Modulation of epididymal delta 4-steroid 5 alpha-reductase activity in vitro by the phospholipid environment

Expression in Escherichia Coli, Purification, and Functional Reconstitution of Human Steroid 5α-Reductases

The potent androgen 5α-dihydrotestosterone irreversibly derives from testosterone via the activity of steroid 5α-reductases (5αRs). The major 5αR isoforms in most species, 5αR1 and 5αR2, have not been purified to homogeneity. We report here the heterologous expression of polyhistidine-tagged, codon-optimized human 5αR1 and 5αR2 cDNAs in Escherichia coli. A combination of the nonionic detergents Triton X-100 and Nonidet P-40 enabled solubilization of these extremely hydrophobic integral membrane proteins and facilitated purification with affinity and cation-exchange chromatography methods. For functional reconstitution, we incorporated the purified isoenzymes into Triton X-100-saturated dioleoylphosphatidylcholine liposomes and removed excess detergent with polystyrene beads. Kinetic studies indicated that the 2 isozymes differ in biochemical properties, with 5αR2 having a lower apparent Km for testosterone, androstenedione, progesterone, and 17-hydroxyprogesterone than 5αR1; however, 5αR1 had a greater capacity for steroid conversion, as reflected by a higher Vmax than 5αR2. Both enzymes preferred progesterone as substrate over other steroids, and the catalytic efficiency of purified reconstituted 5αR2 exhibited a sharp pH optimum at pH 5. Intriguingly, we found that the prostate-cancer drug-metabolite 3-keto-∆ 4-abiraterone is metabolized by 5αR1 but not 5αR2, which may serve as a structural basis for isoform selectivity and inhibitor design. The functional characterization results with the purified reconstituted isoenzymes paralleled trends obtained with HEK-293 cell lines stably expressing native 5αR1 and 5αR2. Access to purified human 5αR1 and 5αR2 will advance studies of these important enzymes and might help to clarify their contributions to steroid anabolism and catabolism.

Importance of membrane structural integrity for RPE65 retinoid isomerization activity

Regeneration of visual chromophore in the vertebrate visual cycle involves the retinal pigment epithelium-specific protein RPE65, the key enzyme catalyzing the cleavage and isomerization of all-trans-retinyl fatty acid esters to 11-cis-retinol. Although RPE65 has no predicted membrane spanning domains, this protein predominantly associates with microsomal fractions isolated from bovine retinal pigment epithelium (RPE). We have re-examined the nature of RPE65 interactions with native microsomal membranes by using extraction and phase separation experiments. We observe that hydrophobic interactions are the dominant forces that promote RPE65 association with these membranes. These results are consistent with the crystallographic model of RPE65, which features a large lipophilic surface that surrounds the entrance to the catalytic site of this enzyme and likely interacts with the hydrophobic core of the endoplasmic reticulum membrane. Moreover, we report a critical role for phospholipid membranes in preserving the retinoid isomerization activity and physical properties of RPE65. Isomerase activity measured in bovine RPE was highly sensitive to phospholipase A(2) treatment, but the observed decline in 11-cis-retinol production did not directly reflect inhibition by products of lipid hydrolysis. Instead, a direct correlation between the kinetics of phospholipid hydrolysis and retinoid isomerization suggests that the lipid membrane structure is critical for RPE65 enzymatic activity. We also provide evidence that RPE65 operates in a multiprotein complex with retinol dehydrogenase 5 and retinal G protein-coupled receptor in RPE microsomes. Modifications in the phospholipid environment affecting interactions with these protein components may be responsible for the alterations in retinoid metabolism observed in phospholipid-depleted RPE microsomes. Thus, our results indicate that the enzymatic activity of native RPE65 strongly depends on its membrane binding and phospholipid environment.

Localization of 5α-reductase in the rat main olfactory bulb

The enzyme steroid 5alpha-reductase catalyzes the production of dihydroprogesterone and dihydrotestosterone, which were recently recognized as neurosteroids in the brain with variably potential neuroactivity. The present study reports for the first time detailed localization of 5alpha-reductase type 1 in the rat main olfactory bulb. The occurrence of 5alpha-reductase in the olfactory bulb was detected by reverse transcription-polymerase chain reaction and Western blotting analyses. In addition, the enzyme activity was also detected by thin layer chromatography. Immunocytochemistry showed that 5alpha-reductase immunoreactive cells of variable intensity were present in all layers of the olfactory bulb. Multiple immunolabeling revealed that 5alpha-reductase was mainly localized in glial cells, namely, in S-100beta- and glial fibrillary acidic protein-immunoreactive astrocytes, 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase)-immunoreactive oligodendrocytes, and in S-100beta- and neuropeptide-Y-immunoreactive olfactory ensheathing cells, whereas the bulbar neurons exhibited little immunoreactivity. Quantitative analysis revealed that the number of 5alpha-reductase-immunoreactive cells was greatest in the olfactory nerve layer. The most intense 5alpha-reductase-immunoreactivity was found in the olfactory ensheathing cells, and next in the CNPase-immunoreactive cells. The 5alpha-reductase in the olfactory bulb was expressed constantly throughout different ages and sexes and in neutered and hypophysectomized rats. Thus, 5alpha-reductase may contribute via 5alpha-reduced metabolites to the formation and maintenance of olfactory inputs and outputs, which were closely associated with the olfactory ensheathing cells and the oligodendrocytes, respectively.

