Delta 5-3 beta-hydroxysteroid dehydrogenase-isomerase activity in canine pancreas

Activity of delta 5-3 beta-hydroxysteroid dehydrogenase coupled with steroid-delta 5-4-isomerase was demonstrated for the first time in the pancreas. The enzyme complex was assayed by measuring the conversion of pregnenolone to progesterone as well as of dehydroepiandrosterone to androstenedione and found to be localized primarily in the mitochondrial fraction of dog pancreas homogenates. The delta 5-3 beta-hydroxysteroid dehydrogenase used either NAD+ or NADP+ as co-substrates, although maximal activity was observed with NAD+. In phosphate buffer, pH 7.0 and 37 degrees C, the apparent Km values of the dehydrogenase were 6.54 +/- 0.7 microM for pregnenolone and 9.61 +/- 0.8 microM for NAD+. The apparent Vmax was determined as 0.82 +/- 0.02 nmol min-1 mg-1. Under the same conditions the Km values for dehydroepiandrosterone and NAD+ were 3.3 +/- 0.2 microM and 9.63 +/- 1.6 microM, respectively, and the apparent Vmax was 0.62 +/- 0.01 nmol min-1 mg-1.     

Substrate and nucleotide specificity of placental microsomal 3 beta-hydroxysteroid dehydrogenase

Recent kinetic studies on the placental microsomal 3 beta-hydroxysteroid dehydrogenase have shown that apparent Km values for 3 beta-hydroxy-5-androsten-17-one (dehydroepiandrosterone) and 3 beta-hydroxy-5-pregnen-20-one (pregnenolone) are 15nM and 40nM respectively, which are orders of magnitude lower than found in earlier studies. The purpose of this study was to investigate the substrate and nucleotide specificity of the 3 beta-hydroxysteroid dehydrogenase, and the ability of various steroids to inhibit the reaction at these lower steroid concentrations. Each steroid inhibited the metabolism of the other competitively, and the Ki values obtained were not significantly different from their respective Km values. The ability of various steroids to inhibit the reaction at concentrations of 100nM was usually less than that found at micromolar concentrations. However, certain steroids showed marked inhibition. For example, estrone and estradiol-17 beta inhibit the oxidation of both substrates competitively with Ki values of between 15 and 24nM. The Km values of dehydroepiandrosterone and pregnenolone with NADP+ as cofactor are higher than those with NAD+ as cofactor and the V values are much lower. These data indicate that in human placental microsomes a single 3 beta-hydroxysteroid dehydrogenase, essentially NAD+ specific, metabolizes dehydroepiandrosterone and pregnenolone.     

3beta-Hydroxysteroid dehydrogenase activity in human fetal membranes.

A 3beta-hydroxysteroid dehydrogenase (3betaHSD) was demonstrated in term human fetal membranes (chorion and amnion) with both dehydroepiandrosterone (3beta-hydroxy-5-androsten-17-one) and pregnenolone (3beta-hydroxy-5-pregnen-20-one as substrates, and the subcellular distribution substrate and nucleotide specificity of the enzyme was studied. In both membranes the microsomal fraction (particles which sedimented at 105,000 g after 90 min) had the highest specific activity. The chorion was more active than the amnion but the enzyme in both tissues had similar substrate and nucleotide specificity. NAD was the preferred cofactor, and pregnenolone was a better substrate than dehydroepiandrosterone in the presence of NAD. However, with NADP as cofactor both steroids were equally good substrates. When the 3beta-hydroxysteroid dehydrogenase activity of chorion microsomes was compared with that of placental microsomes, the specific activities were found to be of the same order of magnitude, and the substrate, nucleotide specificity and steroid binding properties were almost identical.     

The specificity of the 3beta-hydroxysteroid dehydrogenase activity of bovine ovaries toward dehydroepiandrosterone and pregnenolone: evidence for multiple enzymes.

