Effect of fasting on substrate specificity of rat liver UDP-glucuronosyltransferase.

The effect of a 3-day period of complete starvation on the hepatic UDPglucuronosyltransferase activity was studied in the rat. The substrate specificity of the enzyme was assayed with bilirubin as a carboxylic acceptor, and phenolphthalein and p-nitrophenol as phenolic acceptors. Starvation increased the bilirubin UDPglucuronosyltransferase specific activity by 33%, whereas no increase in specific activities appeared when the phenolic substrates were used. However, on a total liver weight basis, all three activities were significantly lower than those of the controls. Kinetic studies of activated microsomal bilirubin UDPglucuronosyltransferase showed that apparent Km values were similar; fasting acted only by increasing V. The results suggest that the changes in bilirubin glucoronosyltransferase activity provoked by starvation may reflect actual enzyme induction; they favour the multiplicity of the UDPglucuronosyltransferase system.