Enzymes In Male Rats Research Articles

Age-Related Changes in Hepatic Activity and Expression of Detoxification Enzymes in Male Rats

Process of aging is accompanied by changes in the biotransformation of xenobiotics and impairment of normal cellular functions by free radicals. Therefore, this study was designed to determine age-related differences in the activities and/or expressions of selected drug-metabolizing and antioxidant enzymes in young and old rats. Specific activities of 8 drug-metabolizing enzymes and 4 antioxidant enzymes were assessed in hepatic subcellular fractions of 6-week-old and 21-month-old male Wistar rats. Protein expressions of carbonyl reductase 1 (CBR1) and glutathione S-transferase (GST) were determined using immunoblotting. Remarkable age-related decrease in specific activities of CYP2B, CYP3A, and UDP-glucuronosyl transferase was observed, whereas no changes in activities of CYP1A2, flavine monooxygenase, aldo-keto reductase 1C, and antioxidant enzymes with advancing age were found. On the other hand, specific activity of CBR1 and GST was 2.4 folds and 5.6 folds higher in the senescent rats compared with the young ones, respectively. Interindividual variability in CBR1 activity increased significantly with rising age. We suppose that elevated activities of GST and CBR1 may protect senescent rats against xenobiotic as well as eobiotic electrophiles and reactive carbonyls, but they may alter metabolism of drugs, which are CBR1 and especially GSTs substrates.

Antioxidative potential of Quercetin against hydrogen peroxide induced oxidative stress in spermatozoa in vitro

This study was carried out to test the antioxidant effects of Quercetin on sperm parameters and antioxidant enzymes in male rats assessed in vitro after H(2) 0(2) -mediated sperm oxidative damage. Spermatozoa were incubated with Quercetin (10, 100 and 200 μm), H(2) O(2) alone (100 μm) and Quercetin (100, 200 μm) + H(2) O(2) (100 μm) repectively, for 3 h at 32 °C. After that, sperm parameters (motility, viability and abnormal morphology), malondialdehyde, superoxide dismutase, catalase and glutathione peroxidase levels were determined. We found that exposure to H(2) O(2) let to significant increase in lipid peroxidation (LP) and abnormal morphology associated with significant decrease in sperm motility, viability and antioxidant enzymes activities. When Quercetin was added in culture medium, it improved activities of antioxidant enzymes and protected spermatozoa against the deleterious effect of H(2) O(2) on sperm parameters and LP. This study demonstrated that supplementation with Quercetin could protect spermatozoa against H(2) O(2) -mediated sperm damage.

Effects of pregnenolone-16α-carbonitrile on vitamin E status and protein levels of antioxidant enzymes in male rats fed a vitamin E-supplemented diet

The aim of this study was to evaluate the effects of the administration of pregnenolone-16α-carbonitrile (PCN), an inducer of the cytochrome P450 3A gene in rats, on vitamin E status and antioxidant enzyme protein levels in rats fed a vitamin E-supplemented diet. Two groups of male Wistar rats were fed for 3 weeks with a basal diet containing 50 ppm of α-tocopherol or the same diet containing 10 times more α-tocopherol. In the final 3 days, each group was divided into two subgroups which were given a single daily intraperitoneal injection of PCN at 75 mg/kg (groups PCN and PCN+VE) or DMSO (groups DS and DS+VE). PCN treatment alone significantly reduced the α-tocopherol content of the liver and plasma and this effect was prevented by supplementation with 10-fold more α-tocopherol. α-Tocopherol levels in the kidneys, lung, heart, and testes were significantly higher in both vitamin E-supplemented groups than in the control groups. TBARS levels in the liver and lung were significantly increased in both PCN-treated groups, as shown by two-way ANOVA analysis. PCN also caused a significant reduction in protein levels of catalase and glutathione peroxidase (GPx) in both groups. Dietary vitamin E supplementation caused a decrease in liver protein levels of GPx and superoxide dismutase, but not catalase, in both groups and protected against PCN-induced lipid peroxidation, which was caused by CYP3A induction and a reduction in antioxidant enzyme levels.