The 3beta-hydroxysteroid dehydrogenase activity in whole bovine ovaries was systematically studied using dehydroepiandrosterone (3beta-hydroxy-5-androsten-17-one) and pregnenolone (3 beta-hydroxy-5-pregnen-20-one) as substrates, in order to determine whether, in this tissue, the same or different 3beta-hydroxysteroid dehydrogenases metabolize these steroids. The majority of the activity, with both substrates was found in the microsomes. Detergent extraction of the microsomes indicated that more than one enzyme was present in this fraction. A number of experiments on the Triton X-100 extract of the microsomes (the stability of the activity, its nucleotide specificity and kinetic analyses) were most simply explained by a single enzyme metabolizing both steroids. However, the stereospecificity of hydride-ion transfer from pregnenolone to NAD+ (B transfer) was different than that from dehydroepiandrosterone to NAD+ (A and B transfer). Thus, as no single enzyme is known to catalyze the transfer of hydride-ion to both sides of NAD+, it is proposed that there are at least two 3beta-hydroxysteroid dehydrogenases in the Triton X-100 extract.     

Rv2131c from Mycobacterium tuberculosis is a CysQ 3'-phosphoadenosine-5'-phosphatase

Mycobacterium tuberculosis ( Mtb) produces a number of sulfur-containing metabolites that contribute to its pathogenesis and ability to survive in the host. These metabolites are products of the sulfate assimilation pathway. CysQ, a 3'-phosphoadenosine-5'-phosphatase, is considered an important regulator of this pathway in plants, yeast, and other bacteria. By controlling the pools of 3'-phosphoadenosine 5'-phosphate (PAP) and 3'-phosphoadenosine 5'-phosphosulfate (PAPS), CysQ has the potential to modulate flux in the biosynthesis of essential sulfur-containing metabolites. Bioinformatic analysis of the Mtb genome suggests the presence of a CysQ homologue encoded by the gene Rv2131c. However, a recent biochemical study assigned the protein's function as a class IV fructose-1,6-bisphosphatase. In the present study, we expressed Rv2131c heterologously and found that the protein dephosphorylates PAP in a magnesium-dependent manner, with optimal activity observed between pH 8.5 and pH 9.5 using 0.5 mM MgCl 2. A sensitive electrospray ionization mass spectrometry-based assay was used to extract the kinetic parameters for PAP, revealing a K m (8.1 +/- 3.1 microM) and k cat (5.4 +/- 1.1 s (-1)) comparable to those reported for other CysQ enzymes. The second-order rate constant for PAP was determined to be over 3 orders of magnitude greater than those determined for myo-inositol 1-phosphate (IMP) and fructose 1,6-bisphosphate (FBP), previously considered to be the primary substrates of this enzyme. Moreover, the ability of the Rv2131c-encoded enzyme to dephosphorylate PAP and PAPS in vivo was confirmed by functional complementation of an Escherichia coli Delta cysQ mutant. Taken together, these studies indicate that Rv2131c encodes a CysQ enzyme that may play a role in mycobacterial sulfur metabolism.     

Human brain short chain L-3-hydroxyacyl coenzyme A dehydrogenase is a single-domain multifunctional enzyme. Characterization of a novel 17beta-hydroxysteroid dehydrogenase.

Human brain short chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD) was found to catalyze the oxidation of 17beta-estradiol and dihydroandrosterone as well as alcohols. Mitochondria have been demonstrated to be the proper location of this NAD+-dependent dehydrogenase in cells, although its primary structure is identical to an amyloid beta-peptide binding protein reportedly associated with the endoplasmic reticulum (ERAB). This fatty acid beta-oxidation enzyme was identified as a novel 17beta-hydroxysteroid dehydrogenase responsible for the inactivation of sex steroid hormones. The catalytic rate constant of the purified enzyme was estimated to be 0.66 min-1 with apparent Km values of 43 and 50 microM for 17beta-estradiol and NAD+, respectively. The catalytic efficiency of this enzyme for the oxidation of 17beta-estradiol was comparable with that of peroxisomal 17beta-hydroxysteroid dehydrogenase type 4. As a result, the human SCHAD gene product, a single-domain multifunctional enzyme, appears to function in two different pathways of lipid metabolism. Because the catalytic functions of human brain short chain L-3-hydroxyacyl-CoA dehydrogenase could weaken the protective effects of estrogen and generate aldehydes in neurons, it is proposed that a high concentration of this enzyme in brain is a potential risk factor for Alzheimer's disease.     

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

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

Human placental 3beta-hydroxysteroid dehydrogenase: delta5-isomerase. Demonstration of an intermediate in the conversion of 3beta-hydroxypregn-5-en-20-one to pregn-4-ene-3,20-dione.

