Binding of Inhibitory Fatty Acids Is Responsible for the Enhancement of UDP-Glucuronosyltransferase 2B7 Activity by Albumin: Implications for in Vitro-in Vivo Extrapolation

Studies were performed to elucidate the mechanism responsi-ble for the reduction in Km values of UDP-glucuronosyltrans-ferase 2B7 (UGT2B7) substrates observed for incubations con-ducted in the presence of albumin. Addition of bovine serum albumin (BSA) and fatty acid-free human serum albumin (HSA-FAF), but not “crude ” HSA, resulted in an approximate 90% reduction in the Km values for the glucuronidation of zidovudine (AZT) by human liver microsomes (HLM) and UGT2B7 and a 50 to 75 % reduction in the S50 for 4-methylumbelliferone (4MU) glucuronidation by UGT2B7, without affecting Vmax. Oleic, lino-leic, and arachidonic acids were shown to be the most abun-dant unsaturated long-chain fatty acids present in crude HSA and in the membranes of HLM and human embryonic kidney (HEK)293 cells, and it was demonstrated that these and othe