission of the 17,20-carbon-carbon bond occurs, when b5 is added to the reconstituted in-vitro system, while b5 promotes the formation of DHEA from 17OH-Preg. When using human SOAT-HEK293 cells expressing CYP17A1 and CPR, we could confirm that PregS is metabolized to 17OH-PregS, strengthening the potential physiological meaning of a pathway for sulfonated steroids.

[Protein-protein interactions of cytochromes P450 3A4 and 3A5 with their intermediate redox partners cytochromes b5]

Molecular interactions between proteins redox partners (cytochromes P450 3A4, 3A5 and cytochrome b5) within the monooxygenase system, which is known to be involved in drug biotransformation, were investigated. Human cytochromes P450 3A4 and 3A5 (CYP3A4 and CYP3A5) form complexes with various cytochromes b5: the microsomal (b5mc) and mitochondrial (b5om) forms of this protein, as well as with 2 "chimeric" proteins, b5(om-mc), b5(mc-om). Kinetic constants and equilibrium dissociation constants were determined by the SPR biosensor. Essential distinction between CYP3A4 and CYP3A5 was only observed upon their interactions with cytochrome b5om. Electroanalytical characteristics of electrodes with immobilized hemoproteins were obtained. The electrochemical analysis of CYP3A4, CYP3A5, b5mc, b5om, b5(om-mc), and b5(mc-om) immobilized on screen printed graphite electrodes modified with membranous matrix revealed that these proteins have very close reduction potentials -0.435 - -0.350 V (vs. Ag/AgCl). Cytochrome b5mc was shown to be capable of stimulating the electrocatalytic activity of CYP3A4 to testosterone.

Whole exome sequencing identifies variation in CYB5A and RNF10 associated with adiposity and type 2 diabetes

OBJECTIVE

Few coding variants in genes associated with type 2 diabetes (T2D) have been identified, and the underlying physiologic mechanisms whereby susceptibility genes influence T2D risk are often unknown. The objective of this study was to identify coding variation that increases risk for T2D via an effect on a pre-diabetic trait.

METHODS

Whole exome sequencing was done in 177 Pima Indians. Selected variants (N = 345) were genotyped in 555 subjects characterized for body fatness, glucose disposal rates during a clamp, acute insulin response to glucose, and 2-h plasma glucose concentrations during an OGTT, and were also genotyped in up to 5,880 subjects with longitudinal measures of BMI. Variants associated with quantitative traits were assessed for association with T2D in 7,667 subjects.

RESULTS

rs7238987 in CYB5A associated with body fatness (P = 7.0 × 10(-6) ). This SNP and a novel SNP in RNF10 also associated with maximum recorded BMI (P = 6.2 × 10(-7) and P = 7.2 × 10(-4) ) and maximum childhood BMI z-score (P = 5.9 × 10(-4) and P = 8.5 × 10(-7) ). The BMI increasing alleles increased the risk for T2D (P = 0.01; OR = 1.13 [1.03-1.24] and 9.5 × 10(-3) ; OR = 1.49 [1.10-2.02]).

CONCLUSIONS

CYB5A, which has a role in stearyl-CoA-desaturase activity, and RNF10, with an unknown role in weight regulating pathways, associated with adiposity and nominally increased the risk for T2D in American Indians.

Association of cytochrome b5 with ETR1 ethylene receptor signaling through RTE1 in Arabidopsis

Ethylene plays important roles in plant growth, development and stress responses, and is perceived by a family of receptors that repress ethylene responses when ethylene is absent. Repression by the ethylene receptor ETR1 depends on an integral membrane protein, REVERSION TO ETHYLENE SENSITIVITY1 (RTE1), which acts upstream of ETR1 in the endoplasmic reticulum (ER) membrane and Golgi apparatus. To investigate RTE1 function, we screened for RTE1-interacting proteins using the yeast split-ubiquitin assay, which yielded the ER-localized cytochrome b(5) (Cb5) isoform D. Cb5s are small hemoproteins that perform electron transfer reactions in all eukaryotes, but their roles in plants are relatively uncharacterized. Using bimolecular fluorescence complementation (BiFC), we found that all four ER-localized Arabidopsis Cb5 isoforms (AtCb5–B, -C, -D and -E) interact with RTE1 in plant cells. In support of this interaction, atcb5 mutants exhibited phenotypic parallels with rte1 mutants in Arabidopsis. Phenotypes included partial suppression of etr1–2 ethylene insensitivity, and no suppression of RTE1-independent ethylene receptor isoforms. The single loss-of-function mutants atcb5–b, -c and -d appeared similar to the wild-type, but double mutant combinations displayed slight ethylene hypersensitivity. Over-expression of AtCb5–D conferred reduced ethylene sensitivity similar to that conferred by RTE1 over-expression, and genetic analyses suggested that AtCb5–D acts upstream of RTE1 in the ethylene response. These findings suggest an unexpected role for Cb5, in which Cb5 and RTE1 are functional partners in promoting ETR1-mediated repression of ethylene signaling.