Phospholipase A2 degradation products modulate epithelial and stromal 5alpha-reductase activity of human benign prostatic hyperplasia in vitro

BACKGROUND

Recent studies have demonstrated the inhibition of 5alpha-reductase activity in human prostate by phospholipases. Among those phospholipases, phospholipase A2 cleaves one of the acyl chains from phospholipids, thereby producing fatty acids and lysophospholipids such as LPC, LPS, and LPE. Therefore, we were interested in the effect of those lysophospholipids on 5alpha-reductase activity in human benign prostatic hyperplasia (BPH).

METHODS

In a first set of experiments, cell homogenates were incubated with phospholipase A2 either in the presence or absence of albumin, which is known to bind fatty acids and lysophospholipids. Thereafter, the effect of lysophospholipids of known structure on 5alpha-reductase activity was investigated.

RESULTS

In epithelium and stroma of human BPH, 5alpha-reductase activity was inhibited in a dose-dependent manner by phospholipase A2. In the presence of albumin, this inhibition was enhanced. In epithelium, LPC at low concentration yielded a dose-dependent stimulation of 5alpha-reductase activity up to 167%. At higher concentrations, epithelial as well as stromal 5alpha-reductase activity was inhibited significantly. As indicated by results of enzyme kinetic analyses, the LPC-mediated activation in the epithelium results from an increase of the active population of 5alpha-reductase. In contrast, LPC reduces the affinity of epithelial 5alpha-reductase to testosterone. LPE had no effect on epithelial 5alpha-reductase, whereas stromal 5alpha-reductase was inhibited in a dose-dependent manner up to 46%. Finally, LPS stimulated epithelial and stromal 5alpha-reductase activity; this stimulation was significantly stronger in epithelium (296%) than in stroma (163%). The LPC-mediated effects could be neutralized by the addition of albumin.

CONCLUSIONS

The present data on BPH tissue suggest that lysophospholipids may play a specific and structure-related role in the posttranslational regulation of human prostatic 5alpha-reductase.

In vitro modulation of steroid 5alpha-reductase activity by phospholipases in epithelium and stroma of human benign prostatic hyperplasia

Membrane components, such as phospholipids, play an important role in the regulation of prostatic 5alpha-reductase activity. To describe in more detail the impact of such regulation on 5alpha-reductase activity, epithelial and stromal cell homogenates of human BPH were treated with phospholipases to specifically alter the structure of cellular phospholipid components. Phospholipase A(2) (PLA(2)) was used to alter the structure of the nonpolar, hydrophobic region of the membrane bilayer. Various types of phospholipase C (PLC) affect the polar, hydrophilic region of phospholipids. In epithelium and stroma, 5alpha-reductase activity was dose-dependently inhibited by PLA(2) and PLC type III. In epithelium and stroma, the mean IC(50) values of PLA(2) were 9.4 +/- 1.1 and 13.9 +/- 2.6 [U/mg protein +/- SEM], respectively. The mean IC(50) values of PLC type III in epithelium and stroma were 4.5 +/- 1.2 and 1.7 +/- 0.2 [U/mg protein +/- SEM], respectively. In epithelium as well as in stroma, 5alpha-reductase activity was more greatly inhibited by PLC type III than by PLA(2). Both in epithelium and stroma, PLA(2) significantly decreased the V(max) of 5alpha-reductase whereas its K(m) remained unaffected. A similar decrease in V(max) was found with PLC type III in epithelium and stroma. Furthermore, the K(m) of epithelial 5alpha-reductase increased significantly following the addition of PLC type III. The two phospholipases, with their specific substrate affinities and sites of hydrolysis, exhibited significantly different effects on 5alpha-reductase, indicating that 5alpha-reductase activity is not unspecifically affected by modification of the hydrophilic milieu. Rather, 5alpha-reductase activity is specifically modulated by various phospholipids and/or phospholipolysis mediated degradation products. These findings suggest that the structural composition of the lipid environment plays a fundamental role in the post-translational regulation of 5alpha-reductase activity in the epithelium and stroma of human BPH. Thus, changes in membrane phospholipid content seem to be instrumental in the expression of DHT-dependent processes.

