Bilirubin UDPglucuronosyltransferase of rat or human liver microsomes was inhibited, in vitro, by triphenylacetic acid and by structurally related arylcarboxylic acids. This inhibition appeared to be competitive towards bilirubin, and mixed-type towards UDPglucuronic acid. A decrease in the number of phenyl rings or the absence of the carboxyl group in the molecule gave structures which did not affect enzyme activity, showing that both the triphenyl moiety and the carboxyl group were necessary for the inhibition. On the other hand, successive additions of methylene groups in the aliphatic chain progressively increased inhibitory potency. Kappi,bilirubin for triphenylacetic acid was 96 microM compared with 5 microM for 7,7,7-triphenylheptanoic acid. The inhibition of bilirubin UDPglucuronosyltransferase was not due to displacement of bilirubin from albumin. On the basis of these results an attempt was made to delineate the molecular events leading to glucuronidation of bilirubin.