The involvement of mitochondrial amidoxime reducing components 1 and 2 and mitochondrial cytochrome b5 in N-reductive metabolism in human cells

The mitochondrial amidoxime reducing component mARC is a recently discovered molybdenum enzyme in mammals. mARC is not active as a standalone protein, but together with the electron transport proteins NADH-cytochrome b5 reductase (CYB5R) and cytochrome b5 (CYB5), it catalyzes the reduction of N-hydroxylated compounds such as amidoximes. The mARC-containing enzyme system is therefore considered to be responsible for the activation of amidoxime prodrugs. All hitherto analyzed mammalian genomes code for two mARC genes (also referred to as MOSC1 and MOSC2), which share high sequence similarities. By RNAi experiments in two different human cell lines, we demonstrate for the first time that both mARC proteins are capable of reducing N-hydroxylated substrates in cell metabolism. The extent of involvement is highly dependent on the expression level of the particular mARC protein. Furthermore, the mitochondrial isoform of CYB5 (CYB5B) is clearly identified as an essential component of the mARC-containing N-reductase system in human cells. The participation of the microsomal isoform (CYB5A) in N-reduction could be excluded by siRNA-mediated down-regulation in HEK-293 cells and knock-out in mice. Using heme-free apo-CYB5, the contribution of mitochondrial CYB5 to N-reductive catalysis was proven to strictly depend on heme. Finally, we created recombinant CYB5B variants corresponding to four nonsynonymous single nucleotide polymorphisms (SNPs). Investigated mutations of the heme protein seemed to have no significant impact on N-reductive activity of the reconstituted enzyme system.

Cytochrome b(5) forms homomeric complexes in living cells

Cytochrome b(5) (cyt-b(5)) is a ubiquitous hemoprotein also associated with microsomal cytochrome P450 17α-hydroxylase/17,20 lyase (CYP17A1). In the steroidogenic pathway CYP17A1 catalyses the metabolism of pregnenolone, yielding both glucocorticoid and androgen precursors. While not affecting the 17α-hydroxylation of pregnenolone, cyt-b(5) augments the 17,20 lyase reaction of 17-hydroxypregnenolone, catalyzing the formation of DHEA, through direct protein-protein interactions. In this study, multimeric complex formation of cyt-b(5) and the possible regulatory role of these complexes were investigated. Cyt-b(5) was isolated from ovine liver and used to raise anti-sheep cyt-b(5) immunoglobulins. Immunochemical studies revealed that, in vivo, cyt-b(5) is primarily found in the tetrameric form. Subsequent fluorescent resonance energy transfer (FRET) studies in COS-1 cells confirmed the formation of homomeric complexes by cyt-b(5) in live cells. Site-directed mutagenesis revealed that the C-terminal linker domain of cyt-b(5) is vital for complex formation. The 17,20-lyase activity of CYP17 was augmented by truncated cyt-b(5), which is unable to form complexes when co-expressed in COS-1 cells, thereby implicating the monomeric form of cyt-b(5) as the active species. This study has shown for the first time that cyt-b(5) forms homomeric complexes in vivo, implicating complex formation as a possible regulatory mechanism in steroidogenesis.

Plant sphingolipid fatty acid 2-hydroxylases have unique characters unlike their animal and fungus counterparts

2-Hydroxy fatty acids mainly contained in sphingolipids are synthesized by a sphingolipid fatty acid 2-hydroxylase (FAH). Recently, two FAH homologs in Arabidopsis thaliana (AtFAH1 and AtFAH2), without any cytochrome b₅(Cb5)-like domains, which are essential for the function of Saccharomyces cerevisiae and mammalian FAH, were identified and both AtFAHs were shown to be activated by the interaction with Cb5. In this study, we compared FAHs of various plants, animals and fungi. Interestingly, only plants had two FAH homologs and none of plant FAHs had any Cb5-like domains. In addition, we showed from the interaction and expression analyses that AtFAHs interacted with multiple Cb5s probably in various tissues. Thus, plant FAHs may have evolved unlike animal and fungus FAHs.

Interfacial hydration, dynamics and electron transfer: multi-scale ET modeling of the transient [myoglobin, cytochrome b5] complex

Formation of a transient [myoglobin (Mb), cytochrome b(5) (cyt b(5))] complex is required for the reductive repair of inactive ferri-Mb to its functional ferro-Mb state. The [Mb, cyt b(5)] complex exhibits dynamic docking (DD), with its cyt b(5) partner in rapid exchange at multiple sites on the Mb surface. A triple mutant (Mb(3M)) was designed as part of efforts to shift the electron-transfer process to the simple docking (SD) regime, in which reactive binding occurs at a restricted, reactive region on the Mb surface that dominates the docked ensemble. An electrostatically-guided brownian dynamics (BD) docking protocol was used to generate an initial ensemble of reactive configurations of the complex between unrelaxed partners. This ensemble samples a broad and diverse array of heme-heme distances and orientations. These configurations seeded all-atom constrained molecular dynamics simulations (MD) to generate relaxed complexes for the calculation of electron tunneling matrix elements (T(DA)) through tunneling-pathway analysis. This procedure for generating an ensemble of relaxed complexes combines the ability of BD calculations to sample the large variety of available conformations and interprotein distances, with the ability of MD to generate the atomic level information, especially regarding the structure of water molecules at the protein-protein interface, that defines electron-tunneling pathways. We used the calculated T(DA) values to compute ET rates for the [Mb(wt), cyt b(5)] complex and for the complex with a mutant that has a binding free energy strengthened by three D/E → K charge-reversal mutations, [Mb(3M), cyt b(5)]. The calculated rate constants are in agreement with the measured values, and the mutant complex ensemble has many more geometries with higher T(DA) values than does the wild-type Mb complex. Interestingly, water plays a double role in this electron-transfer system, lowering the tunneling barrier as well as inducing protein interface remodeling that screens the repulsion between the negatively-charged propionates of the two hemes.