Down-regulation of hepatic cytochrome P450 enzymes associated with cisplatin-induced acute renal failure in male rats

Hepatic drug metabolism is impaired in experimental animals and humans with renal diseases. An anticancer drug, cisplatin induces acute renal failure (ARF) in rats. Under the same experimental conditions, cisplatin causes down-regulation of hepatic cytochrome P450 (P450) enzymes in an isozyme selective manner. The present study examined the pathological role of ARF in the down-regulation of hepatic P450 enzymes in the cisplatin-treated rats. Male rats with single dose of intraperitoneally cisplatin (5 mg/kg) caused marked changes in renal parameters, BUN and serum creatinine but not hepatic parameters, serum alanine aminotransferase or aspartate aminotransferase. The rats also suffered from down-regulation of hepatic microsomal CYP2C11 and CYP3A2, male specific P450 isozymes, but not CYP1A2, CYP2E1, or CYP2D2. The decrease in serum testosterone level was also observed in injured rats, which was consistent with the selective effects on male specific P450 enzymes. Protection of rats against cisplatin-induced ARF by dimethylthiourea, a hydroxyl radical scavenger, also protected rats against the decrease in serum testosterone levels and the down-regulation of CYP2C11 and CYP3A2. Carboplatin, an analogue to cisplatin but no ARF inducer, did not cause decrease in serum testosterone levels and down-regulation of hepatic male specific P450 enzymes. These results suggest that down-regulation of hepatic P450 enzymes in male rats given cisplatin is closely related to the cisplatin-induced ARF and the resultant impairment of testis function.

Absence of P450 effect with a novel phospholipid emulsion (GR270773)

GR270773 (773) is a novel phospholipid emulsion with endotoxin binding activity. Alterations in dietary lipids have been shown to alter P450 activity. We therefore examined 773′s effects on protein content and activity of selected hepatic P450 enzymes in male rats. 773 was given intravenously by continuous infusion to three male rats/group at doses of 0, 800, and 1600 mg/kg/day for 3 days. Two hours after the infusion, liver samples were removed, frozen, and stored at -60C. Microsomal fractions were prepared for each animal by differential centrifugation. Protein yield, total P450 content, and activities of 7-ethoxyresorufin O-deethylase (CYP1A), testosterone 6β- and 16β-hydroxylase (CYP3A and CYP2B, respectively), and lauric acid 11- and 12-hydroxylase (CYP2E and CYP4A, respectively) were examined. Treatment with 773 at doses of 800 and 1600 mg/kg/day for 3 days caused no notable changes in the mean hepatic microsomal protein yields (p>0.8) and total cytochrome P450 content (p>0.3). Treatment of male rats with 773 caused no notable change in the mean activity of CYP1A, CYP3A or CYP4A (p>0.4). Treatment with 773 caused a slight non-significant (p>0.2) increase in the mean activities of CYP2B and CYP2E in the 1600-mg/kg/day dose group (1.6 and 1.4-times control, respectively). Intravenous infusion of 773 at 800 and 1600 mg/kg/day for 3 days to male rats had no notable effect on hepatic microsomal protein, total cytochrome P450 levels or the activities of 5 P450 subfamilies. Clinical Pharmacology & Therapeutics (2004) 75, P87–P87; doi: 10.1016/j.clpt.2003.11.333

