Abstract

Like other basic amphiphilic drugs, the (S)-enantiomer of the antiallergic drug ketotifen exhibited biphasic kinetics when it was converted to two isomeric quaternary ammonium-linked glucuronides in human liver microsomes. For (R)-ketotifen this applied when incubations were carried out in the absence of a detergent. Two UDP-glucuronosyltransferases (UGTs) present in human liver, UGT1A4 and UGT1A3, were previously shown to catalyze tertiary amineN-glucuronidation when expressed in HK293 cells. Therefore, the conjugation kinetics of (R)- and (S)-ketotifen were investigated with the two expressed proteins. When homogenates of HK293 cells expressing UGT1A4 were incubated without detergent, N-glucuronidation kinetics were monophasic with K_M values of 59 ± 5 μM for (R)- and 86 ± 26 μM for (S)-ketotifen. In experiments with membranes containing expressed UGT1A3, somewhat higher K_M values were obtained. These values correspond to the high rather than to the low K_M components of ketotifen glucuronidation in liver microsomes, the latter exhibitingK_M values around 2 and 1 μM, respectively, with (R)- and (S)-ketotifen. With amitriptyline as the substrate, N-glucuronidation kinetics in the absence of detergent were biphasic in human liver microsomes and monophasic with a high K_Mvalue in cell homogenates containing UGT1A4. The results suggest that UGT1A4 and UGT1A3 catalyze high-K_MN-glucuronidation of tertiary amine drugs, whereas the low-K_M reaction requires either an alternative enzyme or a special conformation of UGT1A4 or UGT1A3 that can be attained in liver microsomes, but not in HK293 cell membranes.