Arabidopsis sucrose transporter SUT4 interacts with cytochrome b5-2 to regulate seed germination in response to sucrose and glucose

It remains unknown whether a sucrose transporter mediates sugar signaling. Here, we report that the Arabidopsis (Arabidopsis thaliana) sucrose transporter SUT4 interacts with five members of the Arabidopsis cytochrome b5 (Cyb5) family, and sucrose represses the interaction between SUT4 and a Cyb5 member Cyb5-2/A. We observed that down-regulation of SUT4 and three cytochrome b5 members (Cyb5-2, Cyb5-4, and Cyb5-6) confers the sucrose- and glucose-insensitive phenotypes in the sucrose/glucose-induced inhibition of seed germination. The sut4 cyb5-2 double mutant displays slightly stronger sucrose/glucose-insensitive phenotypes than either the sut4 or cyb5-2 single mutant. We showed that the SUT4/Cyb5-2-mediated signaling in the sucrose/glucose-induced inhibition of seed germination does not require ABA or the currently known ABI2/ABI4/ABI5-mediated signaling pathway(s). These data provide evidence that the sucrose transporter SUT4 interacts with Cyb5 to positively mediate sucrose and glucose signaling in the sucrose/glucose-induced inhibition of seed germination.

Arabidopsis sucrose transporter SUT4 interacts with cytochrome b5-2 to regulate seed germination in response to sucrose and glucose

It remains unknown whether a sucrose transporter mediates sugar signaling. Here, we report that the Arabidopsis (Arabidopsis thaliana) sucrose transporter SUT4 interacts with five members of the Arabidopsis cytochrome b5 (Cyb5) family, and sucrose represses the interaction between SUT4 and a Cyb5 member Cyb5-2/A. We observed that down-regulation of SUT4 and three cytochrome b5 members (Cyb5-2, Cyb5-4, and Cyb5-6) confers the sucrose- and glucose-insensitive phenotypes in the sucrose/glucose-induced inhibition of seed germination. The sut4 cyb5-2 double mutant displays slightly stronger sucrose/glucose-insensitive phenotypes than either the sut4 or cyb5-2 single mutant. We showed that the SUT4/Cyb5-2-mediated signaling in the sucrose/glucose-induced inhibition of seed germination does not require ABA or the currently known ABI2/ABI4/ABI5-mediated signaling pathway(s). These data provide evidence that the sucrose transporter SUT4 interacts with Cyb5 to positively mediate sucrose and glucose signaling in the sucrose/glucose-induced inhibition of seed germination.

Reconstitution of plant alkane biosynthesis in yeast demonstrates that Arabidopsis ECERIFERUM1 and ECERIFERUM3 are core components of a very-long-chain alkane synthesis complex

In land plants, very-long-chain (VLC) alkanes are major components of cuticular waxes that cover aerial organs, mainly acting as a waterproof barrier to prevent nonstomatal water loss. Although thoroughly investigated, plant alkane synthesis remains largely undiscovered. The Arabidopsis thaliana ECERIFERUM1 (CER1) protein has been recognized as an essential element of wax alkane synthesis; nevertheless, its function remains elusive. In this study, a screen for CER1 physical interaction partners was performed. The screen revealed that CER1 interacts with the wax-associated protein ECERIFERUM3 (CER3) and endoplasmic reticulum-localized cytochrome b5 isoforms (CYTB5s). The functional relevance of these interactions was assayed through an iterative approach using yeast as a heterologous expression system. In a yeast strain manipulated to produce VLC acyl-CoAs, a strict CER1 and CER3 coexpression resulted in VLC alkane synthesis. The additional presence of CYTB5s was found to enhance CER1/CER3 alkane production. Site-directed mutagenesis showed that CER1 His clusters are essential for alkane synthesis, whereas those of CER3 are not, suggesting that CYTB5s are specific CER1 cofactors. Collectively, our study reports the identification of plant alkane synthesis enzymatic components and supports a new model for alkane production in which CER1 interacts with both CER3 and CYTB5 to catalyze the redox-dependent synthesis of VLC alkanes from VLC acyl-CoAs.

A missense mutation in the human cytochrome b5 gene causes 46,XY disorder of sex development due to true isolated 17,20 lyase deficiency

CONTEXT

Isolated 17,20 lyase deficiency is commonly defined by apparently normal 17α-hydroxylase activity but severely reduced 17,20 lyase activity of the bifunctional enzyme cytochrome P450 (CYP) enzyme 17A1 (CYP17A1), resulting in sex steroid deficiency but normal glucocorticoid and mineralocorticoid reserve. Cytochrome b5 (CYB5A) is thought to selectively enhance 17,20 lyase activity by facilitating the allosteric interaction of CYP17A1 with its electron donor P450 oxidoreductase (POR).

OBJECTIVE

We investigated a large consanguineous family including three siblings with 46,XY disorder of sex development (DSD) presenting with isolated 17,20 lyase deficiency.

DESIGN

We investigated the clinical and biochemical phenotype, conducted genetic analyses, and functionally characterized the identified CYB5A mutation in cell-based CYP17A1 coexpression assays.

RESULTS

All three siblings presented with 46,XY DSD, sex steroid deficiency, normal mineralocorticoids and glucocorticoids, and a urine steroid metabolome suggestive of isolated 17,20 lyase deficiency. CYP17A1 and POR sequences were normal, but we detected a homozygous CYB5A missense mutation (g.28,400A→T; p.H44L). Functional in vitro analysis revealed normal CYP17A1 17α-hydroxylase activity but severely impaired 17,20 lyase activity. In silico analysis suggested the disruption of CYB5A heme binding by p.H44L.

