The induction, purification and characterization of 17 beta-hydroxy-C19-steroid dehydrogenase of the female guinea pig kidney

Epimers of Vitamin D: A Review

In this review, we discuss the sources, formation, metabolism, function, biological activity, and potency of C3-epimers (epimers of vitamin D). We also determine the role of epimerase in vitamin D-binding protein (DBP) and vitamin D receptors (VDR) according to different subcellular localizations. The importance of C3 epimerization and the metabolic pathway of vitamin D at the hydroxyl group have recently been recognized. Here, the hydroxyl group at the C3 position is orientated differently from the alpha to beta orientation in space. However, the details of this epimerization pathway are not yet clearly understood. Even the gene encoding for the enzyme involved in epimerization has not yet been identified. Many published research articles have illustrated the biological activity of C3 epimeric metabolites using an in vitro model, but the studies on in vivo models are substantially inadequate. The metabolic stability of 3-epi-1α,25(OH)₂D3 has been demonstrated to be higher than its primary metabolites. 3-epi-1 alpha, 25 dihydroxyvitamin D3 (3-epi-1α,25(OH)₂D3) is thought to have fewer calcemic effects than non-epimeric forms of vitamin D. Some researchers have observed a larger proportion of total vitamin D as C3-epimers in infants than in adults. Insufficient levels of vitamin D were found in mothers and their newborns when the epimers were not included in the measurement of vitamin D. Oral supplementation of vitamin D has also been found to potentially cause increased production of epimers in mice but not humans. Moreover, routine vitamin D blood tests for healthy adults will not be significantly affected by epimeric interference using LC-MS/MS assays. Recent genetic models also show that the genetic determinants and the potential factors of C3-epimers differ from those of non-C3-epimers.Most commercial immunoassays techniques can lead to inaccurate vitamin D results due to epimeric interference, especially in infants and pregnant women. It is also known that the LC-MS/MS technique can chromatographically separate epimeric and isobaric interference and detect vitamin D metabolites sensitively and accurately. Unfortunately, many labs around the world do not take into account the interference caused by epimers. In this review, various methods and techniques for the analysis of C3-epimers are also discussed. The authors believe that C3-epimers may have an important role to play in clinical research, and further research is warranted.

Measurement and characterization of C-3 epimerization activity toward vitamin D3

Recently, epimerization of the hydroxyl group at C-3 has been identified as a unique metabolic pathway of vitamin D compounds. We measured C-3 epimerization activity in subcellular fractions prepared from cultured cells and investigated the basic properties of the enzyme responsible for the epimerization. C-3 epimerization activity was detected using a NADPH-generating system containing glucose-6-phosphate, NADP, glucose-6-phosphate dehydrogenase, and Mg(2+). The highest level of activity was observed in a microsomal fraction prepared from rat osteoblastic UMR-106 cells but activity was also observed in microsomal fractions prepared from MG-63, Caco-2, Hep G2, and HUH-7 cells. In terms of maximum velocity (V(max)) and the Michaelis constant (K(m)), 25-hydroxyvitamin D(3) [25(OH)D(3)] exhibited the highest specificity for the epimerization at C-3 among 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], 25(OH)D(3), 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)], and 22-oxacalcitriol (OCT). The epimerization activity was not inhibited by various cytochrome P450 inhibitors and antiserum against NADPH cytochrome P450 reductase. Neither CYP24, CYP27A1, CYP27B1 nor 3(alpha-->beta)hydroxysteroid epimerase (HSE) catalyzed the epimerization in vitro. Based on these results, the enzyme(s) responsible for the epimerization of vitamin D(3) at C-3 are thought to be located in microsomes and different from cytochrome P450 and HSE.

Molecular cloning and characterization of mouse estradiol 17 beta-dehydrogenase (A-specific), a member of the aldoketoreductase family

Several mammalian livers contain monomeric 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) with A-stereospecificity in hydrogen transfer, which differs from the B-specific dimeric enzyme of human placenta in its ability to catalyze the oxidoreduction of xenobiotic trans-dihydrodiols of aromatic hydrocarbons and carbonyl compounds. Here, we report the isolation and characterization of a mouse cDNA clone encoding monomeric 17 beta-HSD of the liver. This clone had an entire coding region for a protein of 323 amino acid residues with a molecular weight of 37,055. The deduced sequence of the protein aligned with a high degree of identity with rat and rabbit 20 alpha-HSDs, rat and human 3 alpha-HSD/dihydrodiol dehydrogenases, and bovine prostaglandin F synthase, which are members of the aldoketoreductase family, but was distinct from human 17 beta-HSD and carbonyl reductase, members of the short chain dehydrogenases. The expression of the cDNA in Escherichia coli resulted in synthesis of a protein that was active toward androgens, estrogens, and xenobiotic substrates. The recombinant and mouse liver 17 beta-HSDs also exhibited low 20 alpha-HSD activity toward progestins, which is similar to bifunctional activity of human placental 17 beta-HSD. Therefore, the mouse enzyme was given the designation of estradiol 17 beta-dehydrogenase (A-specific). Northern analysis of mouse tissues revealed the existence of a single 1.7-kilobase 17 beta-HSD mRNA species in the liver, kidney, testis, and stomach. The liver mRNA content was considerably more abundant than those found in the other tissues, as 17 beta-HSD protein was mainly detected in the liver by Western analysis.

