Formation of highly analgesic morphine-6-glucuronide following physiologic concentration of morphine in human brain.

3H-Morphine at physiologic concentration was metabolized in vitro to its 3- and 6-glucuronides by human brain homogenate. Recombinant UGT2B7, one of the UDP-glucuronosyltransferase (UGT) isoforms, is able to glucuronidate the 3- and 6-hydroxy groups of morphine at nanomolar concentrations. These results suggest that endogenous morphine is converted to its 6-glucuronide, a more highly analgesic substance than the parent compound, to suppress effectively pain symptoms in humans.