3beta-Hydroxypregn-5-en-20-one (pregnenolone) and NAD+ were incubated with a solubilized preparation of the coupled enzyme 3beta-hydroxysteroid:NAD(P) oxidoreductase-3-ketosteroid delta4,delta5-isomerase (3beta-hydroxysteroid dehydrogenase: delta5-isomerase) from the mitochondrial fraction of human placenta. Unconverted pregnenolone, pregn-4-ene-3,20-dione (rogesterone), and a small but detectable amount of pregn-5-ene-3,20-dione were isolated from the medium by Sephadex LH-20 chromomatography. The identification of pregn-5-ene-3,20-dione, confirmed by mass fragmentography, has provided the first direct evidence for the formation of the hypothetical delta5,3-ketone intermediate in the conversion of pregnenolone to progesterone. When tritium-labeled pregnenolone and [4-14C]pregnenolone were incubated simultaneously the 3H:14C ratio in isolated pregn-5-ene-3,20-dione was 4.6 times greater than in isolated progesterone and pregnenolone, indicating a kinetic isotope effect in the enzymatic isomerization of tritium-labeled pregn-5-ene-3,20-dione. Exposure of the enzyme to two steroids which inhibit the overall enzyme reaction, 2alpha-cyano-17beta-hydroxy-4,4,17alpha-trimethylandrost-5-en-3-one (cyanoketone) and 3-hydroxyestra-1,3,5(10),6,8-pentaen-17-one (equilenin), increased the relative yield of labeled pregn-5-ene-3,20-dione as well as the recovery of radioactivity remaining as unconverted pregnenolone, suggesting that both the dehydrogenase and isomerase activities were inhibited. Exposure of the enzyme to equilenin increased the ratio of isolated pregn-5-ene-3,20-dione radioactivity to progesterone radioactivity as progesterone synthesis was inhibited. Equilenin also diminished the tritium isotope effect on the isomerase reaction. Both findings suggest that it is possible to inhibit the isomerase to a greater extent than the dehydrogenase. In order to measure the rate of progesterone produced by the coupled enzymes, we have modified a radiochemical method which involves precipitation of pregnenolone by digitonin. Digitonin precipitation proved to be effective in separating unconverted pregnenolone from the steroid products of both enzyme reactions, progesterone and pregn-5-ene-3,20-dione. Neither the steroidal inhibitors nor the kinetic isotope effect altered the accuracy of the method for routine measurement of the overall rate of conversion of delta5,3beta-hydroxysteroid to delta4,3-ketosteroid.     

Cholesterol is not an essential source of nutrition for Mycobacterium tuberculosis during infection

Rv1106c (hsd; 3β-hydroxysteroid dehydrogenase) is required by Mycobacterium tuberculosis for growth on cholesterol as a sole carbon source, whereas Rv3409c is not. Mutation of Rv1106c does not reduce Mycobacterium tuberculosis growth in infected macrophages or guinea pigs. We conclude that cholesterol is not required as a nutritional source during infection.     

A quantitative cytochemical method for the demonstration of delta5,3beta-hydroxysteroid dehydrogenase activity in unfixed tissue sections of rat ovary.

A method for the quantitative measurement of delta5,3beta-hydroxysteroid dehydrogenase activity in unfixed tissue sections of rat ovary has been described. The method depends on the oxidation of dehydroepiandrosterone (DHEA) and uses nitroblue tetrazolium as the final electron acceptor. Although the dehydrogenase is not a soluble enzyme, polyvinyl alcohol is included in the reaction medium to allow the use of a high substrate concentration whilst employing a low concentration (5%) of dimethyl formamide. The enzyme is equally dependent on NAD+ or NADP+ for its activity and this activity is significantly enhanced by the presence of cyanide. The NADP+ dependence is not abolished by inhibiting nonspecific alkaline phomonoesterase. The activity of delta5,3beta-hydroxysteroid dehydrogenase is completely dependent on a functional sulphydryl group. Furthermore, the enzyme activity is totally inhibited in the presence of a steroid substrate analogue at 10(-4) M.     

Steroid metabolism in vitro by gonadal tissue from a true hermaphrodite.

