Induction of UDP-glucuronosyltransferase isozymes in male and female rat liver microsomes by an isoquinoleine derivative (52028 RP)

A Toxicologist's Guide to the Preclinical Assessment of Hepatic Microsomal Enzyme Induction

The assessment of hepatic microsomal enzyme induction at the completion of preclinical toxicology studies in rodents and large mammals provides a wealth of information to the toxicologist and pharmacokineticist regarding how the drug-metabolizing system of the hepatocyte endoplasmic reticulum responded to high-dose levels of a xenobiotic designed for a specific pharmacological target in any of several target organs. The interpretation of these data can be greatly enhanced by a clear understanding of how this system functions and what the immediate and long-term ramifications are to organs and organ systems. This review focuses on how drugs modify the hepatic cytochrome P450 system, how those modifications are detected, the various consequences of these modifications, and some differences in the induction response among species.

Heterogeneity of hepatic UDP-glucuronosyltransferase activities: investigations of isoenzymes involved in p-nitrophenol glucuronidation

1. UDP-Glucuronosyltransferase (UGT, EC 2.1.4.17) has been measured routinely with para-nitrophenol (pNP) because the UGT assay using this substrate is easy to assess and run. 2. This compound has been used in several studies as a substrate for purification of the enzyme. 3. In the present work, we characterize the para-nitrophenol-conjugating activity. 4. An analysis of kinetics of para-nitrophenol conjugation obtained from various biological sources (various tissues and various species) leads us to the conclusion that at least three isoenzymes are responsible for this activity in the rat. 5. Both UGT-(testosterone) and the 3-methylcholanthrene-inducible form previously described in the literature, may be responsible for the activity, whilst a highly specific form (UGT-phenol) is reported here for the first time. 6. This work is intended to lay down the basis of further investigations, including purification of the highly specific isoform.