Effect of food restriction on UDP-glucuronosyltransferase (UGT) enzymes in liver

Purpose The influence of diet and nutritional status on drug metabolism has been widely documented. Hepatic drug metabolism enzymes can be profoundly altered in patients suffering from malnutrition and starvation. In this research we have studied the effect of food restriction on UDP-glucuronosyltransferase (UGT) enzymes in rat liver. Methods Male Sprague-Dawley rats were maintained on a powdered commercial diet. Rats in control group (C, n=4) had ad libitum access to food and water. Animals in food restriction group (FRG, n=5) were fed with 60% of food intake of the control group. After 5 weeks, the rats were sacrificed and liver microsomes were prepared. Protein content was assessed by Western blot using polyclonal antibody for rat UGT1A1, human UGT1A6, and rat UGT1A7. Results The body weight of the FRG rats was reduced by 35% compared to the C rats. Hepatic and renal organ function was normal between the two groups. Western blot results revealed that antibody against human UGT1A6 cross-reacted with rat UGT1A6. The polyclonal antibodies were found to be specific for the UGTs. Food restriction did not have any influence on protein content of hepatic UGT1A1 and UGT1A7. However, the UGT1A6 protein levels were significantly lower (p<0.05) in the FRG rats (~3-fold reduction) compared to control animals. Conclusion Food restriction appears to selectively downregulate hepatic UGT enzymes in male rats. Clinical Pharmacology & Therapeutics (2004) 75, P85–P85; doi: 10.1016/j.clpt.2003.11.326

Effect of prenatal exposure to hydroxyprogesterone on steroidogenic enzymes in male rats.

Pregnant Wistar strain albino rats were administered hydroxyprogesterone and the steroidogenic potential of the testis was analysed in the next generation adult male rats. In utero exposure to supranormal levels (10 mg/kg body weight or 25 mg/kg body weight) of hydroxyprogesterone decreased the activity levels of marker steroidogenic enzymes (3beta-hydroxysteroid dehydrogenase and 17beta-hydroxysteroid dehydrogenase) of adult (mature) rat testis. Hydroxyprogesterone mediation in the suppression of rat testicular steroidogenesis is suggested, which can be correlated to the observed decrease in male reproductive potential in mammals exposed to female hormones during embryonic development.

Effects of conjugated linoleic acid isomers on lipid-metabolizing enzymes in male rats.

Male weanling Wistar rats (n = 15), weighing 200-220 g, were allocated for 6 wk to diets containing 1% (by weight) of conjugated linoleic acid (CLA), either as the 9c,11 t-isomer, the 10t,12c-isomer, or as a mixture containing 45% of each of these isomers. The five rats of the control group received 1% of oleic acid instead. Selected enzyme activities were determined in different tissues after cellular subfractionation. None of the CLA-diet induced a hepatic peroxisome-proliferation response, as evidenced by a lack of change in the activity of some characteristic enzymes [i.e., acyl-CoA oxidase, CYP4A1, but also carnitine palmitoyltransferase-I (CPT-I)] or enzyme affected by peroxisome-proliferators (glutathione S-transferase). In addition to the liver, the activity of the rate-limiting beta-oxidation enzyme in mitochondria, CPT-I, did not change either in skeletal muscle or in heart. Conversely, its activity increased more than 30% in the control value in epididymal adipose tissue of the animals fed the CLA-diets containing the 10t,12c-isomer. Conversely, the activity of phosphatidate phosphohydrolase, a rate-limiting enzyme in glycerolipid neosynthesis, remained unchanged in adipose tissue. Kinetic studies conducted on hepatic CPT-I and peroxisomal acyl-CoA oxidase with CoA derivatives predicted a different channeling of CLA isomers through the mitochondrial or the peroxisomal oxidation pathways. In conclusion, the 10t,12c-CLA isomer seems to be more efficiently utilized by the cells than its 9c,11t homolog, though the Wistar rat species appeared to be poorly responsive to CLA diets for the effects measured.