The case of a true hermaphrodite, with a normal ovary and an ovotestis is presented. The ovotestis was removed and incubated in vitro with tritiated steroids (testosterone, dehydroepiandrosterone, pregnenolone and 17 alpha-hydroxyprogesterone). Labeled metabolites were isolated and identified. Based upon these findings, a pathway of steroid biogenesis in this abnormal gonadal tissue is suggested. The ovotestis studied did not contain all the enzymes involved in ovarian steroidogenesis: 3 beta-hydroxysteroid dehydrogenase, isomerase, 17--20 desmolase and 17 beta-hydroxysteroid dehydrogenase were present, but other important enzymes, such as 16 and 17-hydroxylases, and aromatizing enzyme systems, were deficient or absent.     

Purification and characterization of pyridoxal 4-dehydrogenase from Aureobacterium luteolum

A pyridoxal dehydrogenase was purified to homogeneity from Aureobacterium luteolum, which can use pyridoxine as a carbon and nitrogen source, and characterized. The enzyme was a dimeric protein with a subunit molecular weight of 38,000. It had several properties distinct from those of the partially purified enzyme from Pseudomonas MA-1. The optimum pH (8.0-8.5) was 0.8-1.3 lower than that of the Pseudomonas enzyme. The Aureobacterium enzyme showed much higher and lower affinities for NAD+ (Km, 0.140 +/- 0.008 mM) and pyridoxal (0.473 +/- 0.109 mM), respectively, than those of the Pseudomonas enzyme. The Aureobacterium enzyme could use NADP+ as a substrate: the reactivity was 6.5% of NAD+. The enzyme was much more tolerant to metal-chelating agents. Irreversibility of the enzymatic reaction was shared by the two enzymes. No aldehyde dehydrogenase showed similarity to the amino-terminal amino acid sequence of the enzyme.     

Human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase: purification from mitochondria and kinetic profiles, biophysical characterization of the purified mitochondrial and microsomal enzymes

In human placenta, 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase, an enzyme complex found in microsomes and mitochondria, synthesizes progesterone from pregnenolone and androstenedione from fetal dehydroepiandrosterone sulfate. The dehydrogenase and isomerase activities of the mitochondrial enzyme were copurified (733-fold) using sequential cholate solubilization, ion exchange chromatography (DEAE-Toyopearl 650S), and hydroxylapatite chromatography (Bio-Gel HT). Enzyme homogeneity was demonstrated by a single protein band in SDS-polyacrylamide gel electrophoresis (monomeric Mr = 41,000), gel filtration at constant specific enzyme activity (Mr = 77,000), and a single NH2-terminal sequence. Kinetic constants were determined for the oxidation of pregnenolone (Km = 1.6 microM, Vmax = 48.6 nmol/min/mg) and dehydroepiandrosterone (Km = 2.4 microM, Vmax = 48.5 nmol/min/mg) and for the isomerization of 5-pregnene-3,20-dione (Km = 9.3 microM, Vmax = 914.2 nmol/min/mg) and 5-androstene-3,17-dione (Km = 27.6 microM, Vmax = 888.4 nmol/min/mg. Mixed substrate studies showed that the dehydrogenase and isomerase activities utilize their respective pregnene and androstene substrates competitively. Dixon analysis demonstrated that the product steroids, progesterone and androstenedione, are competitive inhibitors of the C-21 and C-19 dehydrogenase activities. Enzyme purified from mitochondria and microsomes had similar kinetic profiles with respect to substrate utilization, product inhibition, and cofactor (NAD+) reduction (mean Km +/- SD using C-19 and C-21 dehydrogenase substrates = 26.4 +/- 0.8 microM, mean Vmax = 73.2 +/- 1.3 nmol/min/mg). Pure enzyme from both organelles exhibited identical biophysical properties in terms of molecular weight and subunit composition, pH optima (pH 9.8, dehydrogenase; pH 7.5, isomerase), temperature optimum (37 degrees C), stability in storage and solution, effects of divalent cations, and the single NH2-terminal sequence of 27 amino acids. These results suggest that the mitochondrial and microsomal enzymes are the same protein localized in different organelles.     

Augmented androgen production is a stable steroidogenic phenotype of propagated theca cells from polycystic ovaries.

