Abstract
After oral administration of rifampicin and 25-desacetylrifampicin, which is a major metabolite of rifampicin in man but not in rat, to male Wister rats for 7 days, hepatic microsomal cytochrome P450, cytochrome b5, and activities of aniline hydroxylase, aminopyrine demethylase, bilirubin-conjugating enzymes and supernatant glutathione S-transferase were measured. Rifampicin induced bilirubin UDP-glucuronyltransferase, bilirubin UDP-glucosyltransferase, bilirubin UDP-xylosyltransferase and glutathione S-transferase activities, but did not induce mixed function oxidase activities. No inductive effect of desacetylrifampicin on any enzymes was observed. Serum bilirubin increased till the third day, and decreased after 7 days of rifampicin treatment. Plasma clearances of indocyanine green and sulfobromophthalein showed a marked delay after 1 day and 7 days of rifampicin treatment. Induction of bilirubin-conjugating enzymes and glutathione S-transferase by rifampicin in rats was different from that in humans, in which selective induction of mixed function oxidase is reported to occur. This species difference does not seem to be derived from the species difference of rifampicin metabolism, because no effect of desacetylrifampicin was observed. These results suggested that in rats rifampicin directly inhibits the hepatic excretion of bilirubin, whereas it enhances bilirubin conjugation due to enzyme induction.
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