The uptake of two androgen substrates by cultured human gingival fibroblasts in response to minocycline and metabolic studies using a cell-free system

The uptake of two androgen substrates by cultured fibroblasts and the anabolic response of homogenates of cultured gingival fibroblasts to minocycline were investigated. Monolayer cultures of confluent fibroblasts were incubated with [14C]testosterone/[14C]4-androstenedione for timed intervals in the presence or absence of optimal concentrations of minocycline. The intracellular uptake of androgens was quantified by radio-isotope counts on cell digests. Confluent gingival fibroblasts were homogenized by snap freezing/rapid thawing and duplicate incubations were made in phosphate-buffered saline (pH 6.5) with radiolabelled androgens, in the presence or absence of minocycline, for 24 h. At the end of the incubation period the buffer was extracted for radioactive metabolites, analysed and quantified with a radio-isotope scanner. There were 30% increases in the uptake of androgen substrates in the presence of minocycline (n=3; p < 0.01; one-way ANOVA). With the metabolic studies there were 2-3-fold increases in the formation of dihydrotestosterone from [14C]testosterone and [14C]4-androstenedione, respectively (n=4; p < 0.001; one-way ANOVA), and 4-fold/2-fold increases in the formation of 4-androstenedione/ testosterone from these substrates (n=4; p < 0.001) in response to an optimal concentration of 20 microg/ml of minocycline, compared with control incubations. The presence of minocycline in the incubate significantly increased the activity of the steroid-metabolizing enzymes. This increase might result from increased intracellular availability of steroid substrate and enhanced metabolic activity. Homogenates of cultured gingival fibroblasts are a useful model for studying the anabolic potential of minocycline in gingiva, using C19 steroid substrates.

5 alpha reductase activity in human gingiva and gingival fibroblasts in response to bacterial culture supernatants, using [14C]4-androstenedione as substrate

5 alpha-Reduction of androgen substrates is increased in inflamed gingiva. It was therefore relevant to study the effect of bacterial culture supernatants derived from Prevotella intermedius (P.i), Porphyromonas gingivalis (P.g) and Actinobacillus actinomycetemcomitans (A.a) on the metabolism of [14C]4-androstenedione to 5 alpha-dihydrotestosterone (DHT) in gingival tissue and cultured fibroblasts. Chronically inflamed human gingival tissue and cultured gingival fibroblasts from the same source were incubated in duplicate with [14C]4-androstenedione and optimal concentrations of P.i, P.g. and A.a culture supernatants in Eagle's minimal essential medium in a CO2 incubator for 24 h at 37 degrees C. The metabolites were then extracted, separated and quantified using a radioisotope scanner. There were 87, 50 and 6% increases in DHT synthesis by human gingival tissue in response to the culture supernatants of P.i, P.g and A.a, respectively, over control incubations (n = 3; p < 0.01: Wilcoxon signed-ranked statistic for paired observations). With the cells in culture, all four fibroblast cell lines produced DHT and testosterone from [14C]4-androstenedione in varying amounts. The production of DHT was enhanced in the presence of each the bacterial culture supernatants to varying degrees (P.i 40%, P.g 35% and A.a 40%; p < 0.01). Combinations of the bacterial extracts: (P.i + P.g), (P.i + A.a), (P.g + A.a) and (P.i + P.g + A.a) showed intermediate or suppressor effects on DHT formed compared with individual incubations. Culture supernatants of these pathogens can influence DHT synthesis in fibroblasts, and effect that is modulated by baseline androgen metabolism and the proportion of virulent pathogens present. This may have some bearing on host susceptibility on host and the progression of the periodontal lesion.

Effect of PCBs on androgen production by suspension of adult rat Leydig cells in vitro

The effects of PCBs (mixture of 2, 3, 4, 5-tetra; 2, 2', 4, 5, 5'-penta; 2, 2', 3, 3', 6, 6'-hexa and 2, 2', 3, 3', 4, 4', 5, 5'-octa congeners) on androgen production were investigated by suspension of Leydig cells from adult rat testis. hCG-stimulated androgen production was significantly inhibited by PCBs while progesterone level was not affected. Progesterone supported testosterone production was also decreased by PCBs, while conversion of androstenedione to testosterone was unchanged. These results suggest that the activity of microsomal enzyme C21 side-chain cleavage P450 was decreased by PCB treatment of Leydig cells in vitro.