To test the hypothesis that the hyperandrogenemia associated with polycystic ovary syndrome (PCOS) results from an intrinsic abnormality in ovarian theca cell steroidogenesis, we examined steroid hormone production, steroidogenic enzyme activity, and mRNA expression in normal and PCOS theca cells propagated in long-term culture. Progesterone (P4), 17alpha-hydroxyprogesterone (17OHP4), and testosterone (T) production per cell were markedly increased in PCOS theca cell cultures. Moreover, basal and forskolin-stimulated pregnenolone, P4, and dehydroepiandrosterone metabolism were increased dramatically in PCOS theca cells. PCOS theca cells were capable of substantial metabolism of precursors into T, reflecting expression of an androgenic 17beta-hydroxysteroid dehydrogenase. Forskolin-stimulated cholesterol side chain cleavage enzyme (CYP11A) and 17alpha-hydroxylase/17,20-desmolase (CYP17) expression were augmented in PCOS theca cells compared with normal cells, whereas no differences were found in steroidogenic acute regulatory protein mRNA expression. Collectively, these observations establish that increased CYP11A and CYP17 mRNA expression, as well as increased CYP17, 3beta-hydroxysteroid dehydrogenase, and 17beta-hydroxysteroid dehydrogenase enzyme activity per theca cell, and consequently increased production of P4, 17OHP4, and T, are stable properties of PCOS theca cells. These findings are consistent with the notion that there is an intrinsic alteration in the steroidogenic activity of PCOS thecal cells that encompasses multiple steps in the biosynthetic pathway.     

Affinity alkylation of human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase by 2 alpha-bromoacetoxyprogesterone: evidence for separate dehydrogenase and isomerase sites on one protein

We have copurified human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase, which synthesize progesterone from pregnenolone and androstenedione from fetal dehydroepiandrosterone sulfate, from microsomes as a homogeneous protein based on electrophoretic and NH2-terminal sequencing data. The affinity alkylator, 2 alpha-bromoacetoxyprogesterone, simultaneously inactivates the pregnene and androstene dehydrogenase activities as well as the C21 and C19 isomerase activities in a time-dependent, irreversible manner following first order kinetics. At four concentrations (50/1-20/1 steroid/enzyme M ratios), the alkylator inactivates the dehydrogenase activity (t1/2 = 1.5-3.7 min) 2-fold faster than the isomerase activity. Pregnenolone and dehydroepiandrosterone protect the dehydrogenase activity, while 5-pregnene-3,20-dione, progesterone, and androstenedione protect isomerase activity from inactivation. The protection studies and competitive kinetics of inhibition demonstrate that the affinity alkylator is active site-directed. Kitz and Wilson analyses show that 2 alpha-bromoacetoxyprogesterone inactivates the dehydrogenase activity by a bimolecular mechanism (k3' = 160.9 l/mol.s), while the alkylator inactivates isomerase by a unimolecular mechanism (Ki = 0.14 mM, k3 = 0.013 s-1). Pregnenolone completely protects the dehydrogenase activity but does not slow the rate of isomerase inactivation by 2 alpha-bromoacetoxyprogesterone at all. NADH completely protects both activities from inactivation by the alkylator, while NAD+ protects neither. From Dixon analysis, NADH competitively inhibits NAD+ reduction by dehydrogenase activity. Mixed cofactor studies show that isomerase binds NAD+ and NADH at a common site. Therefore, NADH must not protect either activity by simply binding at the cofactor site. We postulate that NADH binding as an allosteric activator of isomerase protects both the dehydrogenase and isomerase activities from affinity alkylation by inducing a conformational change in the enzyme protein. The human placental enzyme appears to express the pregnene and androstene dehydrogenase activities at one site and the C21 and C19 isomerase activities at a second site on the same protein.     

17 beta-hydroxysteroid dehydrogenase activity in canine pancreas

The mitochondrial fraction of the dog pancreas showed NAD(H)-dependent enzyme activity of 17 beta-hydroxysteroid dehydrogenase. The enzyme catalyzes oxidoreduction between androstenedione and testosterone. The apparent Km value of the enzyme for androstenedione was 9.5 +/- 0.9 microM, the apparent Vmax was determined as 0.4 nmol mg-1 min-1, and the optimal pH was 6.5. In phosphate buffer, pH 7.0, maximal rate of androstenedione reduction was observed at 37 degrees C. The oxidation of testosterone by the enzyme proceeded at the same rate as the reduction of the androstenedione at a pH of 6.8-7.0. The apparent Km value and the optimal pH of the enzyme for testosterone were 3.5 +/- 0.5 microM and 7.5, respectively.     

