Effect of genetic obesity and experimental diabetes on hepatic microsomal mixed function oxidase activities.

In male rats, genetic obesity and experimental diabetes are associated with altered activities of several of the hepatic microsomal P-450 isozymes concerned with steroid and xenobiotic oxidation. The present study examined the roles of insulin and ketonaemia in effecting these changes. In obese male Zucker rats, androstenedione 6 beta-, 16 alpha- and 16 beta-hydroxylase activities (mediated by P450PCN-E, P-450UT-A and P450PB-B, respectively) were significantly decreased to 21%, 20% and 43% of lean control. Obesity was also associated with a significant decrease in the activities of N-nitrosodimethylamine demethylase (P-450j) and aniline p-hydroxylase to about 70%. A similar decrease in total microsomal P-450 was also observed. Androstenedione 7 alpha-hydroxylase activity (mediated by P-450UT-F) was unchanged in these animals. In streptozotocin-induced diabetic male Wistar rats, androstenedione 7 alpha- and 16 beta-hydroxylase activities were significantly elevated to 230% and 270% of control, respectively. Significant increases in the rates of N-nitrosodimethylamine demethylase and aniline p-hydroxylase were also noted in diabetic rat liver. In contrast, the activity of P-450UT-A was reduced to 30% of control and P-450PCN-E-specific 6 beta-hydroxylation was unchanged. Control of the diabetic state with insulin treatment reversed all the changes in P-450-mediated activities. Significant correlations were found between serum concentrations of insulin and catalytic activities of P-450PB-B (rho = -0.46), P-450UT-F (rho = -0.65) and P-450j (rho = -0.71). Positive correlations of the same magnitude were also found between these mixed function oxidase activities and beta-hydroxybutyrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of antithrombitic and antimycotic N-substituted imidazoles on rat hepatic microsomal steroid and xenobiotic hydroxylases in vitro.

N-Substituted imidazoles have been shown to be potent inhibitors of microsomal mixed-function oxidase activities in vitro and in vivo. In the present study the effects of two antithrombitic (dazmegrel and dazoxiben) and four antimycotic (ketoconazole, econazole, miconazole and clotrimazole) imidazoles on microsomal cytochrome P-450-mediated steroid and xenobiotic hydroxylases were studied in vitro. Despite the presence of the N-substituted imidazole moiety, the antithrombitic agents were essentially non-potent as inhibitors of all of the oxidase activities evaluated. In contrast, the antimycotic drugs were potent inhibitory compounds. Binding studies revealed that all six imidazoles elicited type II optical difference spectra and exhibited relatively high affinity for ferricytochrome P-450 in microsomal suspensions (Ks range 0.26-0.73 microM for the antimycotic agents and 6.5 microM and 21 microM for dazmegrel and dazoxiben, respectively). The structural feature that the antithrombitic compounds share is a carboxylate function so that, at physiological pH, less than 1% of the drug would be present in the unionised form. This functionality is absent from the structures of the antimycotic agents which possess much greater hydrophobic character. Even though the antithrombitic imidazoles elicit type II binding interactions of quite high affinity it would appear from this study that significant inhibition potency does not necessarily follow. The present findings also suggest that interesting differences exist between the active site binding regions in the cytochrome P-450 that catalyse thromboxane synthetase activity and those involved in microsomal drug oxidation. Inhibitor hydrophobicity is clearly an important factor in the inhibition of microsomal cytochromes P-450 whereas effective thromboxane synthetase inhibitors may be quite hydrophilic at physiological pH.

Impaired androgen 16 alpha-hydroxylation in hepatic microsomes from carbon tetrachloride-cirrhotic male rats.

