3 alpha-hydroxysteroid dehydrogenase activity catalyzed by purified pig adrenal 20 alpha-hydroxysteroid dehydrogenase

Porcine aldo-keto reductase 1C subfamily members AKR1C1 and AKR1C4: Substrate specificity, inhibitor sensitivity and activators

Most members of the aldo-keto reductase (AKR) 1 C subfamily are hydroxysteroid dehydrogenases (HSDs). Similarly to humans, four genes for AKR1C proteins (AKR1C1-AKR1C4) have been identified in the pig, which is a suitable species for biomedical research model of human diseases and optimal organ donor for xenotransplantation. Previous study suggested that, among the porcine AKR1Cs, AKR1C1 and AKR1C4 play important roles in steroid hormone metabolism in the reproductive tissues; however, their biological functions are still unknown. Herein, we report the biochemical properties of the two recombinant enzymes. Kinetic and product analyses of steroid specificity indicated that AKR1C1 is a multi-specific reductase, which acts as 3α-HSD for 3-keto-5β-dihydro-C₁₉/C₂₁-steroids, 3β-HSD for 3-keto-5α-dihydro-C₁₉-steroids including androstenone, 17β-HSD for 17-keto-C₁₉-steroids including estrone, and 20α-HSD for progesterone, showing K_m values of 0.5-11 µM. By contrast, AKR1C4 exhibited only 3α-HSD activity for 3-keto groups of 5α/β-dihydro-C₁₉-steroids, 5β-dihydro-C₂₁-steroids and bile acids (K_m: 1.0-1.9 µM). AKR1C1 and AKR1C4 also showed broad substrate specificity for nonsteroidal carbonyl compounds including endogenous 4-oxo-2-nonenal, 4-hydroxy-nonenal, acrolein, isocaproaldehyde, farnesal, isatin and methylglyoxal, of which 4-oxo-2-nonenal was reduced with the lowest K_m value of 0.9 µM. Moreover, AKR1C1 had the characteristic of reducing aliphatic ketones and all-trans-retinal. The enzymes were inhibited by flavonoids, synthetic estrogens, nonsteroidal anti-inflammatory drugs, triterpenoids and phenolphthalein, whereas only AKR1C4 was activated by bromosulfophthalein. These results suggest that AKR1C1 and AKR1C4 function as 3α/3β/17β/20α-HSD and 3α-HSD, respectively, in metabolism of steroid hormones and a sex pheromone androstenone, both of which also play roles in metabolism of nonsteroidal carbonyl compounds.

Structure-function relationships and molecular genetics of the 3 beta-hydroxysteroid dehydrogenase gene family

The isoenzymes of the 3 beta-hydroxysteroid dehydrogenase/5-ene-4-ene-isomerase (3 beta-HSD) gene family catalyse the transformation of all 5-ene-3 beta-hydroxysteroids into the corresponding 4-ene-3-keto-steroids and are responsible for the interconversion of 3 beta-hydroxy- and 3-keto-5 alpha-androstane steroids. The two human 3 beta-HSD genes and the three related pseudogenes are located on the chromosome 1p13.1 region, close to the centromeric marker D1Z5. The 3 beta-HSD isoenzymes prefer NAD+ to NADP+ as cofactor with the exception of the rat liver type III and mouse kidney type IV, which both prefer NADPH as cofactor for their specific 3-ketosteroid reductase activity due to the presence of Tyr36 in the rat type III and of Phe36 in mouse type IV enzymes instead of Asp36 found in other 3 beta-HSD isoenzymes. The rat types I and IV, bovine and guinea pig 3 beta-HSD proteins possess an intrinsic 17 beta-HSD activity specific to 5 alpha-androstane 17 beta-ol steroids, thus suggesting that such "secondary" activity is specifically responsible for controlling the bioavailability of the active androgen DHT. To elucidate the molecular basis of classical form of 3 beta-HSD deficiency, the structures of the types I and II 3 beta-HSD genes in 12 male pseudohermaphrodite 3 beta-HSD deficient patients as well as in four female patients were analyzed. The 14 different point mutations characterized were all detected in the type II 3 beta-HSD gene, which is the gene predominantly expressed in the adrenals and gonads, while no mutation was detected in the type I 3 beta-HSD gene predominantly expressed in the placenta and peripheral tissues. The mutant type II 3 beta-HSD enzymes carrying mutations detected in patients affected by the salt-losing form exhibit no detectable activity in intact transfected cells, at the exception of L108W and P186L proteins, which have some residual activity (approximately 1%). Mutations found in nonsalt-loser patients have some residual activity ranging from approximately 1 to approximately 10% compared to the wild-type enzyme. Characterization of mutant proteins provides unique information on the structure-function relationships of the 3 beta-HSD superfamily.

