UDP-glucuronyltransferase in perfused rat liver and in microsomes: V. Studies with Gunn rats

Induction of drug-metabolizing enzymes in Gunn rat liver. Effect of polycyclic aromatic hydrocarbons on cytochrome P-450 regulation

Response of congenitally jaundiced rats (Gunn rats) to administration of polycyclic aromatic hydrocarbons (PAH) was investigated and compared to that of Wistar rats. Unlike Wistar, Gunn males did not exhibit changes in the overall cytochrome P-450 content of hepatic microsomes. The first step in the induction process (i.e. presence of cytosolic receptors for PAH) was found present and functionally similar (number of sites, Kd) to that of Wistar rats from which the Gunn strain is derived. An increase in monooxygenase activities related to P-450c and P-450d isoenzymes specifically induced by PAH was noticed, whereas no effect could be detected on the glucuronidation rate of either 4-nitrophenol, testosterone or estrone. As determined by immunoquantification after Western blotting, the isoenzymatic profile of P-450 from PAH-treated male Gunn rats showed an increase of P-450c and P-450d accompanied by an equivalent decrease in P-4502c (major male-specific isoenzyme). The balance between increase in P-450c and P-450d and decrease in P-4502c may explain the absence of increase in the total P-450 in PAH-treated male Gunn rats. Such a response was not observed in PAH-treated male Wistar rats or in female rats of both strains. In contrast, the response of male Gunn rats to PB treatment was similar to that observed in Wistar rats, i.e. increase in overall cytochrome P-450 content of hepatic microsomes and of specific isoenzyme P-450b/e. A possible regulation of P-450 isoenzyme synthesis by the intracellular haem pool might be involved.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferases activities: use and comparison of differential inductions in some mammalian species

1. The co-injection in rats of the inducers 3-methylcholanthrene or phenobarbital and of a protein synthesis inhibitor (cycloheximide) shows that two clusters of hepatic UDP-Glucuronosyltransferases (GT1 and GT2) are under separate genomic expression and differentially regulated. 2. The administration of cycloheximide alone even suggests a distinct turn-over for these two groups of isoenzymes. 3. Indirect evidence for a UDPGT isoform specialized for some structurally-related exogenous substrates, the monoterpenoid alcohols, is brought. Their conjugation exhibits a small deficiency and a marked response to phenobarbital treatment in the Gunn rat and an exclusive inducibility by phenobarbital in the guinea-pig.

Hepatic oxidative drug metabolism and the microsomal milieu in a rat model of congenital hyperbilirubinemia

The aims of this study were to evaluate the hypothesis that impaired glucuronidation of bilirubin and possibly of drug oxidation in the liver of homozygous (jj) Gunn rats may be due to an altered microsomal milieu. Accordingly, we investigated and compared in vivo and in vitro demethylation of aminopyrine, hepatic cytochrome P-450 levels, microsomal lipid composition, and microsomal membrane fluidity in icteric, homozygous (jj) Gunn rats and in their anicteric heterozygous (jJ) littermates. In both males and females, [14C]aminopyrine demethylation in vivo, using the 14CO2 breath test, was unimpaired in the icteric animals. Likewise, cytochrome P-450 levels in the icteric and nonicteric groups were similar, and aminopyrine kinetics in vitro in the females were comparable in icteric and nonicteric littermates. The main lipid classes were also similar in the homozygous and heterozygous female Gunn rats, whereas only minor changes were seen in the phospholipid fatty acyl composition with a small, but significant, increase in the unsaturated index in the icteric group. Despite this, there was no apparent effect on hepatic microsomal membrane fluidity as measured by the order parameter of I[12,3] and the rotational correlation time of I[1,14] in either female or male sets of homozygous and heterozygous Gunn rats. Our data, therefore, do not support an alteration of composition or fluidity of the microsomal milieu as a mechanism of impaired bilirubin glucuronidation and possibly of oxidation in these animals. They also absolve long-term unconjugated hyperbilirubinemia as a mechanism of hepatic microsomal dysfunction. Our study, therefore, indirectly suggests that abnormal glucuronidation of bilirubin and some other aglycones in homozygous Gunn rats is due to genetic abnormalities involving the enzyme(s) itself.

Separation of four glucuronides in a single sample by high-pressure liquid chromatography and its use in the determination of UDP glucuronosyltransferase activity toward four aglycones

A single-solvent, reverse-phase HPLC system that separates glucuronides has been developed, and its applicability to the separation of glucuronides of four substrates (1-naphthol, morphine, testosterone, and estrone), representative of different UDP-glucuronosyltransferase enzymes, has been demonstrated. The combined supernatants from four separate microsomal incubations from which the protein has been precipitated can be used directly for the quantitation of the glucuronides formed. Variations in the four enzyme activities, upon treatment of rats with microsomal enzyme-inducing agents, after liver trauma and in the Gunn strain of rats have been investigated.