A novel membrane associated carbonyl reducing enzyme is present in smooth endoplasmic reticulum of mouse liver

Evidence supporting the existence of two distinct carbonyl (metyrapone) reducing enzymes which differ in subcellular localization and immunological homology has been provided. A soluble enzyme, designated as carbonyl reductase (EC 1.1.1.184) is located in the cytosol. The distribution of the second, membrane associated, MPON-reductase shows an excellent linear correlation to NADPH-cytochrome c reductase and, on the other hand, is reciprocal to the RNA/protein ratio of submicrosomal preparations. This indicates that the membrane associated MPON-reductase is exclusively located in the smooth endoplasmic reticulum. Using antibodies against the purified membrane associated enzyme the extent of immunological crossreaction corresponds well to the specific activities of MPON-reductase in the granular fractions, thus further confirming the localization of this enzyme within this organelle. The absence of antigenic crossreaction to cytosolic MPON-reductase indicates differences also in terms of the immunological relationship between the two enzymes.

Metabolites of testosterone: significance in the vital economy

Testosterone is metabolized by practically every tissue in the body to a large variety of related steroids. The metabolites vary with each tissue and appear to be formed to meet the specific needs of the particular tissue and animal. The many biologic actions of testosterone do not change in parallel in the various metabolites. Many of the metabolites show interesting differences in their relative biologic effects. A large number and variety of steroid-metabolizing enzymes (oxidoreductases, reductases, hydroxylases, aromatases) are involved. Some have been partially purified. The 17 beta-oxidoreductases of the cytosol of the guinea pig liver and kidney have been purified and characterized.

3 beta-hydroxysteroid isomerase dehydrogenase in guinea-pig kidney: possible involvement in 11-deoxycorticosterone formation in situ

3 beta-Hydroxysteroid isomerase dehydrogenase, capable of acting on C21- and C19-3 beta-hydroxy-5-ene-steroids has been found in guinea-pig kidney at equivalent levels to those in guinea pig testes. Of the 3 beta-hydroxy-5-ene-steroids present in guinea pig serum, 21-hydroxypregnenolone occurs in highest concentration (17 nM) followed by pregnenolone (10 nM), whereas 17 alpha-hydroxy-pregnenolone and dehydroepiandrosterone occur in very low concentrations (less than 0.5 nM). Furthermore, the concentration of 21-hydroxypregnenolone relative to 11-deoxycorticosterone (the mineralocorticoid of the guinea pig), is 10:1 (Nishikawa and Strott, Steroids 41 (1983) 105-120). The apparent Km value for 21-hydroxypregnenolone, for the reaction yielding 11-deoxycorticosterone as catalysed by guinea pig kidney microsomes, was 85 nM and the Vmax 33 pmol/min per mg protein. Pregnenolone was a competitive inhibitor (apparent Ki = 5 microM) in the above reaction. A sex difference in the level of the enzyme in the kidney was found (activity in the female was one-third of that in the male) which may indicate that the enzyme is under partial androgen control. 3 beta-Hydroxysteroid isomerase dehydrogenase activity was also detected in guinea pig liver and again it was lower in the female. Whilst the exact role of 3 beta-hydroxysteroid isomerase dehydrogenase in guinea-pig kidney remains uncertain, the data suggest that it may utilise blood-borne 21-hydroxypregnenolone, the later then playing the role of a prohormone.

Conversion of testosterone and androstenedione to 5 beta-androstanes by adult male hamster liver cytosol

The incubation of [4-14C]testosterone with adult male hamster liver cytosol at pH 6.7 yielded 5 beta-androstane-3 alpha, 17 beta-diol and small quantities of 5 beta-androstane-3 beta, 17 beta-diol, 17 beta-hydroxy-5 beta-androstan-3-one, 3 alpha-hydroxy-5 beta-androstan-17-one and androstenedione. The use of [4-14C]androstenedione as substrate yielded the same 5 beta-metabolites and also testosterone and a trace of epitestosterone. 5 beta-Androstane-3 alpha, 17 beta-diol was the major metabolite at "low" concentrations of substrate but testosterone and 3 alpha-hydroxy-5 beta-androstan-17-one became the major metabolites as the concentration of the substrate was increased. Small quantities of 5 beta-androstane-3,17-dione and 3 beta-hydroxy-5 beta-androstan-17-one were detected at "high" while 5 beta-androstane-3 alpha, 17 alpha-diol was detected at "low" concentrations of androstenedione. NADPH was more effective than NADH except in the formation of the 3 beta-steroids. Furthermore, the 3 beta-steroids were formed in maximum quantities at a lower pH than the other metabolites. The relative production of the metabolites was consistent with their respective spectrophotometrically determined degree of hydroxyl dehydrogenation.

Steroidal control of rat uterine 17 beta-hydroxysteroid dehydrogenase activity

The interconversion of estradiol-17 beta and estrone in the rat uterus is due to the action of 17 beta-hydroxysteroid dehydrogenase. Whole uteri or 800 x g supernatant fractions of the uteri were incubated in the presence of [3H] estradiol-17 beta and NAD at 37 degrees C for 3 h or 1 h, respectively. In the mature rat uterus the oxidation of estradiol-17 beta and estrone was dependent on the stage of the estrous cycle, suggesting hormonal control. The 17 beta-hydroxysteroid dehydrogenase activity was highest at estrus (200 fmol estrone) and lowest at diestrus (80 fmol estrone). An enhancement of activity occurred when adult rats at each stage of the estrous cycle were administered estradiol-17 beta, while progesterone administration at each stage resulted in decreased enzyme activity. The uterine 17 beta-hydroxysteroid dehydrogenase activity of estradiol-17 beta treated ovariectomized rats was time and dose dependent but decreased when progesterone was administered with or without estradiol-17 beta administration. These results suggest that estradiol-17 beta caused an increase in enzyme activity that was inhibitable by progesterone in the rat uterus. The increased 17 beta-hydroxysteroid dehydrogenase activity may reflect a specific response of the rat uterus to estradiol-17 beta.