The specificity of glucuronidation of entacapone and tolcapone by recombinant human UDP-glucuronosyltransferases

The COMT inhibitors entacapone and tolcapone are rapidly metabolized in viv o, mainly by glucuronidation. In this work, the main UGT isoforms responsib le for their glucuronidation in vitro were characterized by using a subset of representative cloned and expressed human UGT isoforms. Entacapone in pa rticular was seen to be an exceptionally good substrate for UGT1A9 with an even higher reaction velocity value at 500 mu M substrate concentration com pared with that of the commonly used substrate, propofol (1.3 and 0.78 nmol min(-1) mg(-1), respectively). Neither entacapone nor tolcapone was glucur onidated by UGT1A6. Tolcapone was not detectably glucuronidated by UGT1A1, and the rate of glucuronidation of entacapone was also low by this isoform. However, UGT1A1 was the only UGT capable of catalyzing the formation of tw o glucuronides of the catecholic entacapone. Both COMT inhibitors were gluc uronidated at low rates by the representative members of the UGT2B family, UGT2B7 and UGT2B15. Michaelis-Menten parameters were determined for entacap one and tolcapone using recombinant human UGT isoforms and human liver micr osomes to compare the kinetic properties of the two COMT inhibitors. The ki netic data illustrates that UGT1A9 exhibited a much greater rate of glucuro nidation and a far lower K-m value for both entacapone and tolcapone than U GT2B15 and UGT2B7 whose contribution is minor by comparison. Entacapone sho wed a 3 to 4 times higher V-max value and a 4 to 6 times lower K-m value co mpared with those of tolcapone both in UGT1A9 cell lysates and in human liv er microsomes.