Biochemical and morphological comparison of microsomal preparations from rat, quail, trout, mussel, and water flea

Differential centrifugation methods already in use were applied to purify rat, quail, and trout liver microsomes and modified as necessary to purify microsomes from mussel digestive gland and whole water flea. All these microsomal preparations were comparatively characterized with respect to protein and RNA content, levels of markers of subcellular contaminants, ultrastructural morphology, differential spectra of cytochromes P-450 and b5, monoxygenase activity, and in vitro metabolism of p-dichlorobenzene. Yields of microsomal proteins of the tested organisms differed widely, with mussel showing the lowest yield. Very low levels of nuclear and mitochondrial contaminants were detected in all microsomal preparations, but cell membrane contaminants were clearly present in most preparations. Daphnia microsomes were significantly contaminated by plasma membranes, and hepatopancreas microsomes contained significant amounts of partially disrupted secretory granules and plasma membrane. From a qualitative standpoint differential spectra of cytochrome b5 were very similar for all the preparations, whereas cytochrome P-450 spectra were largely dependent on the microsomal preparation as well as on the assay method used. The content of cytochrome P-450 was highest for rat liver microsomes and very low or absent in Daphnia and mussel preparations; the range of cytochrome b5 contents was much narrower. Significant differences were observed among monoxygenase activities of the different preparations. In Daphnia and mussel microsomes, aniline hydroxylase was absent and benzo[a]pyrene hydroxylase activity was much lower than in rat and quail microsomes. Benzo[a]pyrene hydroxylase activity of trout liver microsomes was similar to that of rat and quail microsomes, whereas hydroxylation of substrates which in rat liver are preferentially metabolized by phenobarbital-inducible forms of cytochrome P-450 was much lower in trout microsomes.

Vitamin A (retinol) status in the Gunn rat. The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Vitamin A (retinol) status and the effect of a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 10 micrograms X kg-1, on vitamin A in the liver and serum, and on the hepatic UDP-glucuronosyltransferase (UDPGT) (EC 2.4.1.17) activity, were studied in heterozygous (GW) and homozygous (GG) Gunn rats. 1) Data from vitamin A analyses demonstrate that the amount of vitamin A stored in the liver of untreated Gunn rats is of the same magnitude as that of Sprague-Dawley rats. 2) The retinol content in the liver of both GG and GW rats was reduced to about 50% by TCDD-treatment. 3) Retinol levels in serum were found to be variable and no significant effect due to TCDD could be observed. 4) No correlation between the TCDD-induced reduction of vitamin A and the induction of UDPGT activity by TCDD could be demonstrated in this study. The vitamin A reduction caused by TCDD was considerably less in the Gunn rat than in the Sprague-Dawley rat, and the results indicate that the Gunn rat is more resistant to TCDD than other strains of rat. TCDD-induced reduction of liver vitamin A seems to some extent to correlate with TCDD-toxicity in different strains of rat. The specific properties of the Gunn rat and its relatively high resistance to TCDD make it a valuable tool in studies about the mechanism of TCDD-toxicity.

Effect of spironolactone on p-nitrophenol glucuronidation in isolated rat hepatocytes

The effect of spironolactone (SP) on p-nitrophenol (PNP) glucuronidation was studied in isolated rat hepatocytes with appropriate viability conditions. A significant increase of protein concentration and PNP glucuronidation was found in the hepatocytes from SP-treated rats. Increased enzyme activity apparently was related to the SP dose. The results favor the conclusion that SP may induce PNP glucuronidation in the hepatocyte.

Reconstitution of purified Wistar rat liver bilirubin UDP-glucuronyltransferase into Gunn-rat liver microsomes

1. Reconstitution of purified bilirubin UDP-glucuronyltransferase from Wistar-rat liver into Gunn-rat liver microsomes provides a better environment than phosphatidylcholine liposomes, such that the final specific activity of the Wistar-rat liver enzyme was increased up to 85 units/mg of protein. 2. Gunn- and Wistar-rat liver microsomes were equally effective for reconstitution of the purified enzyme. 3. The transferase activity does not appear to be fully expressed in the more rigid environment of foetal Wistar-rat liver microsomes. 4. These reconstitution experiments reveal a final specific activity for the purified bilirubin UDP-glucuronyltransferase consistent with the capacity of the whole rat liver to glucuronidate bilirubin and indicate that the absence of this enzyme activity in Gunn-rat liver microsomes is not due to an abnormal microenvironment.

Quantification of naphthyl conjugates. Comparison of high-performance liquid chromatography and selective enzyme hydrolysis methods

1. An h.p.l.c. method for the simultaneous determination of naphthol, naphthyl glucuronide and naphthyl sulphate in urine is described. This procedure is based on the selective formation of an ion pair between the sulphate conjugate and tetrabutyl lammonium, which allows its resolution from the glucuronide on a reverse phase column. 2. The h.p.l.c. method is sed to assess the selective enzyme hydrolysis method which is frequently employed in drug conjugate assays. Results suggest that the use of sulphatase and beta-glucuronidase may give data which are quantitatively and qualitatively erroneous.

Purification of rat-liver microsomal UDP-glucuronyltransferase. Separation of two enzyme forms inducible by 3-methylcholanthrene or phenobarbital

Glucuronidation reactions catalysed by rat liver microsomal UDP-glucuronyltransferase are differentially inducible by 3-methylcholanthrene and phenobarbital. To elucidate the molecular basis of this functional heterogeneity the enzyme was purified from livers of rats pretreated with the inducing agents. Using cholate solubilization, chromatography on Bio-Gel A-1.5m and on DEAE-cellulose in the presence of the nonionic detergent Brij 58, two enzyme forms could be separated. Both forms were subsequently purified to apparent homogeneity by affinity chromatography on UDP-hexanolamine Sepharose 4B, 3-Methylcholanthrene-inducible enzyme activity towards 1-naphthol, 4-nitrophenol, 3-hydroxybenzo(a)pyrene and N-hydroxy-2-naphthylamine copurified with one enzyme form (enzyme 1). In contrast phenobarbital-inducible enzyme activity towards morphine, chloramphenicol and 4-hydroxybiphenyl was associated with the other enzyme fraction (enzyme 2). Sodium dodecylsulfate/polyacrylamide gels showed similar molecular weights of 54000 for enzyme 1 and 56000 for enzyme 2. The results suggest the presence of at least two forms of UDP-glucuronyltransferase in rat liver. Factors affecting enzyme activity in purified and membrane-bound states are discussed.