Inter-species comparison of 7-hydroxycoumarin glucuronidation and sulfation in liver S9 fractions

Abstract

UDP glycosyltransferases (UGTs) and sulfotransferases (SULTs) are phase II enzymes that interact with a number of xenobiotics in humans and animals. Species differences in enzymatic characteristics have seldom been investigated. Liver S9 fractions are commonly used for studying phase II metabolism in vitro. The objective of this study was to characterize the UGT and SULT activities in liver S9 fractions from various species including humans, monkeys, dogs, and rats. A single substrate, 7-hydroxycoumarin (7-HC), at several concentrations was incubated at 37 degrees C with the S9 reaction matrices along with necessary cofactors. The rate of formation of two metabolites, 7-HC-glucuronide (7-HC-G) and 7-HC-sulfate (7-HC-S), was determined with Liquid Chromatography/Tanderm Mass Spectrometry (LC/MS/MS). Apparent Km and Vmax values were calculated for each species. For the UGTs, the apparent Km and Vmax for 7-HC-G formation varied greatly among different species, with dog UGTs having both the highest Km and Vmax values. In contrast to UGTs, the Km for 7-HC-S formation showed no significant difference among humans, monkeys, and rats (approximately 3 microM). However, the Km in dog was 8.7 microM. Species differences with respect to phase II metabolism must be carefully considered when selecting an in vitro model system to study various aspects of drug metabolism.