Characteristics of microsomal enzyme controls in primary cultures of rat hepatocytes

Abstract Cytochrome P -450, NADPH-cytochrome c reductase, biphenyl hydroxylase, and epoxide hydratase have been compared in intact rat liver and in primary hepatocyte cultures. After 10 days in culture, microsomal NADPH-cytochrome c reductase and epoxide hydratase activities declined to a third of the liver value, while cytochrome P -450 decreased to less than a tenth. Differences in the products of benzo[ a ]pyrene metabolism and gel electrophoresis of the microsomes indicated a change in the dominant form(s) of cytochrome P -450 in the cultured hepatocytes. Exposure of the cultured cells to phenobarbital for 5 days resulted in a threefold induction in NADPH-cytochrome c reductase and epoxide hydratase activities which was typical of liver induction of these enzymes. Exposure of the cells to 3-methylcholanthrene did not affect these activities. Cytochrome P -450 was induced over two times by phenobarbital and three to four times by 3-methylcholanthrene. The λ max of the reduced carbon monoxide complex (450.7 nm) and analysis of microsomes by gel electrophoresis showed that the phenobarbital-induced cytochrome P -450 was different from the species induced by 3-methylcholanthrene (reduced carbon monoxide λ max = 447.9 nm). However, metabolism of benzo[ a ]pyrene (specific activity and product distribution) was similar in microsomes of control and phenobarbital- and 3-methylcholan-threne-induced hepatocytes and the specific activity per nmole of cytochrome P -450 was higher than in liver microsomes. The activities for 2- and 4-hydroxylation of biphenyl were undetectable in all hepatocyte microsomes even though both activities were induced by 3-methylcholanthrene in the liver. Substrate-induced difference spectra and gel electrophoresis indicated an absence in phenobarbital-induced hepatocytes of most forms of cytochrome P -450 which were present in phenobarbital-induced rat liver microsomes. It is concluded that the control of cytochrome P -450 synthesis in these hepatocytes is considerably different from that found in whole liver, while other microsomal enzymes may be near to normal. Hormonal deficiencies in the culture medium and differential hormonal control of the various microsomal enzymes provide a likely explanation of these effects.