Tissue distribution, toxicokinetics and induction of hepatic drug metabolizing enzymes in male rats after a single S.C. dose of 3,4,3′,4′-tetrachlorobiphenyl (PCB-77)

The tissue distribution and toxicokinetics of PCB-77 after a single s.c. dose of 6 mg/kg body weight was investigated in male adult rats as well as the concurrent induction of hepatic drug-metabolizing enzymes. During five days after treatment, the PCB-77 concentrations on fat weight basis in testis and in whole blood were found to be about in the same range as the concentrations in adipose tissue, whereas in liver fat PCB-77 accumulated. EROD and MROD activities were induced by about 8 and 4 times, respectively, during a period of two weeks after treatment. Concentrations in adipose tissue and in liver as well as the hepatic enzyme activities rapidly declined during the investigation period of eight weeks. An elimination half-life for PCB-77 of approximately 7–9 days was estimated.

Effect of fluvastatin, an inhibitor of 3-hydroxy-3-methyl glutaryl CoA reductase, on drug-metabolizing enzymes in rats

Fluvastatin (FV), a new cholesterol-lowering agent, has been studied for its effects on hepatic microsomal drug-metabolizing enzymes in male rats. FV was orally administered in dosages of 1, 5, and 30 mg/kg/day for 7 consecutive days. 3-Hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activities were markedly decreased at all dose levels. The amount of microsomal protein and the contents of cytochromes P450 and b5 did not change. No induction of aniline hydroxylase, aminopyrine N-demethylase, testosterone hydroxylases (15 alpha-, 7 alpha-, 6 beta-, 16 alpha-, and 16 beta-), and UDP-glucuronosyltransferase were found. On the other hand, 7-ethoxycoumarin o-deethylase activity was slightly increased and lauric acid omega-1-hydroxylase activity tended to be decreased after treatment with FV.

Biocompatibility studies with perfluorochemical oxygen carriers.

The effects of injecting either a novel perfluorodecalin (FDC)-based emulsion or various perfluorochemical (PFC) oils on liver cytochromes P-450 (P-450) and aryl esterase (LAE) enzymes in male rats have been studied. Mean P-450 concentration increased (P less than 0.05) following injection of the novel emulsion or FDC. In contrast, LAE increased in response to injection of FDC (P less than 0.05) and the C-16 oil, perfluoroperhydrofluoranthrene (P less than 0.01), added to stabilize the emulsion. No corresponding changes in either P-450 or LAE occurred in rats injected with either perfluorotripropylamine, perfluorotributylamine or perfluorooctylbromide. The "sleeping time" of animals injected with the pentobarbital-based anaesthetic, Equithesin, following pretreatment with the novel emulsion was lower (P less than 0.05) than in saline-injected controls. The results are discussed in relation to the effects of PFCs on liver metabolic functions.

Effects of tetrachlorvinphos on hepatic microsomal enzymes in the rat.

The inducement of the hepatic microsomal enzymes in male rats following oral administration of tetrachlorvinphas (TCVP) for 7 days with the dosage of 150 mg/kg and the mechanism of its inducing effect were studied. It was confirmed that TCVP could enhance significantly the activity of hepatic microsomal enzymes, such as aniline hydroxylase, P-nitroanisole O-demethylase and uridine diphosphoglucuronyltransferase (UDP-GAT). Besides, it could also increase contents of microsomal cytochrome P-450 and microsomal protein. In rats it shortened pentobarbital sleeping time. These results demonstrated that repeated administration of TCVP might induce the activity of hepatic microsomal enzymes in rats. The inducing effect of TCVP could be blocked by DL-ethionine, an inhibitor of aminoacids. Furthermore, TCVP failed to increase enzymic activity in vitro. These findings suggested that the inducing effect of TCVP on the activity of hepatic microsomal enzymes could be explained by accelerating synthesis of microsomal protein.

Effect of gossypol on liver metabolic enzymes in male rats

The effects of gossypol on liver metabolism were examined in male rats. Gossypol acetic acid was administered to Sprague-Dawley rats intraperitoneally (i.p.) at 5 mg/kg daily, 5 days/week for 2 weeks. The rats were killed 24 h after the last injection. The liver/body weight ratio (−42%), concentration of liver glutathione (−34%), activities of liver α-naphthtylacetate esterase (−30%) and DNase (−39%) were significantly decreased when compared to controls. Hepatic β-glucuronidase (+37%), RNase (+35%) and serum alkaline RNase (+23%) activities were significantly increased. No changes were found in serum transaminases (SGPT, SCOT) or in hepatic RNA and DNA concentration. Elevation of liver and serum RNase activities suggest that gossypol treatment produces some catabolic effects. The depletion of hepatic glutathione and the elevation of β-glucuronidase activity indicate that gossypol is hepatotoxic when given at this dose for 2 weeks.

