Sulfinpyrazone C-glucuronidation is catalyzed selectively by human UDP-glucuronosyltransferase 1A9

The uricosuric agent sulfinpyrazone (SFZ) is metabolized via C-glucuronidation, an uncommon metabolic pathway, in humans. The present study aimed to characterize SFZ glucuronidation by human liver microsomes (HLMs) and identify the hepatic forms of UDP-glucuronosyltransferase responsible for this pathway. Incubations of SFZ with HLMs formed a single glucuronide that was resistant to beta-glucuronidase and acid hydrolysis, consistent with formation of a C-glucuronide. Mass spectral analysis confirmed the identity of the metabolite as SFZ glucuronide (sulfinpyrazone beta-D-glucuronide; SFZG). SFZ C-glucuronidation by HLMs exhibited Michaelis-Menten kinetics, with mean (+/- S.D.) K-m and V-max values of 51 +/- 21 mu M and 2.6 +/- 0.6 pmol/min center dot mg, respectively. Fifteen recombinant human UDP-glucuronosyltransferases (UGTs), expressed in HEK293 cells, were screened for their capacity to catalyze SFZ C-glucuronidation. Of the hepatically expressed enzymes, only UGT1A9 formed SFZG. UGTs 1A7 and 1A10, which are expressed in the gastrointestinal tract, also metabolized SFZ, but rates of metabolism were low compared with UGT1A9. SFZ glucuronidation by UGT1A9 exhibited weak negative cooperative kinetics, which was modeled by the Hill equation (S-50 16 mu M). The data indicate that UGT1A9 is the enzyme responsible for hepatic SFZ C-glucuronidation and that SFZ may be used as a substrate probe for UGT1A9 activity in HLMs.