Identification of human UDP-glucuronosyltransferase responsible for the glucuronidation of niflumic acid in human liver.

PURPOSE

To assess the uridine diphosphate (UDP)-glucuronosyltransferase (UGT) isozymes involved in the glucuronidation of niflumic acid in human liver.

METHODS

The glucuronidation activity of niflumic acid was determined in liver microsomes and recombinant UGT isozymes by incubation of niflumic acid with UDP-glucuronic acid (UDPGA).

RESULTS

Incubation of niflumic acid with liver microsomes and UDPGA produced one peak, which was identified as a glucuronide from mass spectrometric analysis. A study involving a panel of recombinant human UGT isozymes showed that glucuronidation activity was highest in UGT1A1 among the isozymes investigated. The glucuronidation in human liver microsomes (HLMs) followed Michaelis-Menten kinetics with a Km value of 16 microM, which is similar to that found with recombinant UGT1A1. The glucuronidation activity of niflumic acid in microsomes from eight human livers significantly correlated with UGT1A1-catalyzed estradiol 3beta-glucuronidation activity (r=0.78, p<0.05). Beta-estradiol inhibited niflumic acid glucuronidation with an IC50 of 25 microM in HLMs, comparable to that for UGT1A1.

CONCLUSIONS

These findings indicate that UGT1A1 is the main isozyme involved in the glucuronidation of niflumic acid in the human liver.