TRIGLYCERIDE LOWERING EFFECT AND INDUCTION OF LIVER ENZYMES IN MALE RATS AFTER ADMINISTRATION OF HYPOLIPIDEMIC DRUGS

Annals of the New York Academy of SciencesVolume 386, Issue 1 p. 449-452 TRIGLYCERIDE LOWERING EFFECT AND INDUCTION OF LIVER ENZYMES IN MALE RATS AFTER ADMINISTRATION OF HYPOLIPIDEMIC DRUGS Karlheinz Stegmeier, Corresponding Author Karlheinz Stegmeier Research Laboratories Boehringer Mannheim GmbH Mannheim, Federal Republic of Germany*Address correspondence to: Dr. K. Stegmeier, Boehringer Mannheim GmbH, Sandhofer Strasse 115, D-6800 Mannheim 31, Federal Republic of Germany.Search for more papers by this authorFelix H. Schmidt, Felix H. Schmidt Research Laboratories Boehringer Mannheim GmbH Mannheim, Federal Republic of GermanySearch for more papers by this authorAndreas Reinicke, Andreas Reinicke Department of Anatomy University of Heidelberg Heidelberg, Federal Republic of GermanySearch for more papers by this authorH. Dariush Fahimi, H. Dariush Fahimi Department of Anatomy University of Heidelberg Heidelberg, Federal Republic of GermanySearch for more papers by this author Karlheinz Stegmeier, Corresponding Author Karlheinz Stegmeier Research Laboratories Boehringer Mannheim GmbH Mannheim, Federal Republic of Germany*Address correspondence to: Dr. K. Stegmeier, Boehringer Mannheim GmbH, Sandhofer Strasse 115, D-6800 Mannheim 31, Federal Republic of Germany.Search for more papers by this authorFelix H. Schmidt, Felix H. Schmidt Research Laboratories Boehringer Mannheim GmbH Mannheim, Federal Republic of GermanySearch for more papers by this authorAndreas Reinicke, Andreas Reinicke Department of Anatomy University of Heidelberg Heidelberg, Federal Republic of GermanySearch for more papers by this authorH. Dariush Fahimi, H. Dariush Fahimi Department of Anatomy University of Heidelberg Heidelberg, Federal Republic of GermanySearch for more papers by this author First published: May 1982 https://doi.org/10.1111/j.1749-6632.1982.tb21449.xCitations: 13AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume386, Issue1Peroxisomes and GlyoxysomesMay 1982Pages 449-452 RelatedInformation

Effect of pyrrolizidine alkaloids from tansy ragwort ( Senecio jacobaea) on hepatic drug-metabolizing enzymes in male rats

Ad lib. consumption of diets containing 5% tansy ragwort ( Senecio jacobaea) for 1–4 weeks produced a 5- to 6-fold increase in hepatic microsomal epoxide hydrase and significant increases in cytosolic glutathione S-transferase activities in male Long-Evans rats. An enhancement of these enzyme activities was also observed when a diet containing 1% tansy ragwort was fed for a period of 3 weeks. Feeding a diet containing 0.5% pyrrolizidine alkaloid (PA) mixture extracted from tansy ragwort for 1 week produced a 5-fold increase in hepatic epoxide hydrase and a 73 per cent increase in glutathione S-transferase activities. In contrast, hepatic microsomal aryl hydrocarbon hydroxylase activity (AHH) was reduced significantly by feeding diets containing 5% tansy ragwort or a 0.5% alkaloid mixture. Hepatic microsomal cytochrome P-450 content was lowered following consumption of the 0.5% alkaloid mixture but not by feeding a 5% tansy ragwort diet, the difference presumably being a result of the lowered PA intake by the latter animals. Exposure to the pyrrolizidine alkaloids, therefore, may influence significantly the capacity of animals to metabolize endogenous or foreign compounds and possibly also affect the subsequent biotransformation and toxicity of these plant constituents.