CONCLUSION

We have identified the first human CYB5A missense mutation as the cause of isolated 17,20 lyase deficiency in three individuals with 46,XY DSD. Detailed review of previously reported cases with apparently isolated 17,20 lyase deficiency due to mutant CYP17A1 and POR reveals impaired 17α-hydroxylase activity as assessed by steroid metabolome analysis and short cosyntropin testing. This suggests that truly isolated 17,20 lyase deficiency is observed only in individuals with inactivating CYB5A mutations.

Amidoxime reductase system containing cytochrome b5 type B (CYB5B) and MOSC2 is of importance for lipid synthesis in adipocyte mitochondria

Reduction of hydroxylamines and amidoximes is important for drug activation and detoxification of aromatic and heterocyclic amines. Such a reductase system was previously found to be of high activity in adipose tissue and liver, and furthermore, in vitro studies using recombinant truncated and purified enzymes suggested the participation of cytochrome b(5) reductase (CYB5R), cytochrome b(5) (CYB5), and molybdenum cofactor sulfurase C-terminal containing 1 and 2 (MOSC1 and -2). Here, we show that purified rat liver outer mitochondrial membrane contains high amidoxime reductase activity and that MOSC2 is exclusively localized to these membranes. Moreover, using the same membrane fraction, we could show direct binding of a radiolabeled benzamidoxime substrate to MOSC2. Following differentiation of murine 3T3-L1 cells into mature adipocytes, the MOSC2 levels as well as the amidoxime reductase activity were increased, indicating that the enzyme is highly regulated under lipogenic conditions. siRNA-mediated down-regulation of MOSC2 and the mitochondrial form of cytochrome b(5) type B (CYB5B) significantly inhibited the reductase activity in the differentiated adipocytes, whereas down-regulation of MOSC1, cytochrome b(5) type A (CYB5A), CYB5R1, CYB5R2, or CYB5R3 had no effect. Down-regulation of MOSC2 caused impaired lipid synthesis. These results demonstrate for the first time the direct involvement of MOSC2 and CYB5B in the amidoxime reductase activity in an intact cell system. We postulate the presence of a novel reductive enzyme system of importance for lipid synthesis that is exclusively localized to the outer mitochondrial membrane and is composed of CYB5B, MOSC2, and a third unknown component (a CYB5B reductase).

Higher plant cytochrome b5 polypeptides modulate fatty acid desaturation

BACKGROUND

Synthesis of polyunsaturated fatty acids (PUFAs) in the endoplasmic reticulum of plants typically involves the fatty acid desaturases FAD2 and FAD3, which use cytochrome b(5) (Cb5) as an electron donor. Higher plants are reported to have multiple isoforms of Cb5, in contrast to a single Cb5 in mammals and yeast. Despite the wealth of information available on the roles of FAD2 and FAD3 in PUFA synthesis, information regarding the contributions of various Cb5 isoforms in desaturase-mediated reactions is limited.

RESULTS

The present functional characterization of Cb5 polypeptides revealed that all Arabidopsis Cb5 isoforms are not similarly efficient in ω-6 desaturation, as evidenced by significant variation in their product outcomes in yeast-based functional assays. On the other hand, characterization of Cb5 polypeptides of soybean (Glycine max) suggested that similar ω-6 desaturation efficiencies were shared by various isoforms. With regard to ω-3 desaturation, certain Cb5 genes of both Arabidopsis and soybean were shown to facilitate the accumulation of more desaturation products than others when co-expressed with their native FAD3. Additionally, similar trends of differential desaturation product accumulation were also observed with most Cb5 genes of both soybean and Arabidopsis even if co-expressed with non-native FAD3.

CONCLUSIONS

The present study reports the first description of the differential nature of the Cb5 genes of higher plants in fatty acid desaturation and further suggests that ω-3/ω-6 desaturation product outcome is determined by the nature of both the Cb5 isoform and the fatty acid desaturases.

The syndrome of 17,20 lyase deficiency

CONTEXT

Disorders of steroidogenesis have been instrumental in delineating human steroidogenic pathways. Each genetic disorder seemed to correspond to a different steroidogenic activity, helping to identify several enzymes. Beginning in 1972, several patients have been reported as having "17,20 lyase deficiency," but there have been inconsistent genetic findings.

OBJECTIVE

This manuscript reviews the biochemistry, genetics, and clinical disorders of 17,20 lyase activity, which converts 21-carbon precursors of glucocorticoids to 19-carbon precursors of sex steroids.

FINDINGS

A single enzyme, cytochrome P450c17, catalyzes both 17α-hydroxylase activity and 17,20 lyase activity. The 17,20 lyase activity is especially sensitive to the activities of the accessory proteins P450 oxidoreductase and cytochrome b(5). The first cases of genetically and biochemically proven 17,20 lyase deficiency were reported in 1997, in which specific P450c17 mutations were identified that lost 17,20 lyase activity but not 17α-hydroxylase activity when assayed in vitro. Subsequent work identified other P450c17 mutations and mutations in the genes encoding P450 oxidoreductase and cytochrome b(5). Recently, the initially reported cases from 1972 were found to carry mutations in two aldo-keto reductases, AKR1C2 and AKR1C4. These AKR1C isozymes catalyze 3α-hydroxysteroid dehydrogenase activity in the so-called "backdoor pathway" by which the fetal testis produces dihydrotestosterone without the intermediacy of testosterone.

CONCLUSIONS

17,20 Lyase deficiency should be considered a syndrome with multiple causes, and not a single disease. Study of this very rare disorder has substantially advanced our understanding of the pathways, mechanisms, and control of androgen synthesis. Mutations in other, as-yet unidentified genes may also cause this phenotype.