Bacterial steroidogenesis by periodontal pathogens and the effect of bacterial enzymes on steroid conversions by human gingival fibroblasts in culture

Studies were performed to investigate the effect of microbial culture supernatants of periodontal pathogens on the metabolism of radiolabelled testosterone in the presence or absence of human gingival fibroblasts. Subgingival plaque samples were obtained on paper points from 3 sites with probing depth values of 6-8 mm. Samples were incubated with 14C-testosterone for 24 h in brain heart infusion (BHI) broth. Similar incubations were also carried out with strains of A. actinomycetemcomitans, P. Intermedius and P. gingivalis to study the metabolism of radiolabelled testosterone by these periodontal pathogens. At the end of a 24 h incubation period with fibroblasts and supernatants or sonicates, the radioactive metabolites were extracted with ethyl acetate, evaporated and subjected to thin layer chromatography. The separated metabolites were quantified by scanning the radioactive plates using a Berthold linear analyser. When three sub-gingival plaque samples were incubated with radiolabelled testosterone there were 50-fold, 10-12-fold and 15-17-fold increases in 5 alpha-dihydrotestosterone (DHT) synthesis over 4-androstenedione production in these mixed microbial cultures. The two strains of P. intermedius produced 3- and 20-fold increases in 4-androstenedione production and DHT synthesis respectively. Both strains of A. actinomycetemcomitans and P. gingivalis showed 3-4-fold and 12-28-fold increases respectively in 4-androstenedione synthesis over that of DHT. Culture supernatants of P. intermedius and P. gingivalis caused 3-fold and 2-fold increases in DHT synthesis by fibroblasts over controls. There was little change in the case of the third pathogen. Since DHT has implications on matrix synthesis by fibroblasts in the environment of plaque associated inflammatory periodontal disease, bacterial metabolism and the effect of bacterial supernatants on human gingival fibroblasts can influence the degree of inflammatory repair.

Phospholipid requirement of epididymal testosterone 5 alpha-reductase and phospholipid composition of epididymal microsomes

We have investigated phospholipid requirement for testosterone 5 alpha-reductase solubilized from microsomal and nuclear fractions of rat epididymis. The 5 alpha-reductase from microsomal fraction was stimulated by phosphatidylcholine (PC) with long acyl-chain lengths, but inhibited by short chain PC. The nuclear enzyme activity was weakly activated by PC with various acyl-chain lengths tested. Synthetic phosphatidylserine (PS), such as dioleoylPS, most strongly stimulated the microsomal enzyme activity, but did not exhibit any activation of the nuclear enzyme activity. Endogenous phospholipids, such as PC, PS, and phosphatidylethanolamine (PE) separated from bovine epididymal microsomes were tested for their stimulatory effects on microsomal and nuclear enzymes. Among these endogenous phospholipids, PS most greatly stimulated the microsomal 5 alpha-reductase activity, whereas both PC and PE weakly activated the enzyme activity. On the other hand, endogenous PC and PS had no ability to support the nuclear enzyme activity. The fatty acid compositions of PC and PS from bovine epididymal microsomes were determined, in order to elucidate the relationship between 5 alpha-reductase activation by these phospholipids and the structure of their acyl chains. The relative content of fatty acids in PC, in a decreasing order, was palmitate > linoleate > oleate; that in PS was stearate > oleate > palmitate. Based on these observations, the roles of microsomal PS and PC in epididymal 5 alpha-reductase reaction will be discussed.

Identification of phospholipids capable of modulating the activities of some enzymes involved in androgen and 16-androstene biosynthesis in the immature pig testis

Testicular steroidogenic enzymes in the microsomal fraction from immature pigs were investigated for the effects of phospholipids of known structure on androgen and 16-androstene biosynthesis. Untreated (control) microsomes metabolized pregnenolone to 17-hydroxypregnenolone, DHA and small quantities of progesterone, 17-hydroxyprogesterone, androstenedione and testosterone; and to 5,16-androstadien-3 beta-ol (andien-beta) and 4,16-androstadienone (dienone) in the 16-androstene pathway. Phosphatidyl(P)-serine, P-glycerol, P-ethanolamine, P-inositol, P-choline and phosphatidic acid did not significantly alter the 17-hydroxylase/C-17,20 lyase or "andien-beta-synthetase" activities. Thus, the C21 side-chain cleavage reactions appeared not to be dependent upon phospholipids for optimal activity. The conversion of pregnenolone to 4-ene steroids (progesterone, 17-hydroxyprogesterone, androstenedione and testosterone) was inhibited by dilinoleoyl-phosphatidyl-choline, but other phospholipids tested were without effect. On the other hand, the conversion of andien-beta to dienone was inhibited by P-serine, P-inositol and P-cholines with short saturated or long polyunsaturated acyl chains. Therefore, the presence of these phospholipids in pregnenolone incubations had different consequences for 3 beta-hydroxysteroid dehydrogenase-isomerase activities. It is concluded that substrate specific 3 beta-HSD-isomerases exist for androgen and 16-androstene biosynthesis and that phospholipids may play an intrinsic role in their catalytic activity.

