Abstract

Bovine serum albumin (BSA) and fatty acid-free human serum albumin (HSAFAF) reduce the K_m values for UGT2B7 substrates by sequestering inhibitory long-chain fatty acids released by incubations of human liver microsomes (HLM) and HEK293 cells expressing this enzyme. However, the scope of the “albumin effect” is unknown. In this investigation we characterized the effects of albumin on the kinetics of 4-methylumbelliferone (4MU) glucuronidation by UDP-glucuronosyltransferase (UGT) 1A1, 1A6, and 1A9, and propofol (PRO) glucuronidation by UGT1A9 and HLM. BSA and HSAFAF, but not human serum albumin, reduced the K_m values for 4MU and PRO glucuronidation by UGT1A9. For example, HSAFAF (2%) reduced the K_m values for 4MU and PRO glucuronidation from 13.4 to 2.9 and 41 to 7.2 μM, respectively. Similarly, HSAFAF (2%) reduced the K_m for PRO glucuronidation by HLM from 127 to 10.6 μM. Arachidonic, linoleic, and oleic acids and a mixture of these decreased the rates of 4MU and PRO glucuronidation by UGT1A9. K_m values for these reactions were increased 3- to 6-fold by the fatty acid mixture. Inhibition was reversed by the addition of BSA (2%). Extrapolation of kinetic constants for PRO glucuronidation by HLM in the presence of HSAFAF predicted in vivo hepatic clearance within 15%. Fatty acids had no effect on 4MU glucuronidation by UGT1A1 and UGT1A6 but, paradoxically, all forms of albumin altered the kinetic model for 4MU glucuronidation by UGT1A6 (from Michaelis-Menten to two-site). Only BSA caused a similar effect on 4MU glucuronidation by UGT1A1. It is concluded that BSA and HSAFAF reduce the K_m values of only those enzymes inhibited by long-chain unsaturated fatty acids.