Hepatic cirrhosis produced by repeated inhalation of carbon tetrachloride is associated with reduced levels of microsomal cytochrome P450. In this study the C19-steroids androstenedione and testosterone were used as specific probes of the functional activity of several forms of cytochrome P450 in microsomal fractions from control and cirrhotic rat liver. The principal finding, that androstenedione 16 alpha-hydroxylation and testosterone 2 alpha-, 16 alpha-, and 17 alpha-hydroxylation were reduced to 14%-38% of control activity, strongly suggests that levels of the male sexually differentiated cytochrome P450 (P(450)16 alpha) are decreased in hepatic cirrhosis. The activity of other cytochrome P450-mediated C19-steroid hydroxylases, with the exception of androstenedione 6 beta-hydroxylase, appeared essentially unaltered in microsomes from cirrhotic rats. Cirrhosis induced by carbon tetrachloride was also associated with greatly decreased activity of the microsomal cytochrome P450-independent 17 beta-oxidoreductase, an enzyme that catalyzes the conversion of androstenedione to testosterone. Consequently, and in view of the impaired activity of cytochrome P450-mediated testosterone 17 alpha-hydroxylation, the capacity of cirrhotic microsomes to catalyze the interconversion of androstenedione and testosterone was much lower than that of control microsomes. The present data confirm and extend earlier observations that selective impairment of drug oxidation pathways occurs in hepatic cirrhosis. These changes are unrelated to the acute toxicity produced by carbon tetrachloride exposure. The available evidence supports the assertion that specific forms of cytochrome P450 are subject to altered regulation in cirrhosis.

Altered regulation of cytochrome P-450 enzymes in choline-deficient cirrhotic male rat liver: impaired regulation and activity of the male-specific androst-4-ene-3,17-dione 16 alpha-hydroxylase, cytochrome P-450UT-A, in hepatic cirrhosis.

Total microsomal cytochrome P-450 levels were decreased, to about 50% of control, in liver of male rats made cirrhotic by the prolonged intake of a choline-deficient diet. We have suggested previously that this decrease in cytochrome P-450 levels is not a generalized one, but is selective for certain forms of the enzyme. In the present study, levels of six cytochrome P-450 forms including the sex-specific cytochrome P-450 forms, P-450UT-A, P-450PCN-E, and P-450UT-l, were quantitated immunochemically in hepatic microsomes prepared from control and cirrhotic male rats and were related to changes in the regioselectivity of cytochrome P-450-mediated androst-4-ene-3,17-dione hydroxylation in these fractions. The principal finding of this study was that the male-specific androst-4-ene-3,17-dione 16 alpha-hydroxylase was decreased in cirrhotic microsomes to about 20% of control. The content of P-450UT-A decreased concurrently from about 0.40 to less than 0.01 nmol/mg of microsomal protein. Other pathways of androst-4-ene-3,17-dione hydroxylation were also affected, but to different extents than the 16 alpha-hydroxylase. 6 beta-Hydroxylation decreased in cirrhotic microsomes to about 45% of control, despite a marked decrease in P-450PCN-E from 0.27 to less than 0.002 nmol/mg of microsomal protein. The rate of androst-4-ene-3,17-dione 7 alpha-hydroxylation underwent a less pronounced reduction in cirrhosis to about two-thirds of control microsomal activity, and levels of the cytochrome P-450 associated with this activity, P-450UT-F, were decreased in proportion with the decrease in total microsomal cytochrome P-450. 16 beta-Hydroxylase activity was unaffected by the cirrhotogenic process. From spectral binding studies it was apparent that androst-4-ene-3,17-dione elicited a high affinity type I interaction in control microsomal fractions (Ks = 4.5 microM), whereas no interaction was apparent in cirrhotic liver microsomes. Levels of three other forms of cytochrome P-450--P-450PB-C (a constitutive form inducible by phenobarbital), P-450ISF-G (a major isosafrole-inducible form), and P-450UT-I (the major female sexually-differentiated isozyme)--were apparently unaltered in cirrhosis. These findings are consistent with the assertion that specific forms of cytochrome P-450 are subject to altered regulation in hepatic cirrhosis.