Circadian rhythm of hepatic steroid metabolizing enzyme activities in the rat

The activity of various rat-liver microsomal steroid metabolizing enzymes was shown to follow a circadian rhythm. NADPH: 4-ene-3-oxosteroid-5α-reductase in female rats exhibited a maximum at 18.00 h and a minimum at 6.00 h. The same enzyme in male rats showed a minimum at 18.00h (substrate testosterone) and 12.00 h (substrate corticosterone) and a maximum at 24.00 h. Variations in testosterone hydroxylation and NADPH-cytochrome c reductase activity were only detectable in male rats (maximum at 24.00 h and minimum at 12.00 h). NADH: 4-ene-3-oxosteroid-5α-reductase activity in male and female rats was highest at 6.00 h and lowest at 18.00 h.

Drug Metabolism under Pathological and Abnormal Physiological States in Animals and Man

The activity of microsomal drug-metabolizing enzymes is altered by several pathological or abnormal physiological states, such as changes in nutritional status, liver, heart or kidney diseases, hormonal disturbances, pregnancy, tumour-bearing state, adjuvant arthritis, changes in reticuloendothelial system and environmental factors (stress, irradiation, heavy metals). The activities of other metabolic pathways, such as glucuronidation, sulphate conjugation, acetylation and alcohol oxidation are generally affected to lesser extents. Rats are most commonly used in drug metabolism studies, and it is important to know that the activity of most of the microsomal drug-metabolizing enzymes is higher in males than in females through androgen action which is readily impaire drug-metabolizing enzymes in male rats are thus manifested by two mechanisms; one is by impairment of androgen action and the other is by depression of the basic enzymic activity. Therefore, those effects of pathological states, observed only in male rats but not in females, are generally not seen in other species of animals, including man. The effects of starvation, hyperthyroidism, adrenal insufficiency, diabetes and morphine administration are cases where changes in metabolism are due solely to impairment of androgen action. In other pathological cases, those drug-metabolizing enzymes showing sex differences are depressed more markedly in male rats than those showing no clear sex difference. The author therefore recommends the use of female rats in the evaluation of the effects of pathological states on hepatic microsomal drug-metabolizing enzymes. Generally, changes in activity of the hepatic enzymes reflect closely the changes in the rates of drug metabolism in vivo. However, the protein-binding of drugs, hepatic blood flow and renal function are also known to affect the rate of drug metabolism and excretion in vivo, and therefore changes of these factors in pathological states should also be taken into consideration.

Effects of ethanol on hepatic microsomal drug-metabolizing enzymes in the rat

The activities of several hepatic microsomal drug-metabolizing enzymes were determined in male and female rats after administration of 20% ethanol or an isocaloric amount of glucose in drinking water for 10–49 days. The aniline hydroxylase activity increased, whereas the activities of both pentobarbital hydroxylase and benzphetamine N-demethylase were decreased in male rats given ethanol and killed without ethanol withdrawal. Twenty-four hr after removal of ethanol, the aniline hydroxylase remained elevated but a striking increase of both pentobarbital hydroxylase and benzphetamine N-demethylase above control values occurred. Six days later, all three of these microsomal enzymes returned to normal control values. The reduction in pentobarbital hydroxylase and benzphetamine N-demethylase could not be attributed simply to high endogenous ethanol levels since: (1) addition of high concentrations of ethanol in vitro inhibited microsomal aniline hydroxylase and pentobarbital hydroxylase but did not reduce benzphetamine N-demethylase; and (2) acute administration of ethanol by gastric tube, which markedly elevated the blood ethanol level, did not result in a decline in the enzyme activities. These two findings suggest that the observed decrease in microsomal enzymes in male rats required persistent ethanol exposure. In contrast to male rats, female rats given ethanol for 28 days showed a significant increase in aniline hydroxylase activity, but the activities of pentobarbital hydroxylase and benzphetamine N-demethylase were not decreased. Moreover, administration of ethanol for 28 days to female rats did not reduce the response of these three enzyme activities to pentobarbital administration. It is concluded that the effects of chronic ethanol ingestion on the hepatic microsomal drug-metabolizing enzymes are complex and depend on sex, exposure to other agents and, most importantly, on the duration and proximity of ethanol intake.

