Received for publication February 19, 2008.
Revised March 18, 2008.
Accepted for publication March 20, 2008.

The 'albumin effect' and drug glucuronidation: Bovine serum albumin and fatty acid free serum albumin enhance the glucuronidation of UGT1A9 substrates but not UGT1A1 and UGT1A6 activities

Abstract

BSA and 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. This investigation characterized the effects of albumin on the kinetics of 4-methylumbelliferone (4MU) glucuronidation by UGT 1A1, 1A6 and 1A9, and propofol (PRO) glucuronidation by UGT1A9 and HLM. BSA and HSAFAF, but not HSA, reduced the K_m values for 4MU and PRO glucuronidation by UGT1A9. For example, HSAFAF (2%) reduced the K_ms for 4MU and PRO glucuronidation from 13.4 to 2.9uM and 41 to 7.2uM, respectively. Similarly, HSAFAF (2%) reduced the K_m for PRO glucuronidation by HLM from 127 to 10.6uM. Arachidonic, linoleic and oleic acids and 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 (Michaelis Menten to 2-site). Only BSA caused a similar effect on 4MU glucuronidation by UGT1A1. It is concluded that BSA and HSAFAF reduce the K_m values only of those enzymes inhibited by long chain unsaturated fatty acids.

Key words: glucuronidation, in vitro-in vivo scaling, phase II drug metabolism