Phospholipases modulate immature pig testicular androgen and 16-androstene biosynthetic pathways in vitro

The role of membrane phospholipids in porcine testicular androgen and 16-androstene biosynthesis was examined by monitoring the effects of phospholipase treatments on the activities of the steroid transforming enzymes. Untreated (control) microsomes from immature pig testes converted pregnenolone to 17-hydroxypregnenolone and DHA to 5,16-androstadien-3 beta-ol (andien-beta) and 4,16-androstadien-3-one (dienone) in the 16-androstene pathway, these metabolites accounting for most (65%) of the pregnenolone converted. The 4-ene steroids in the androgen pathway (progesterone, 17-hydroxyprogesterone, androstenedione and testosterone) totalled less than 10% of the pregnenolone metabolites. No estrogens or 5 alpha-reduced metabolites were detected. Treatment with phospholipase A2 or C, decreased the conversion of pregnenolone to 4-ene-3-oxo steroids but did not decrease the quantities of 5-ene-3 beta-hydroxysteroids. Confirmation of these findings was obtained by measuring the individual enzymatic steps. Phospholipases A2 and C significantly reduced the conversion of DHA to androstenedione and andien-beta to dienone but did not affect 17-hydroxylase or 'andien-beta-synthetase'. However, when the C-17, 20 lyase step was measured alone, phospholipase C decreased the quantity of androstenedione produced indicating that the side-chain cleavage reaction may involve a lipid component. The different effects of phospholipases on these enzymes suggests that pregnenolone metabolism may be regulated by alterations in the membrane microenvironment.

Phospholipid requirement of progesterone 5 alpha-reductase from gastric mucosa microsomes of guinea pig

Progesterone 5 alpha-reductase partially purified from gastric mucosa microsomes was stimulated by short-chain synthetic phosphatidylcholines (PC), such as dilauroyl PC, but not by various PC from biological sources. Phosphatidylserine (PS) activated the gastric 5 alpha-reductase to a limited extent compared to the liver 5 alpha-reductase described previously [Ichihara, K., and Tanaka, C. (1987) Biochem. Biophys. Res. Commun. 149, 482-487]. In search of more effective phospholipid activators, we tested the effects of various lysophospholipids on 5 alpha-reductase activity. Strongly stimulatory effects were observed when lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) were used instead of PC and phosphatidylethanolamine. Examination of synthetic lysoPC and lysoPE differing in acyl chain lengths showed that fatty chains of 12 to 16 carbons were effective in stimulating the 5 alpha-reductase. By contrast, other lysophospholipids such as lysophosphatidic acid, lysophosphatidylglycerol or lysophosphatidylserine (lysoPS) greatly inhibited 5 alpha-reductase activity. These findings suggest that gastric 5 alpha-reductase may be under dual regulation; lysoPC and lysoPE may play important roles as positive effectors, whereas lysophosphatidic acid, lysophosphatidylglycerol and lysoPS act as negative effectors in progesterone 5 alpha-reductase regulation.

Solubilization of nuclear steroid 5 alpha-reductase from rat ventral prostate

delta 4-Steroid-5 alpha-reductase (3-oxo-5 alpha-steroid:NADP+ delta 4-oxidoreductase, EC 1.3.1.22), is a membrane-bound enzyme. In the ventral prostate of the rat, its activity is found within the nuclear envelope. Solubilization of this enzyme can only be achieved in the presence of detergents. We studied the inhibitory effect of various detergents on 5 alpha-reductase activity as a function of detergent concentration, of pH, of incubation time, of salt concentration and of additives to the buffer system. Four detergents (Lubrol WX, CHAPS, L-alpha-lysophosphatidylcholine and octyl D-glucopyranoside) were selected for subsequent solubilization studies. The overall recovery of solubilized enzyme activity was about 30% when compared to 100% of 5 alpha-reductase activity found in freshly prepared nuclei. Up to 20-30% of the nuclear proteins were extracted during the solubilization procedure. Among the various treatments tested, a concentration of 3 mg/ml L-alpha-lysophosphatidylcholine per 10 mg/ml of nuclear protein in the presence of 5 mM MgCl2, 0.1 M KCl, 0.1 M sodium citrate and 5 mM NADPH yielded the maximal enzymic activity of 56%, 15% of the nuclear proteins being solubilized in an active and stable form. The activity in these extracts could be kept stable for 2 days at 4 degrees C with a recovery of 75% of enzymic activity. A 3-fold increase of specific 5 alpha-reductase activity was obtained during solubilization under optimal conditions.