Neonatal Imprinting of Liver Microsomal Hydroxylation and Reduction of Steroids

Abstract The metabolism of 4-[4-14C]androstene-3,17-dione, 4-[4-14C]pregnene-3,20-dione, 5α-[4-14C]androstane-3α,17β-diol, 4,16-[7α-3H]androstadien-3-one, [4-14C]cholesterol, and 7α-hydroxy-4-[6β-3H]cholesten-3-one was studied in the microsomal fraction of livers from adult male and female rats which had been castrated neonatally or postpubertally. The effect of testosterone propionate treatment on postpubertally castrated male and female rats was also studied with respect to the hepatic microsomal metabolism of the mentioned steroid substrates. It was found that the different microsomal enzyme activities could be grouped into three categories according to their specific characteristics. One category of enzymes (represented by the 2β-hydroxylase active on 5α-androstane-3α,17β-diol, the 6β-hydroxylase active on 4-androstene-3,17-dione and 4-pregnene-3,20-dione, and the 18-hydroxylase active on 5α-androstane-3α, 17β-diol) was more active in male than in female rats. This sexual difference was completely abolished both by postpubertal and neonatal testectomy. The activities of the enzymes in this group were stimulated by treatment with testosterone propionate. A second category of enzymes (the 2α-hydroxylase active on 4-pregnene-3,20-dione and 5α-androstane-3α,17β-diol, the 16α-hydroxylase active on 4-androstene-3,17-dione, the Δ16-C19 steroid epoxidase active on 4,16-androstadien-3-one, and the 17β-hydroxysteroid reductase active on 4-androstene-3,17-dione) was generally much more active in male than in female rats. Only neonatal testectomy completely abolished this sexual difference, whereas postpubertal testectomy reduced but did not abolish the differences in enzyme activities between male and female rats. Testosterone propionate treatment led to increased activities of these enzymes. The third category of enzymes (the 7α-hydroxylase active on 4-androstene-3,17-dione and cholesterol, the 12α-hydroxylase active on 7α-hydroxy-4-cholesten-3-one, and the 16α-hydroxylase active on 4-pregnene-3,20-dione) was generally slightly more active in female than in male rats. The activities of the enzymes in this group were not significantly affected by neonatal or postpubertal gonadectomy or by treatment with testosterone propionate. Based on these findings the microsomal steroid-metabolizing enzyme activities may be grouped into three classes with regard to the mechanisms regulating their activity: (a) enzymes with a basal activity level regulated by nongonadal factors but reversibly inducible by androgens; (b) enzymes irreversibly imprinted or programmed by androgens during the prepubertal period and reversibly stimulated by androgens postpubertally; and (c) enzymes primarily regulated by nongonadal factors and only slightly affected by androgens. The 5α-reductase active on 4-androstene-3,17-dione, 4-pregnene-3,20-dione and 7α-hydroxy-4-cholesten-3-one is basally regulated by nongonadal factors in both male and female rats. In addition, the activity of this enzyme in male rats, which is about 8 to 10 times lower than that in female rats, is suppressed by testicular androgens by two mechanisms: (a) irreversibly by prepubertal imprinting or programming; and (b) reversibly by postpubertal suppression.