Selective reactivation of steroid hydroxylases following dissociation of the isosafrole metabolite complex with rat hepatic cytochrome P-450.

In order to elucidate the isozyme specificity of complex formation between cytochrome P-450 and the isosafrole metabolite the effect of complex dissociation on different steroid hydroxylation pathways was studied in hepatic microsomal fractions. Isosafrole induction was found to increase the 16 beta- and 7 alpha-hydroxylation of androst-4-ene-3,17-dione approximately 2.8- and 1.7-fold, respectively, whereas the 16 alpha-hydroxylation pathway was decreased to about one-quarter of control activity; 6 beta-hydroxylation was unchanged from control activity. More striking changes were apparent following dissociation of the isosafrole metabolite from its complex with ferricytochrome P-450 by the steroid substrate. Thus an approximate fourfold elevation of 16 beta-hydroxylase activity was observed after displacement and 6 beta-hydroxylation increased about twofold; 7 alpha-hydroxylase activity was decreased to 0.75-fold of undisplaced activity and 16 alpha-hydroxylase activity was unchanged. These data provide convincing evidence that at least two forms of phenobarbital-inducible cytochrome P-450 (cytochromes P-450PB-B and P-450PB/PCN-E) are present to some extent in a catalytically inactive complexed state in isosafrole-induced rat hepatic microsomes. Furthermore, there is now evidence to suggest that the constitutive isozymes cytochrome P-450UT-A and cytochrome P-450UT-F are not complexed to any degree in hepatic microsomes from isosafrole-induced rats.

Drug metabolism in cirrhosis. Selective changes in cytochrome P-450 isozymes in the choline-deficient rat model.

The effect of a choline-deficient diet on microsomal cytochrome P-450 and mixed-function oxidase (MFO) activity was investigated in relation to the development of nutritional cirrhosis. In rats that received the choline-deficient diet for 28 weeks cirrhosis was evident macroscopically and histologically; control rats that received an identical diet supplemented with choline had normal livers. Microsomal cytochrome P-450 and cytochrome b5 were reduced in cirrhotic liver to 50% of control levels. Three MFO activities (ethylmorphine N-demethylase, aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase) were also reduced to 40-70% of control levels. However, the turnover number for the O-deethylation of 7-ethoxycoumarin was not reduced in cirrhotic liver. This finding suggested that certain drug oxidations may be selectively depressed in nutritional cirrhosis. To examine the possibility that selective changes in MFO activity may reflect the suppression of certain cytochrome P-450 isozymes, partially purified fractions of the cytochrome were prepared after solubilisation and hydrophobic affinity chromatography (on n-octylamino-Sepharose 4B) of cirrhotic and control liver microsomes. Analysis of these fractions by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and laser densitometry indicated that a protein band of apparent minimum molecular weight 50.5 kD was primarily affected in cirrhotic rat liver microsomes. Levels of two other bands (apparent minimum molecular weight 48 and 52.5 kD) appeared essentially unaltered. Additional electrophoretic studies, conducted under non-reduced conditions, indicated the haemoprotein nature of protein bands in the 48-55 kD region. These data strongly suggest that cirrhosis produced in rats by a choline-deficient diet is associated with selective decreases in oxidative drug metabolism and individual cytochrome P-450 isozymes.

Effects of portal vein ligation on sex hormone metabolism in male rats: relationship to lowered hepatic cytochrome P450 levels.