Formation and degradation of dihydrotestosterone by recombinant members of the rat 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase family

The structures of cDNA clones encoding four members of the rat 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase (3 beta-HSD) family were characterized. The rat type I, type II and the novel type IV are genuine NAD+/H-dependent 3 beta-HSD isoenzymes. On the other hand, the liver-specific type III protein is a specific 3-keto-reductase (3-KSR) that catalyzes the conversion of 5 alpha-androstane-3-one-17 beta-ol (DHT) and 5 alpha-androstane 3,17-dione (A-dione) into their 3 beta-hydroxy metabolites. The aim of the present study was to further characterize the enzymatic properties of rat types I, III and IV, especially their role in the formation and degradation of DHT after transient expression in intact human HeLa cervical carcinoma, JEG-3 choriocarcinoma or SW-13 adrenal cortex adenocarcinoma cells in culture. The expressed type III 3-KSR in intact HeLa cells catalyzed the reduction of DHT into 3 beta-diol, whereas expression of type I 3 beta-HSD in these cell lines had no significant effect on the basal conversion of DHT into 3 beta-diol, but it did increase the formation of DHT from 3 beta-diol. A-dione is the predominant product obtained when DHT and 5 alpha-androstane-3 beta, 17 beta-diol (3 beta-diol) are used as substrates in intact JEG-3 and SW-13 cells transfected with rat type I 3 beta-HSD. Furthermore, this predominant 17 beta-HSD activity was also observed in SW-13 cells transfected with the novel rat type IV 3 beta-HSD. The predominance of this 'secondary' 17 beta-HSD activity is also reflected in HeLa cells transfected with type I 3 beta-HSD by the deduced predominant pathway 3 beta-diol-->DHT-->5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-diol)-->androsterone (ADT), in which formation of 3 alpha-HSD activity of HeLa cells, whereas the other reactions are catalyzed by the type I 3 beta-HSD isoenzyme. This observation thus demonstrates that rat type I 3 beta-HSD may also catalyze the conversion of 3 alpha-diol into ADT through its intrinsic 17 beta-HSD activity. The predominant metabolic pathways observed in the present study could be attributed to preponderant bioavailability of NAD+ and NADPH in the intact transfected cells used.

Purification and characterization of a novel dimeric 20 alpha-hydroxysteroid dehydrogenase from Tetrahymena pyriformis

Tetrahymena pyriformis was found to exhibit high NADPH-dependent 20-oxosteroid reductase activity that converted 17 alpha-hydroxyprogesterone into 17 alpha,20 alpha-dihydroxypregn-4-en-3-one. The enzyme was purified 400-fold from the cytosolic fraction. The purified enzyme with a specific activity of 6.4 mumol/min per mg of protein had an isoelectric point of 4.9 and M(r) of 68,000, and was composed of two subunits of equal size. The N-terminal sequence was determined to be LAKTVPLNDGTNFPIFGG. The enzyme reduced pregnanes and pregnanes possessing a 17 alpha-hydroxy group to a greater extent than those without the hydroxy group, and oxidized 20 alpha-hydroxy groups of the steroids in the presence of NADP+. The Km values for 17 alpha-hydroxyprogesterone and 17 alpha-hydroxypregnenolone were 2.9 and 3.4 microM respectively. Although the enzyme was inactive towards androgens and oestrogens with 3- or 17-oxo groups, it reduced several nonsteroidal carbonyl compounds and oxidized trans-benzene dihydrodiol. The enzyme activity was inhibited by synthetic oestrogens, barbiturates, aldose reductase inhibitors and quercitrin. Thus, this enzyme is a novel form of 20 alpha-hydroxysteroid dehydrogenase (EC 1.1.1.149) which structurally and functionally differs from the mammalian and bacterial enzymes.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.