Comparison of rate of hepatic metabolism in vitro and half-life for antipyrine in vivo in three species.

1. A radiometric assay for the total metabolism of antipyrine in vitro by hepatic microsomal preparations has been developed. 2. Apparent Km and V values for the process were determined in rats, rabbits and in a marsupial (the quokka; Setomix brachyurus). Km values were similar in rats and rabbits (3.5 and 5 mM respectively) but were somewhat lower in quokkas (1.4--1.5 mM). Estimates of V ranged from 49--64 nmol/mg microsomal protein/10 min and were similar in all three species. Pretreatment of rabbits with phenobarbitone significantly increased V without change in Km. 3. Studies in individual rabbits and quokkas showed a good correlation between rate of hepatic microsomal metabolism of antipyrine in vitro and its half-life in vivo.