Novobiocin-inhibition and magnesium-interaction of rat liver microsomal bilirubin UDP-glucuronosyltransferase

Novobiocin inhibited bilirubin UDP-glucuronosyltransferase (EC 2.4.1.17) from rat liver both in vitro and in vivo, in a dose-dependent fashion. This inhibition was immediate, and was fully reversed when novobiocin was removed by dialysis or by ultracentrifugation through 0.6 M sucrose. The inhibition could not be explained by an alteration in the membrane conformation of this enzyme, since the same kinetic changes were observed in digitonin-activated and in non-activated microsomes. Novobiocin exerted a non-competitive inhibition of bilirubin UDP-glucuronosyltransferase with either bilirubin or UDP-glucuronic acid as the substrate. Kinetic studies demonstrated uncompetitive inhibition of novobiocin or bilirubin UDP-glucuronosyltransferase as a function of Mg²⁺ concentration, whether the assays were EDTA-free or not. Thus, similarities seem to exist between the known effect of novobiocin on membrane-bound enzymes of the bacterial wall and its inhibitory effect on bilirubin UDP-glucuronosyltransferase: both these enzymic systems require metal divalent cations for maximal activity. The uncompetitive inhibition pattern observed with novobiocin with regard to Mg²⁺ suggests that this antibiotic acts on bilirubin conjugation by affecting Mg²⁺-enzyme complexes.