Simple and rapid ion-pair high-performance liquid chromatographic assay for p-nitrophenyl glucuronide in rat-liver microsomal incubations

Release of latent glucuronosyltransferase activity contributes to the sparing of glucuronidation in experimental liver injuries

In patients with parenchymal liver disease, glucuronidation of drugs is generally preserved but oxidation is impaired. This study explores the effects of liver injuries induced in rats by the administration of acute carbon tetrachloride, chronic bile duct ligation and chronic choline deficiency for 30 weeks on the glucuronidation of p-nitrophenol and 1-naphthol, both before and after solubilization of the microsomes and compares this to three measures of oxidation. Cytochrome P450 content was reduced to 17% of control values after acute carbon tetrachloride and to 35% of control values after bile duct ligation. Cytochrome C reductase fell to 58 and 32% of control and aniline hydroxylase to 46 and 13%, respectively, after acute carbon tetrachloride and bile duct ligation. P-nitrophenol glucuronidation by native microsomes was 206 and 73% of controls in the respective models, while 1-naphthol glucuronidation was 167 and 66% of control. Latent uridine diphosphate-glucuronosyltransferase (UDP-GT) activity, that is, differences between solubilized and native activity, was decreased by each liver injury. Chronic choline deficiency had little effect on the oxidation and native glucuronidation activity, although latent glucuronidation activity was lower. These studies suggest a preservation of glucuronidation compared to oxidation at the microsomal level in these experimental liver injuries. The data also support the hypothesis that a release of latent UDP-GT activity may contribute to this phenomenon.

Mechanism of cadmium-decreased glucuronidation in the rat

In isolated rat hepatocytes, cadmium (0-200 microM) decreased the overall glucuronidation of both isopropyl N-(3-chloro-4 hydroxyphenyl)carbamate (4-hydroxychlorpropham, 4-OHCIPC) and 4-nitrophenol in a concentration-dependent manner. In contrast, in native rat liver microsomes, glucuronidation of 4-OHCIPC was increased by cadmium through activation of microsomal 4-OHCIPC glucuronosyl transferase. In addition, in rat microsome incubations, the net amount of 4-OHCIPC glucuronide was also indirectly increased by cadmium through a reduction in the activity of beta-glucuronidase. As the effect of cadmium on the activity of 4-OHCIPC glucuronosyl transferase could not account for the decrease in glucuronide formation in intact hepatocytes, the influence of cadmium on the availability of UDP-glucuronic acid (UDPGA) was investigated further. In isolated rat hepatocytes, cadmium depleted the UDPGA content in a dose-dependent manner without a change in the UDP glucose (UDPG) content. Cadmium did not increase the breakdown of UDPGA by microsomal UDPGA pyrophosphatase but strongly decreased (30-66%) the synthesis of the cofactor in the cytosol by inhibiting UDP-glucose dehydrogenase (UDPGDH). Cadmium (10-50 microM) was found to inhibit the purified enzyme from bovine liver (EC 1.1.1.22) non-competitively. In vivo in the absence of a substrate undergoing glucuronidation, cadmium administration, 1.5 and 2.5 mg Cd/kg i.v., to normally fed rats resulted in a 15 and 30% decrease of hepatic UDPGA, respectively. However, in the liver, neither the NAD+/NADH ratio nor the UDPG content was significantly changed following cadmium treatment. Both in vitro and in vivo results support the conclusion that in intact cells the reduction in overall 4-OHCIPC glucuronidation caused by cadmium was due to a decrease in UDPGA availability which results from the inhibiting effect of cadmium on UDPGDH.

Mixed-function oxidase activity in seabirds and its relationship to oil pollution

1. The hepatic activity of epoxide hydrolase, aldrin epoxidase, aminopyrine N-demethylase, 7-ethoxyresorufin O-deethylase, benzo(a)pyrene 3-hydroxylase and UDP glucuronyl transferase was determined in adult herring gulls (Larus argentatus) at various stages of the breeding season. 2. MFO activity was measured for adult Leach's storm-petrels (Oceanodroma leucorhoa), guillemot (Uria aalge) and Atlantic puffins (Fratercula arctica). For most assays the values were highest for the puffin. 3. MFO activity in both nestling and adult Atlantic puffins was determined. The degree of induction caused by a single internal dose of Prudhoe Bay crude oil in adult puffins and that caused by multiple internal doses in nestling puffins was measured.