Effects of phenobarbital, beta-naphthoflavone, dexamethasone, and formamidoxime on the turnover of inducible microsomal proteins in cultured hepatocytes.

Microsomal proteins from cultured chick embryo hepatocytes were separated by polyacrylamide gel electrophoresis and their rate constants of degradation (Kd) were estimated using double isotope techniques. The proteins were found to be heterogeneous in their turnover rates, proteins, or subunits, with higher molecular weights being more rapidly degraded than those with lower molecular weights. The Kd values of three selected microsomal proteins with Mr 52,000, 54,000, and 56,000 presumed to represent cytochrome P-450 apoproteins were found unaltered following treatment of cultured hepatocytes with inducers of cytochrome P-450 such as phenobarbital or beta-naphthoflavone. Based on their Kd values, the half-lives of these inducible microsomal proteins were estimated to be 10.4, 9.6, and 11.3 h. The response to phenobarbital was markedly modified when cells were either pretreated with dexamethasone or formamidoxime, agents inhibiting replicative DNA synthesis. Under these conditions, the phenobarbital-mediated de novo synthesis of several microsomal proteins was enhanced. Our turnover data may offer an explanation for the rapid changes in cytochrome P-450 concentration and function following drug exposure of cultured chick embryo hepatocytes.