The formation of morphine-3-glucuronide (M-3-G, pharmacologically inactive) and morphine-6-glucuronide (M-6-G, active metabolite) by liver microsomes from humans and rodents, including chimeric mice carrying human liver, was evaluated in the presence of fatty acyl-CoAs. Medium- to long-chain fatty acyl-CoAs, including oleoyl-CoAs, at a physiologic level (around 15 microM) markedly enhanced M-3-G formation catalyzed by rat liver microsomes. A separate experiment indicated that 15 microM oleoyl-CoA enhanced (14)C-UDP-glucuronic acid (UDPGA) uptake by microsomes. The activation by acyl-CoAs disappeared or was greatly reduced by either pre-treating microsomes with detergent or freezing/thawing the rat liver before preparation. Many of the microsomes prepared from frozen human livers (N=14) resisted oleoyl-CoA-mediated activation of UDP-glucuronosyltransferase (UGT) activity, including M-6-G formation, which is highly specific to humans. In sharp contrast, the activity of M-6-G and M-3-G formation in freshly-prepared hepatic microsomes from chimeric mice with humanized liver was potently activated by oleoyl-CoA. Thus, acyl-CoAs activate morphine glucuronidation mediated by human as well as rat UGTs. This activation is assumed to be due to the acyl-CoA-facilitated transportation of UDPGA, and microsomes need to maintain the intact conditions required for the activation. The function of UGT appears to be dynamically changed depending on the cellular acyl-CoA level in many species.