Partial characterization of testosterone 5 alpha-reductase solubilized from rat testicular microsomes

Testosterone 5 alpha-reductase was successfully solubilized by the use of digitonin from rat testicular microsomes and then partially purified by polyethylene glycol fractionation and DEAE-Sephacel column chromatography. The 5 alpha-reductase activity of the partially purified preparation was significantly stimulated by addition of phosphatidylserine (bovine brain). Synthetic dilauroylphosphatidylcholine also increased the reductase activity to a somewhat lesser extent than did phosphatidylserine, whereas natural phosphatidylcholine from bovine liver did not exhibit any stimulation. When synthetic phosphatidylcholines with varying acyl chain lengths were tested for their stimulatory effects on the reductase activity, dilauroylphosphatidylcholine was most active; dimyristoylphosphatidylcholine was less active; dioleoylphosphatidylcholine was almost inactive.

Inhibition of nuclear T3 binding by fatty acids

Studies were performed to evaluate a possible modulatory role of lipids on the binding of T3 to rat liver nuclear receptors in vitro. Unsaturated fatty acids were potent inhibitors of the binding of [125I] T3 to isolated rat liver nuclei. Doses (in mumol/L) causing a 50% inhibition of nuclear T3 binding were 10 for palmitoleic acid, 11 for linoleic acid, 22 for oleic acid, 24 for arachidonic acid, and 37 for linolenic acid. Other lipids had less or no inhibitory activity. Unsaturated fatty acids reduced the affinity constant (Ka) of the binding of T3 to nuclear receptors to 57.4% +/- 11.0% that of controls (mean +/- SE 1.04 +/- 0.14 v 1.97 +/- 0.23 10(9) L/M, n = 5; P less than .02) but did not affect the maximal binding capacity (MBC) (1.47 +/- 0.20 v 1.55 +/- 0.10 10(-10) M/L; NS). Evaporated ether extracts of rat liver homogenate pretreated with phospholipase A2 for five to 20 minutes (that liberates unsaturated fatty acids from phospholipids) demonstrated a progressive inhibition of nuclear T3 binding with time when compared with ether extracts of untreated rat liver homogenate (F = 16.1; P less than .01). Evaporated, fatty-acid-rich ether extracts of human sera caused a dose-dependent inhibition in the binding of [125I] T3 to nuclear T3 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of 4-ene-5 alpha-reductase and studies on its solubilization from porcine testicular microsomes

The activity of 4-ene-5 alpha-reductase was assayed in porcine testis homogenates and subcellular fractions, using testosterone as substrate. 'Marker' enzyme activities were utilized to indicate the purity of the subcellular fractions. 4-Ene-5 alpha-reductase activity was associated with the microsomal fraction; there was no activity in the purified nuclear fraction. Enzyme activity was higher in the testes of 6 week old pigs than those of 3 and 17 week old animals, and a range of activity was found. The enzyme was unstable when stored at -20 degrees C but the addition of albumin (0.1%, w/v) or glycerol (20%, v/v) to the buffer and storage at -70 degrees C or in liquid nitrogen ensured that maximal activity was retained for at least 35 days. In addition to 5 alpha-DHT, other 5 alpha-reduced metabolites and 4-androstenedione were formed in this reaction; NADPH was the preferred cofactor, but 40% of the 4-ene-5 alpha-reductase activity was retained when NADH was used. Solubilization of the microsomal enzyme was achieved using sodium citrate (0.1 M); 4-ene-5 alpha-reductase activity was enhanced to greater than 120% and 60% of this activity was in the soluble fraction. The optimum pH and temperature for both soluble and membrane-bound 4-ene-5 alpha-reductase were 6.9 and 32 degrees C, respectively. The mean apparent Km and Vmax were 0.6 mumol/l and 158 pmol/min/mg microsomal protein for the microsomal enzyme and 1.42 mumol/l and 212.0 pmol/min/mg soluble protein for the solubilized 4-ene-5 alpha-reductase. The estimated sedimentation coefficient was 11.6.