Hepatic cytochrome P450 levels in male rats fall after portal vein ligation, a procedure that produces total hepatic bypass of portal blood. The present study was undertaken to examine whether changes in sex hormone metabolism could account for these lowered cytochrome P450 levels. Portal vein ligation resulted in testicular atrophy and low serum testosterone concentrations. Serum luteinizing hormone levels were also reduced, suggesting that testicular atrophy was secondary to suppression of the hypothalamic-pituitary-gonadal axis. Serum estrone and estradiol concentrations were significantly increased after portal vein ligation, while the magnitude and delayed onset of increases in urinary total estrogen excretion suggested that this was due largely to increased estrogen production. In male rats, both castration (at 12 wk) and exogenous estrogen administration resulted in changes in hepatic cytochrome P450 levels and ethylmorphine N-demethylase activity that were qualitatively similar to those seen after portal vein ligation. In female and castrated male rats, however, cytochrome P450 was not affected by portal vein ligation. Testosterone supplementation corrected the changes of cytochrome P450 levels in castrated male rats but did not have this effect in portal vein-ligated male rats. It is concluded that changes in sex hormone metabolism do occur after portal vein ligation and may contribute to alterations in cytochrome P450 and drug-metabolizing enzyme activity. Decreased levels of serum testosterone, however, do not alone account for the changes in hepatic drug metabolism in this model, and suppression of a hypothalamic-pituitary factor appears to be important.

Hepatic heme metabolism and cytochrome P450 in cirrhotic rat liver.

Cytochrome P450 levels are often low in the cirrhotic liver but the reason for this has not been established. Because changes in heme metabolism may reduce hepatic levels of cytochrome P450, the relationship of heme turnover to cytochrome P450 levels has been examined in rats with cirrhosis. Cirrhosis was produced by repeated carbon tetrachloride inhalation. In animals with cirrhosis, hepatic microsomal cytochrome P450 content was significantly less (32%) than in controls. Heme synthesis was assessed by measuring the activity of mitochondrial delta-amino-levulinic acid synthetase and also by determining the incorporation (within 30 min) of radiolabeled delta-aminolevulinic acid into the microsomal heme fraction. Both these parameters were normal in rats with CCl4-induced cirrhosis. In addition, the activity of microsomal heme oxygenase, the rate-limiting enzyme in catabolism of heme to bilirubin, was not altered. Cytochrome P450 heme degradation was then determined directly by injecting radiolabeled delta-aminolevulinic acid and measuring radioactivity in CO-binding particles (microsomes incubated with protease to remove cytochrome b5) prepared at various times thereafter. By this method, the degradation rate of cytochrome P450 heme did not differ between rats with cirrhosis and controls. Finally, the availability of hepatic heme for formation of hemoproteins was deemed to be satisfactory in cirrhotic liver because tryptophan pyrrolase saturation was comparable with controls, and also because heme administered in vivo did not enhance hepatic clearance of the cytochrome P450 substrate antipyrine. The failure to find defective heme biosynthesis or accelerated heme breakdown and the evidence that heme is available in amounts that do not restrict hemoprotein formation indicate that aberrant heme metabolism is not the cause of low cytochrome P450 levels in this rat model of cirrhosis.

Accuracy and clinical utility of simplified tests of antipyrine metabolism.

High-performance liquid chromatography (h.p.l.c.) was used to measure plasma antipyrine concentrations in sixteen healthy subjects; five males, six females taking oral contraceptive steroids (OCS) and five age-matched females not taking OCS. Following oral administration of the drug, antipyrine clearance could be determined with similar precision and accuracy from plasma concentrations at two selected times (4 h and 24 h) ('two-sample antipyrine clearance test') as from six samples taken over two to three elimination half-lives (t1/2) of the drug ('conventional antipyrine clearance test'). Provided times for plasma sampling were modified appropriately, the two-sample antipyrine clearance test also gave reliable results in four patients taking phenytoin (sampling times 4 h and 8 h) who had significantly enhanced antipyrine clearance, and in nine patients with severe liver disease (using 4 h and 48 h) in whom antipyrine clearance was impaired. Urinary excretion of antipyrine metabolites (from 0-24 h) was determined in the above groups. Antipyrine systemic clearance correlated best with the percentage of administered dose excreted as 4-hydroxyantipyrine (r = 0.66, P less than 0.001) but also with 3-hydroxymethylantipyrine (r = 0.54, P less than 0.001) and with total metabolites excreted (r = 0.60, P less than 0.001). Total antipyrine metabolites excreted in urine in 24 h were significantly different from controls only in patients with liver disease (14 +/- 7.6% of administered dose vs 62 +/- 14%, P less than 0.001). The relative proportion of antipyrine metabolites did not appear to be altered when hepatic mixed function oxidation was induced by phenytoin or inhibited by OCS or by severe liver disease.