Interaction of modified tail-anchored proteins with liposomes: effect of extensions of hydrophilic segment at the COOH-terminus of holo-cytochromes b₅

A group of membrane proteins having a single COOH-terminal hydrophobic domain capable of post-translational insertion into lipid bilayer is known as tail-anchored (TA) proteins. To clarify the insertion mechanism of the TA-domain of human cytochrome b(5) (Hcytb5) into ER membranes, we produced and purified various membrane-bound forms of Hcytb5 with their heme b-bound, in which various truncated forms of NH(2)-terminal bovine opsin sequence were appended at the COOH-terminus of the native form. We analyzed the integration of the TA-domains of these forms onto protein-free liposomes. The integration occurred efficiently even in the presence of a small amount of sodium cholate and, once incorporated, such proteoliposomes were very stable. The mode of the integration was further analyzed by treatment of the proteoliposomes with trypsin either on the extravesicular side or on the luminal side. LC-MS analyses of the trypsin digests obtained from the proteoliposomes indicated that most of the C-terminal hydrophilic segment of the native Hcytb5 were exposed towards the lumen of the vesicles and, further, a significant part of the population of the extended C-terminal hydrophilic segments of the modified Hcytb5 were exposed in the lumen as well, suggesting efficient translocation ability of the TA-domain without any assistance from other protein factors. Present results opened a route for the use of the C-terminal TA-domain as a convenient tool for the transport of proteins as well as short peptides into artificial liposomes.

Human adrenal cells that express both 3β-hydroxysteroid dehydrogenase type 2 (HSD3B2) and cytochrome b5 (CYB5A) contribute to adrenal androstenedione production

Androstenedione is one of several weak androgens produced in the human adrenal gland. 3β-Hydroxysteroid dehydrogenase type 2 (HSD3B2) and cytochrome b5 (CYB5A) are both required for androstenedione production. However, previous studies demonstrated the expression of HSD3B2 within the zona glomerulosa (ZG) and fasciculata (ZF) but low levels in the zona reticularis. In contrast, CYB5A expression increases in the zona reticularis (ZR) in human adrenal glands. Although their colocalization has been reported in gonadal theca and Leydig cells this has not been studied in the human adrenal. Therefore, we immonolocalized HSD3B2 and CYB5A in normal human adrenal glands and first demonstrated their co-expression in the cortical cells located at the border between the ZF and ZR in normal human adrenal. Results of in vitro studies using the human adrenal H295R cells treated with the HSD3B2 inhibitor, trilostane, also demonstrated a markedly decreased androstenedione production. Decreasing CYB5A mRNA using its corresponding siRNA also resulted in significant inhibition of androstenedione production in the H295R cells. These findings together indicate that there are a group of cells co-expressing HSD3B2 and CYB5A with hybrid features of both ZF and ZR in human adrenal cortex, and these hybrid cortical cells may play an important role in androstenedione production in human adrenal gland.

Cytochrome b5 null mouse: a new model for studying inherited skin disorders and the role of unsaturated fatty acids in normal homeostasis

Microsomal cytochrome b (5) is a ubiquitous, 15.2 kDa haemoprotein implicated in a number of cellular processes such as fatty acid desaturation, drug metabolism, steroid hormone biosynthesis and methaemoglobin reduction. As a consequence of these functions this protein has been considered essential for life. Most of the ascribed functions of cytochrome b (5), however, stem from in vitro studies and for this reason we have carried out a germline deletion of this enzyme. We have unexpectedly found that cytochrome b (5) null mice were viable and fertile, with pups being born at expected Mendelian ratios. However, a number of intriguing phenotypes were identified, including altered drug metabolism, methaemoglobinemia and disrupted steroid hormone homeostasis. In addition to these previously identified roles for this protein, cytochrome b (5) null mice displayed skin defects closely resembling those observed in autosomal recessive congenital ichthyosis and retardation of neonatal development, indicating that this protein, possibly as a consequence of its role in the de novo biosynthesis of unsaturated fatty acids, plays a central role in skin development and neonatal nutrition. Results from fatty acid profile analysis of several tissues suggest that cytochrome b (5) plays a role controlling saturated/unsaturated homeostasis. These data demonstrate that regional concentrations of unsaturated fatty acids are controlled by endogenous metabolic pathways and not by diet alone.

RLF, a cytochrome b(5)-like heme/steroid binding domain protein, controls lateral root formation independently of ARF7/19-mediated auxin signaling in Arabidopsis thaliana

Lateral root (LR) formation is important for the establishment of root architecture in higher plants. Recent studies have revealed that LR formation is regulated by an auxin signaling pathway that depends on auxin response factors ARF7 and ARF19, and auxin/indole-3-acetic acid (Aux/IAA) proteins including SOLITARY-ROOT (SLR)/IAA14. To understand the molecular mechanisms of LR formation, we isolated a recessive mutant rlf (reduced lateral root formation) in Arabidopsis thaliana. The rlf-1 mutant showed reduction of not only emerged LRs but also LR primordia. Analyses using cell-cycle markers indicated that the rlf-1 mutation inhibits the first pericycle cell divisions involved in LR initiation. The rlf-1 mutation did not affect auxin-induced root growth inhibition but did affect LR formation over a wide range of auxin concentrations. However, the rlf-1 mutation had almost no effect on auxin-inducible expression of LATERAL ORGAN BOUNDARIES-DOMAIN16/ASYMMETRIC LEAVES2-LIKE18 (LBD16/ASL18) and LBD29/ASL16 genes, which are downstream targets of ARF7/19 for LR formation. These results indicate that ARF7/19-mediated auxin signaling is not blocked by the rlf-1 mutation. We found that the RLF gene encodes At5g09680, a protein with a cytochrome b(5)-like heme/steroid binding domain. RLF is ubiquitously expressed in almost all organs, and the protein localizes in the cytosol. These results, together with analysis of the genetic interaction between the rlf-1 and arf7/19 mutations, indicate that RLF is a cytosolic protein that positively controls the early cell divisions involved in LR initiation, independent of ARF7/19-mediated auxin signaling.

