We characterized the in vitro glucuronidation of
prunetin, a
prodrug of
genistein that is a highly active
cancer prevention agent. Metabolism studies were conducted using expressed human UGT
isoforms and microsomes/S9 fractions prepared from intestine and liver of rodents and humans. The results indicated that human intestinal microsomes were more efficient than liver microsomes in glucuronidating
prunetin, but rates of metabolism were dependent on time of incubation at 37 degrees C. Human liver and intestinal microsomes mainly produced metabolite 1 (prunetin-5- O-glucuronide) and metabolite 2 (prunetin-4'- O-glucuronide), respectively. Using 12 human UGT
isoforms, we showed that UGT1A7,
UGT1A8, and UGT1A9 were mainly responsible for the formation of metabolite 1, whereas UGT1A1,
UGT1A8, and
UGT1A10 were mainly responsible for the formation of metabolite 2. This
isoform-specific metabolism was consistent with earlier results obtained using human liver and intestinal microsomes, as the former (liver) is UGT1A9-rich whereas the latter is UGT1A10-rich. Surprisingly, we found that the thermostability of the microsomes was
isoform- and organ-dependent. For example, human liver microsomal UGT activities were much more heat-stable (37 degrees C) than intestinal microsomal UGT activities, consistent with the finding that
human UGT1A9 is much more thermostable than human
UGT1A10 and
UGT1A8. The organ-specific thermostability profiles were also evident in rat microsomes and mouse S9 fractions, even though human intestinal glucuronidation of
prunetin differs significantly from rodent intestinal glucuronidation. In conclusion,
prunetin glucuronidation is species-, organ-, and UGT-
isoform-dependent, all of which may be impacted by the thermostability of specific UGT
isoforms involved in the metabolism.