Microsomal azoreduction and glucuronidation in the metabolism of dimethylaminoazobenzene by the rat liver

1. Enzymic azoreduction of the hepatocarcinogen, N, N- dimethyl-4-aminoazobenzene (DAB) and glucuronidation of its ring-hydroxylation product, 4′-hydroxy-DAB, by hepatic microsomal fractions in vitro were studied during an eight day period of hepatic regeneration following partial hepatectomy in Wistar rats. Azoreduction of DAB and its N-demethylated metabolites did not significantly change during hepatic regeneration in contrast to N-demethylation of these dyes which is profoundly suppressed during regeneration. UDP-Glucuronosyltransferase (UDP-GT) activity towards 4′-hydroxy-DAB was partially depressed during the regeneration period, but the depression was considerably less than that for bilirubin. Transferase activity towards 4-nitrophenol, after initial depression, returned to normal levels after the third day of partial hepatectomy. 2. In Gunn rats, microsomal UDP-GT activity towards bilirubin was undetectable, whereas transferase activity toward 4-nitrophenol was 50% of normal. Addition of diethylnitrosamine (DEN) in vitro restored transferase activity towards 4-nitrophenol to normal levels, but the activity towards bilirubin was unaffected. Gunn rat UDP-GT activity towards 4′-hydroxy-DAB was 25% of normal and was partially activated upon addition of DEN in vitro. 3. Treatment with clofibrate of β-naphthoflavone induced hepatic microsomal bilirubin- and 4-nitrophenol-specific UDP-GT activities, respectively; both agents induced transferase activity towards 4′-hydroxy-DAB. Triiodothyronine, which induces 4′-nitrophenol-specific UDP-GT and depresses bilirubin-specific UDPG, had little effect on 4′-hydroxy-DAB UDP-GT activity. 4. The results indicate that microsomal reductive metabolism of DAB is affected differently from that of its oxidative metabolism during liver regeneration. Glucuronidation of 4′-hydroxy-DAB appears to be catalysed by both bilirubin- and 4-nitrophenolspecific UDP-GT isoforms which are differentially expressed during liver regeneration.