Photoinitiated singlet and triplet electron transfer across a redesigned [myoglobin, cytochrome b5] interface

We describe a strategy by which reactive binding of a weakly bound, 'dynamically docked (DD)' complex without a known structure can be strengthened electrostatically through optimized placement of surface charges, and discuss its use in modulating complex formation between myoglobin (Mb) and cytochrome b(5) (b(5)). The strategy employs paired Brownian dynamics (BD) simulations, one which monitors overall binding, the other reactive binding, to examine [X --> K] mutations on the surface of the partners, with a focus on single and multiple [D/E --> K] charge reversal mutations. This procedure has been applied to the [Mb, b(5)] complex, indicating mutations of Mb residues D44, D60, and E85 to be the most promising, with combinations of these showing a nonlinear enhancement of reactive binding. A novel method of displaying BD profiles shows that the 'hits' of b(5) on the surfaces of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) progressively coalesce into two 'clusters': a 'diffuse' cluster of hits that are distributed over the Mb surface and have negligible electrostatic binding energy and a 'reactive' cluster of hits with considerable stability that are localized near its heme edge, with short Fe-Fe distances favorable to electron transfer (ET). Thus, binding and reactivity progressively become correlated by the mutations. This finding relates to recent proposals that complex formation is a two-step process, proceeding through the formation of a weakly bound encounter complex to a well-defined bound complex. The design procedure has been tested through measurements of photoinitiated ET between the Zn-substituted forms of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) and Fe(3+)b(5). Both mutants convert the complex from the DD regime exhibited by Mb(WT), in which the transient complex is in fast kinetic exchange with its partners, k(off) >> k(et), to the slow-exchange regime, k(et) >> k(off), and both mutants exhibit rapid intracomplex ET from the triplet excited state to Fe(3+)b(5) (rate constant, k(et) approximately 10(6) s(-1)). The affinity constants of the mutant Mbs cannot be derived through conventional analysis procedures because intracomplex singlet ET quenching causes the triplet-ground absorbance difference to progressively decrease during a titration, but this effect has been incorporated into a new procedure for computing binding constants. Most importantly, these measurements reveal the presence of fast photoinduced singlet ET across the protein-protein interface, (1)k(et) approximately 2 x 10(8) s(-1).

A single amino acid residue, Ala 105, confers 16alpha-hydroxylase activity to human cytochrome P450 17alpha-hydroxylase/17,20 lyase

In adrenal steroidogenesis, CYP17 catalyses the 17alpha-hydroxylation of pregnenolone and progesterone and the subsequent 17,20-lyase reaction, yielding adrenal androgens. The enzyme exhibits distinctly different selectivities towards these substrates in various species. CYP17 has also been shown to exhibit 16alpha-hydroxylase activity towards progesterone in some species, with only human and chimp CYP17 catalysing the biosynthesis of substantial amounts of 16-OHprogesterone. The 16alpha-hydroxylase activity was investigated by introducing an Ala105Leu substitution into human CYP17. The converse mutation, Leu105Ala was introduced into the baboon, goat and pig enzymes. Wt human CYP17 converted approximately 30% progesterone to 16-OHprogesterone while the Ala105Leu mutant converted negligible amounts to 16-OHprogesterone ( approximately 9%), comparable to wt CYP17 of the other three species when expressed in COS-1 cells. The ratio of 17-hydroxylated products to 16-OHprogesterone of human CYP17 was 2.7 and that of the mutant human construct 10.5. Similar ratios were observed for human and goat CYP17 with the corresponding Ala or Leu residues. Although the Leu105Ala mutation of both baboon and pig CYP17 exhibited the same trend regarding the ratios, the rate of progesterone conversion was reduced. Coexpression with cytochrome b(5) significantly decreased the ratio of 17-hydroxylated products to 16-OHprogesterone in the Leu105 constructs, while effects were negligible with Ala at this position. Homology models show that Ala105 faces towards the active pocket in the predicted B'-C domain of CYP17. The smaller residue allows more flexibility of movement in the active pocket than Leu, presenting both the C16 and C17 of progesterone to the iron-oxy complex.

Isolated 17,20-lyase deficiency due to the cytochrome b5 mutation W27X

CONTEXT

Cytochrome P450c17 (P450c17) is a bifunctional enzyme necessary for the production of glucocorticoids (17-hydroxylase activity) and sex steroids (17,20-lyase activity). Isolated 17,20-lyase deficiency is a rare condition characterized by a deficient production of androgens resulting in 46,XY disorders of sex development (DSD) while the production of glucocorticoids is intact. Several missense mutations in the CYP17A1 gene are known to cause this condition. Cytochrome b(5) (CytB5) is an important factor in 17,20-lyase activity, probably by acting as an allosteric factor.

OBJECTIVE

The aim of this study was to investigate the role of CytB5 in a patient with defective 17,20-lyase activity.

SETTING

We conducted the study in a pediatric outpatient clinic of a University Hospital.

PATIENTS

We studied a 46,XY DSD patient with 17,20-lyase deficiency without missense mutation in the CYP17A1 gene and his parents.