Microsomal protein synthesis and induction of cytochrome P-450 in cirrhotic rat liver.

In order to determine whether non-specific defects of protein synthesis account for reduced levels of cytochrome P-450 in cirrhotic liver, total microsomal protein synthesis and response to microsomal enzyme-inducing agents have been examined in rats. Cirrhosis was produced by administration of carbon tetrachloride (CCl4) and phenobarbitone for 10 weeks. Ten days after stopping these agents, cytochrome P-450 levels were 30% lower in cirrhotic liver than in controls (p less than 0.0001). However, total microsomal protein synthesis, determined in vivo by administration of [3H]-leucine, was similar in cirrhotic (1347 +/- 420 dpm/mg protein) and control (1317 +/- 303 dpm/mg protein) liver. Three separate types of microsomal enzyme-inducing agents, phenobarbitone, beta-naphthoflavone, and pregnenolone 16 alpha-carbonitrile, were administered to cirrhotic and normal rats. In both groups of animals increases of total cytochrome P-450 and selective changes of cytochrome P-450 isoenzymes (assessed by mixed function oxidase activity towards four substrates) were qualitatively and quantitatively similar. It is concluded that hepatocytes of cirrhotic rat liver synthesize microsomal protein at a normal rate but less of it is cytochrome P-450, and the entire process of enzyme induction is intact. Thus, it appears likely that altered regulation of basal levels of cytochrome P-450 rather than an altered response of the liver is responsible for the lowered cytochrome P-450 content of cirrhotic rat liver.

Portal vein ligation selectively lowers hepatic cytochrome P450 levels in rats.

In rats, surgical creation of a portacaval shunt leads to hepatic atrophy and lowered levels of cytochrome P450, the key component of liver enzymes involved with drug metabolism. These effects are largely attributable to diversion of portal blood away from the liver and not to decreased hepatic blood flow. The present study has established a simpler model of portal blood diversion in order to examine the role of portal blood constituents in the regulation of hepatic cytochrome P450. Portal vein ligation was performed on male Wistar rats in which portasystemic anastomoses had been produced by subcutaneous transposition of the spleen. Portal vein ligation resulted in portal hypertension, as evidenced by splenomegaly, and in hepatic atrophy. In liver of rats with portal vein ligation, microsomal cytochrome P450 levels were significantly less than in sham-operated control rats, but cytochrome b5, NADPH-cytochrome c reductase, and glucose-6-phosphatase were unaltered. The activities of four mixed function oxidases also were reduced significantly in the liver of rats with portal vein ligation, the changes being greatest for ethylmorphine N-demethylase, a prototype substrate for the phenobarbital-inducible isoenzyme of cytochrome P450. In contrast, the activity of microsomal heme oxygenase, the rate-limiting step in catabolism of heme to bilirubin, was enhanced after portal vein ligation. Experiments in pair-fed rats showed that the changes observed in liver from rats with portal vein ligation could not be attributed to caloric deprivation. Administration of phenobarbital increased liver mass, cytochrome P450 levels, and mixed function oxidase activities both in rats with portal vein ligation and in controls, indicating that the liver of the ligated rats retained considerable protein synthetic capacity. It appears that hepatic atrophy and lowering of cytochrome P450 levels that follow portal vein ligation are consequences of altered exposure of the liver to factors normally present in portal blood, and that the same alterations may also enhance heme oxygenase activity.