Specific stimulation of steroid 5 alpha-reductase solubilized from rat liver microsomes by endogenous phosphatidylserine

Dilauroylphosphatidylcholine caused a marked increase in progesterone 5 alpha-reductase activity solubilized from rat liver microsomes, whereas naturally occurring phosphatidylcholines from biological sources as well as dioleoylphosphatidylcholine had not effect on the activity. Therefore, the stimulatory effect of phospholipids normally found in rat liver microsomes was examined. The lipid extracts were prepared from the fraction which was freed from 5 alpha-reductase activity by DEAE-cellulose chromatography, and found to exhibit a strong stimulatory effect. The lipid extracts were then separated into phosphatidylserine, phosphatidylcholine and phosphatidylethanolamine by chromatography on silicic acid column and preparative thin-layer plate. Among these endogenous phospholipids, only phosphatidylserine stimulated the 5 alpha-reductase, suggesting that the lipid requirement is specific for phosphatidylserine in steroid 5 alpha-reductase from liver microsomes.

Differential effects of combinations of phospholipase A2 and phospholipase C on the activity of rat epididymal nuclear and microsomal 4-ene steroid 5 alpha-reductase

Epididymal 4-ene steroid 5 alpha-reductase converts testosterone to 5 alpha-dihydrotestosterone. The enzyme is localized to the nuclear and microsomal fractions, and the activity can be altered by modifying the phospholipids in the membrane environment. To investigate the membrane dependence of 4-ene steroid 5 alpha-reductase, we have treated nuclear and microsomal membranes with combinations of phospholipase A2 and phospholipase C, and examined the effects on 4-ene steroid 5 alpha-reductase activity. Sequential addition of phospholipase A2 and phospholipase C to the nuclear fraction, reduced the 4-ene steroid 5 alpha-reductase activity to approx 25% of the control level. Neither the nature of the phospholipase, nor the sequence of addition altered the inhibition. When both phospholipases were added simultaneously, nuclear 4-ene steroid 5 alpha-reductase activity was inhibited in a linear fashion, and in tests for cooperativity, the effects of phospholipase A2 and phospholipase C were clearly additive. The microsomal enzyme responded differently to sequential phospholipase treatments; if phospholipase A2 was followed by phospholipase C, or phospholipase C followed by phospholipase A2, the 4-ene steroid 5 alpha-reductase activity was, respectively, 13 and 27% of the control. In contrast, sequential addition of the same phospholipase reduced the activity of 4-ene steroid 5 alpha-reductase to approx 40% of the control level. Furthermore, simultaneous addition of phospholipase A2 and phospholipase C to the microsomal fraction, resulted in non-linearity of 4-ene steroid 5 alpha-reductase activity with time, whereas when added individually, linearity of 4-ene steroid 5 alpha-reductase was maintained. Consequently, it was not possible to test for cooperative effects of phospholipases on the microsomal 4-ene steroid 5 alpha-reductase. These findings suggest that for the nuclear 4-ene steroid 5 alpha-reductase, the polar and non-polar regions of the membrane environment have similar functions, which are most likely involved in the maintenance of the structural integrity of the enzyme. For the microsomal enzyme, the polar and non-polar regions of the membrane appear to have different functions, not only for the maintenance of enzyme integrity, but also in the mechanism at the active site.

The mechanism of rat epididymal 4-ene steroid 5 alpha-reductase

Epididymal nuclear 4-ene steroid 5 alpha-reductase catalyses the bisubstrate reaction between testosterone and NADPH to produce 5 alpha-dihydrotestosterone (DHT) and NADP+. Previous studies from this laboratory have demonstrated that the 4-ene steroid 5 alpha-reductase reaction proceeds through the direct transfer of protons from NADPH to testosterone, and that while the product DHT does not affect 4-ene steroid 5 alpha-reductase activity, NADP+ is a potent inhibitor of this enzyme. In the present studies we have investigated the mechanism of 4-ene steroid 5 alpha-reductase with respect to the binding of the substrates, testosterone and NADPH. Kinetic analyses revealed that testosterone does not alter the Kmapp for NADPH, and that NADPH does not alter the Kmapp for testosterone. These findings excluded the possibility that the mechanism of 4-ene steroid 5 alpha-reductase is of the ping-pong variety, and that the sequential addition of both substrates is required before any products are released. The lack of change in Kmapp, observed for either substrate, further suggests that both testosterone and NADPH are able to bind to the free enzyme, negating the possibility that substrate addition occurs in an ordered manner. Indeed the kinetic profiles are entirely consistent with the mechanism of 4-ene steroid 5 alpha-reductase being a rapid equilibrium random sequential process in which the binding of the first substrate has no affect on the binding of the second. Mean values for the dissociation constants, Ktestosterone and KNADPH, were 200 nmol/l and 50 nmol/l, respectively. These findings, coupled with those from earlier studies, suggest that the mechanism of epididymal nuclear 4-ene steroid 5 alpha-reductase is a rapid equilibrium random bireactant process, with the possible dead-end complex: testosterone-4-ene steroid 5 alpha-reductase-NADP+.