MAIN OUTCOME MEASURES

We sequenced the CYB5 gene and measured steroid hormone levels.

RESULTS

Analysis of the CYB5 gene in our patient revealed a homozygous W27X mutation, leading to the formation of a premature stop codon; his parents were both heterozygous carriers of this mutation. This mutation results in the absence of residues E48 and E49 of CytB5, which are necessary for an intact 17,20-lyase activity.

CONCLUSION

We demonstrated 17,20-lyase deficiency due to an aberrant CytB5. Our findings thus provide evidence for an alternative etiology for this disorder.

A web-based resource for the Arabidopsis P450, cytochromes b5, NADPH-cytochrome P450 reductases, and family 1 glycosyltransferases (http://www.P450.kvl.dk)

Gene and genome duplication is a key driving force in evolution of plant diversity. This has resulted in a number of large multi-gene families. Two of the largest multi-gene families in plants are the cytochromes P450 (P450s) and family 1 glycosyltransferases (UGTs). These two families are key players in evolution, especially of plant secondary metabolism, and in adaption to abiotic and biotic stress. In the model plant Arabidopsis thaliana there are 246 and 112 cytochromes P450 and UGTs, respectively. The Arabidopsis P450, cytochromes b(5), NADPH-cytochrome P450 reductases, and family 1 glycosyltransferases website (http://www.P450.kvl.dk) is a sequence repository of manually curated sequences, multiple sequence alignments, phylogenetic trees, sequence motif logos, 3D structures, intron-exon maps, and customized BLAST datasets.

Approach to predict the contribution of cytochrome P450 enzymes to drug metabolism in the early drug-discovery stage: The effect of the expression of cytochromeb5with recombinant P450 enzymes

In order to evaluate the potential adverse effects due to genetic polymorphism and/or inter-individual variation, it is necessary to calculate the cytochrome P450 (CYP) contribution to the metabolism of new drugs. In the current study, the in vitro intrinsic clearance (CL(int)) values of marker substrates and drugs were determined by measuring metabolite formation and substrate depletion, respectively. Recombinant CYP microsomes expressing CYP2C9, CYP2C19 and CYP3A4 with co-expressed cytochrome b(5) were used, but those expressing CYP1A2 and CYP2D6 did not have co-expressed cytochrome b(5). The following prediction methods were compared to determine the CL(int) value using data from recombinant CYP enzymes: (1) relative CYP enzyme content in human liver microsomes; (2) relative activity factor (RAF) estimated from the V(max) value; and (3) RAF estimated from the CL(int) value. Estimating RAF from CL(int) proved the most accurate prediction method among the three tested, and differences in the CYP3A4 marker reactions did not affect its accuracy. The substrate depletion method will be useful in the early drug-discovery stage when the main metabolite and/or metabolic pathway has not been identified. In addition, recombinant CYP microsomes co-expressed with cytochrome b(5) might be suitable for the prediction of the CL(int) value.

The developmental increase in adrenocortical 17,20-lyase activity (biochemical adrenarche) is driven primarily by increasing cytochrome b5 in neonatal rhesus macaques

Adrenarche is thought to be experienced only by humans and some Old World primates despite observed regression of an adrenal fetal zone and establishment of a functional zona reticularis (ZR) in other species like rhesus macaques. Adrenal differentiation remains poorly defined biochemically in nonhuman primates. The present studies defined ZR development in the neonatal rhesus by examining androgen synthetic capacity and factors affecting it in rhesus and marmoset adrenals. Western immunoblots examined expression of 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17), cytochrome b5 (b5), and 3beta-hydroxysteroid dehydrogenase (3betaHSD), among other key enzymes. 17,20-lyase activity was quantified in adrenal microsomes, as was the contribution of b5 to 17,20-lyase activity in microsomes and cell transfection experiments with rhesus and marmoset P450c17. Expression of b5 increased from birth to 3 months, and was positively correlated with age and 17,20-lyase activity in the rhesus. Recombinant b5 addition stimulated 17,20-lyase activity to an extent inversely proportional to endogenous levels in adrenal microsomes. Although 3betaHSD expression also increased with age, P450c17, 21-hydroxylase cytochrome P450, and the redox partner, reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 oxidoreductase, did not; nor did recombinant cytochrome P450 oxidoreductase augment 17,20-lyase activity. Cotransfection with b5 induced a dose-dependent increase in dehydroepiandrosterone synthesis by both nonhuman primate P450c17 enzymes. We conclude that the increase in 17,20-lyase activity characteristic of an adrenarche in rhesus macaques is driven primarily by increased b5 expression, without the need for a decrease in 3betaHSD, as suggested from human studies. The rhesus macaque is a relevant and accessible model for human ZR development and adrenal function.

The role of porcine cytochrome b5A and cytochrome b5B in the regulation of cytochrome P45017A1 activities

Male pigs are routinely castrated to prevent the accumulation of testicular 16-androstene steroids, in particular 5alpha-androst-16-en-3-one (5alpha-androstenone), which contribute to an off-odour and off-flavour known as boar taint. Cytochrome P450C17 (CYP17A1) catalyses the key regulatory step in the formation of the 16-androstene steroids from pregnenolone by the andien-beta synthase reaction or the synthesis of the glucocorticoid and sex steroids via 17alpha-hydroxylase and C17,20 lyase pathways respectively. We have expressed CYP17A1, along with cytochrome P450 reductase (POR), cytochrome b5 reductase (CYB5R3) and cytochrome b5 (CYB5) in HEK-293FT cells to investigate the importance of the two forms of porcine CYB5, CYB5A and CYB5B, in both the andien-beta synthase as well as the 17alpha-hydroxylase and C17,20 lyase reactions. Increasing the ratio of CYB5A to CYP17A1 caused a decrease in 17alpha-hydroxylase (p<0.013), a transient increase in C17,20 lyase, and an increase in andien-beta synthase activity (p<0.0001). Increasing the ratio of CYB5B to CYP17A1 also decreased 17alpha-hydroxylase, but did not affect the andien-beta synthase activity; however, the C17,20 lyase, was significantly increased. These results demonstrate the differential effects of two forms of CYB5 on the three activities of porcine CYP17A1 and show that CYB5B does not stimulate the andien-beta synthase activity of CYP17A1.