Isolation and amino acid sequence analysis of bovine adrenal 3 beta-hydroxysteroid dehydrogenase/steroid isomerase.

3 beta-Hydroxysteroid dehydrogenase/steroid isomerase has been purified to homogeneity from bovine adrenal glands. A single protein of molecular weight 42,090 +/- 40 containing both enzyme activities has been isolated. Approximately 86% of the amino acid sequence of the bovine adrenal 3 beta-hydroxysteroid dehydrogenase/steroid isomerase has been obtained by sequencing peptides isolated from digests with trypsin and lysyl endopeptidase and by chemical cleavage with CNBr. The sequence obtained is identical with that of the deduced amino acid sequence of the bovine ovarian 3 beta-hydroxysteroid dehydrogenase/steroid isomerase [Zhao et al. (1989) FEBS Lett. 259, 153-157], with the exception that the N-terminal methionine residue found in the bovine ovarian sequence is not present in the mature bovine adrenal enzyme. On the basis of the primary structure and comparisons with other NAD+ binding proteins, we propose a structural model of the bovine adrenal 3 beta-hydroxysteroid dehydrogenase/steroid isomerase localizing the NAD+ binding site as well as the membrane-anchoring segment.     

Development of a quantitative assay method for 3 beta-hydroxy-delta 5-steroid dehydrogenase in the rat testis

We investigated the first step of the sex steroid hormone biosynthesis pathway by assaying the activities of 3 beta-hydroxy-delta 5-steroid dehydrogenase, the rate-limiting enzyme in this pathway. We have developed a simple and rapid colorimetric assay for 3 beta-hydroxy-delta 5-steroid dehydrogenase in rat testis. The supernatant from rat testis tissue homogenates were used for the enzyme assay. The enzyme activity was determined by measuring the absorbance at 570nm which indicates the rate of conversion of pregnenolone into progesterone in the presence of NAD, using phenazine methosulfate and nitro blue tetrazolium as the color reagent. The activity of this enzyme ranged from 4.57+/-1.34 to 10.56+/-2.13 nmol/mg protein/min with a mean activity of 8.96+/-1.27 nmol/mg protein/min. The K(m) of the enzyme at an optimum pH of 7.25 was about 4.7+/-0.12 nM.     

At5g50600 encodes a member of the short-chain dehydrogenase reductase superfamily with 11beta- and 17beta-hydroxysteroid dehydrogenase activities associated with Arabidopsis thaliana seed oil bodies

In a previous work, we presented evidence for the presence of a protein encoded by At5g50600 in oil bodies (OBs) from Arabidopsis thaliana [P. Jolivet, E. Roux, S. D'Andrea, M. Davanture, L. Negroni, M. Zivy, T. Chardot, Protein composition of oil bodies in Arabidopsis thaliana ecotype WS, Plant Physiol. Biochem. 42 (2004) 501-509]. Using specific antibodies and proteomic techniques, we presently confirm the existence of this protein, which is a member of the short-chain steroid dehydrogenase reductase superfamily. We have measured its activity toward various steroids (cholesterol, dehydroepiandrosterone, cortisol, corticosterone, estradiol, estrone) and NAD(P)(H), either within purified OBs or as a purified bacterially expressed chimera. Both enzymatic systems (OBs purified from A. thaliana seeds as well as the chimeric enzyme) exhibited hydroxysteroid dehydrogenase (HSD) activity toward estradiol (17beta-hydroxysteroid) with NAD+ or NADP+, NADP+ being the preferred cofactor. Low levels of activity were observed with cortisol or corticosterone (11beta-hydroxysteroids), but neither cholesterol nor DHEA (3beta-hydroxysteroids) were substrates, whatever the cofactor used. Similar activity profiles were found for both enzyme sources. Purified OBs were found to be also able to catalyze estrone reduction (17beta-ketosteroid reductase activity) with NADPH. The enzyme occurring in A. thaliana OBs can be classified as a NADP+-dependent 11beta-,17beta-hydroxysteroid dehydrogenase/17beta-ketosteroid reductase. This enzyme probably corresponds to AtHSD1, which is encoded by At5g50600. However, its physiological role and substrates still remain to be determined.