Functional characteristics of nuclear 5 alpha-reductase from rat ventral prostate

The subcellular distribution and functional characteristics of 5 alpha-reductase (3-oxo-5 alpha-steroid: NADP+ 4-ene-oxidoreductase, EC 1.3.1.22) from rat ventral prostate were studied and compared to the 5 alpha-reductase from female rat liver. Tissue fractionation retained main enzymic activity in the microsomal fraction of rat liver, while 5 alpha-reductase from rat prostate was localized in the nuclear membrane with a specific activity 160 times that of the initial homogenate. The purity of nuclear envelopes was checked by electron microscopy. Solubilization experiments indicated that the hepatic 5 alpha-reductase is attached to the endoplasmic reticulum as a peripheral protein, while the nuclear prostatic enzyme is an integral membrane protein. Incubation experiments with phospholipases suggested a decisive role of the surrounding phospholipids for the prostatic enzyme activity. To elucidate the characteristics of hydrogen transfer of the enzyme, the effect of flavins and different other cofactors on 5 alpha-reductase activity in isolated prostatic nuclei were studied. Our findings indicate that in rat ventral prostate the conversion of testosterone to 5 alpha-dihydrotestosterone proceeds by a direct hydrogen transfer from NADPH to testosterone. Concerning these parameters the behaviour of hepatic 5 alpha-reductase is absolutely different from the prostatic enzyme. The localization of 5 alpha-reductase within the nuclear envelope of rat ventral prostate as an integral membrane protein seems to be of physiological significance with regard to the action of androgens.

The effects of diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 beta-carboxamide (4-MA) and (4R)-5,10-SECO-19-norpregna-4,5-diene-3,10,20-trione (SECO) on androgen biosynthesis in the rat testis and epididymis

We have investigated the effects of two 4-ene-steroid 5 alpha-reductase inhibitors, diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 beta-carboxamide (4-MA) and (4R)-5,10-seco-19-norpregna-4, 5-diene-3,10,20-trione (SECO), on testicular and epididymal androgen biosynthesis. Kinetic analyses revealed that both compounds inhibited epididymal DHT biosynthesis. 4-MA was a competitive inhibitor of epididymal nuclear and microsomal 4-ene-steroid 5 alpha-reductases (3-oxo-5 alpha-steroid: NADP 4-ene-oxidoreductase EC 1.3.1.22) with Kiapp values of 12.8 and 15.1 nmol/l compared to the respective Kmapp values of 185 and 240 nmol/l. Values for the Vmaxapp were always within 70-130% of the control. SECO at 1.0 mumol/l, also inhibited epididymal nuclear and microsomal 4-ene-steroid-5 alpha-reductases, causing respectively 2.9 and 5.2-fold increases in Kmapp. The Vmaxapp values were unchanged. However, SECO concentrations of 5 and 25 mumol/l abolished 4-ene-steroid 5 alpha-reductase activity at all testosterone concentrations. To examine the specificity of these compounds, we investigated their effects on the enzymes that convert pregnenolone to testosterone. Rat testis microsomes converted pregnenolone to testosterone via the 4-ene-3-oxo pathway, with the major metabolites being progesterone, 17-hydroxyprogesterone, 4-androstenedione and testosterone; some 17-hydroxypregnenolone was also formed. Very small amounts of dehydroepiandrosterone (DHA) and 5-androstenediol were detected. SECO, at a concentration that completely inhibited epididymal 4-ene-steroid 5 alpha-reductase activity, did not alter the metabolic profile of pregnenolone metabolism. However, 4-MA prevented the appearance of 4-ene steroids, and large quantities of 17-hydroxypregnenolone and DHA accumulated, suggesting that inhibition of the 3 beta-hydroxysteroid: NAD(P)+ oxidoreductase (EC 1.1.1.51) and 3-oxosteroid 5-ene-4-ene-isomerase (EC 5.3.3.1) [3 beta-hydroxysteroid dehydrogenase-isomerase] was occurring. Optimal conditions for the microsomal conversion of DHA to 4-androstenedione were determined; kinetic analyses of the 3 beta-hydroxysteroid dehydrogenase-isomerase activity revealed that 4-MA inhibited this reaction non-competitively, reducing Vmaxapp values to 25% of the control. The Kiapp determined from the intercept replot, was 121 nmol/l, and the Kmapp was always between 90 and 130% of the control value. It is concluded that SECO is more specific than 4-MA in its effects on androgen biosynthesis in the testis and epididymis and that both these drugs should provide useful tools in assessments of the relative contributions of 5 alpha-reduced androgens to androgen dependent processes.