Preparation of apo-cytochrome b5 utilizing heme transfer to apo-myoglobin

OBJECTIVES

Cytochrome b5 (cyt b5), a component of endoplasmic reticulum membrane, plays a role in modulation of activity of several cytochromes P450 (CYP). To elucidate the mechanism of such modulations it is necessary to evaluate not only the effect of native cyt b5, but also that of apo-cyt b5. To prepare apo-cyt b5, heme transfer from native cyt b5 to a protein with higher affinity toward the heme, the horse heart apo-myoglobin, was utilized.

METHODS

Butanone extraction was employed to prepare apo-myoglobin. Apo-cyt b5 was separated from myoglobin by chromatography on DEAE-Sepharose. Mass spectrometry was utilized to characterize proteins eluted from DEAE- Sepharose.

RESULTS

The prepared apo-myoglobin was incubated with the cyt b5 at pH 4.2 that is the optimal pH for heme transfer from cyt b5 into apo-myoglobin. The apo-cyt b5 protein was separated from myoglobin present in the reaction mixture by chromatography on a column of DEAE-Sepharose. Using such a procedure, 16% yield of apo-cyt b5 that did not contain any heme in its molecule was obtained from the native rabbit cyt b5. Oxidized and reduced forms of the apo-b5 reconstituted with heme exhibit the same absorbance spectra as native cyt b5. The prepared apo-cyt b5 reconstituted with heme can receive electrons from NADPH:CYP reductase.

CONCLUSION

A biologically active apo-cyt b5 was prepared using transfer of heme from cyt b5 to horse heart apo-myoglobin by the procedure described here.

Anchorage to the cytosolic face of the endoplasmic reticulum membrane: a new strategy to stabilize a cytosolic recombinant antigen in plants

The levels of accumulation of recombinant vaccines in transgenic plants are protein specific and strongly influenced by the subcellular compartment of destination. The human immunodeficiency virus protein Nef (negative factor), a promising target for the development of an antiviral vaccine, is a cytosolic protein that accumulates to low levels in transgenic tobacco and is even more unstable when introduced into the secretory pathway, probably because of folding defects in the non-cytosolic environment. To improve Nef accumulation, a new strategy was developed to anchor the molecule to the cytosolic face of the endoplasmic reticulum (ER) membrane. For this purpose, the Nef sequence was fused to the C-terminal domain of mammalian ER cytochrome b5, a long-lived, tail-anchored (TA) protein. This consistently increased Nef accumulation by more than threefold in many independent transgenic tobacco plants. Real-time polymerase chain reaction of mRNA levels and protein pulse-chase analysis indicated that the increase was not caused by higher transcript levels but by enhanced protein stability. Subcellular fractionation and immunocytochemistry indicated that Nef-TA accumulated on the ER membrane. Over-expression of mammalian or plant ER cytochrome b5 caused the formation of stacked membrane structures, as observed previously in similar experiments performed in mammalian cells; however, Nef-TA did not alter membrane organization in tobacco cells. Finally, Nef could be removed in vitro by its tail-anchor, taking advantage of an engineered thrombin cleavage site. These results open up the way to use tail-anchors to improve foreign protein stability in the plant cytosol without perturbing cellular functions.

Screening and purification for novel cytochrome b5 from uncultured environmental micro-organisms

AIMS

We describe a sequence-based PCR method suitable for the isolation of a novel soluble heme-binding domain of cytochrome b(5) (cyt b(5)) gene directly from metagenomic DNA is described.

METHODS AND RESULTS

Using the degenerate primer set, a cyt b(5) gene was isolated directly from metagenomic DNA. Based on the sequence-based PCR method, the similar conserved motif of cyt b(5) from Rhodopseudomonas palustris strain makes the novel target gene. The gene encoding cyt b(5) was cloned and expressed in Escherichia coli BL21 (DE3) using pET expression system. The expressed recombinant enzyme was purified by Ni-nitrilotriacetic acid affinity chromatography and characterized.

CONCLUSIONS

Sequence-based strategy is an effective method for application of the novel gene from metagenomic DNA.

SIGNIFICANCE AND IMPACT OF THE STUDY

Investigation of novel genes from metagenome, most of the micro-organism species are largely untapped, could represent an interesting and useful reservoir for biological processes.

The identification of two CYP17 alleles in the South African Angora goat

South African Angora goats (Capra hircus) are susceptible to cold stress, due to the inability of the adrenal cortex to produce sufficient levels of cortisol. Two CYP17 isoforms were identified, cloned and characterized in this study. Sequence analysis revealed three amino acid differences between the two CYP17 isoforms, which resulted in a significant difference in 17,20 lyase activity of the expressed enzymes in both the presence and absence of cytochrome b(5). Furthermore, cotransfections with 3 beta HSD revealed that one CYP17 isoform strongly favours the Delta(5) steroid pathway. Our data implicates CYP17 as the primary cause of the observed hypoadrenocorticoidism in the South African Angora goat.