Effects of polyriboinosinic acid · polyribocytidylic acid and a mouse interferon preparation on cytochrome P-450-dependent monooxygenase systems in cultures of primary mouse hepatocytes

Previous studies from our laboratory have shown that a variety of interferon-inducing agents depress cytochrome P-450-dependent monooxygenase systems when administered to rats. With the expectation that the use of cultured hepatocytes would provide a more accessible means of studying the mechanism by which interferon-inducing agents depress these enzyme systems, measurements were made of the effects of the interferon inducer poly rI.rC and a crude preparation of mouse interferon on the cytochrome P-450 content and aminopyrine N-demethylase and benzo[a]pyrene hydroxylase activities of primary, nonreplicating mouse hepatocytes maintained on floating collagen membranes. During the first 24 hr of culture, hepatocytes lost about 80% of their cytochrome P-450, 97% of their aminopyrine N-demethylase activity, and 90% of their benzo[a]pyrene hydroxylase activity. The specific activity of cytochrome P-450 (nanomoles of [¹⁴C]formaldehyde formed from the N-demethylation of aminopyrine per nanomole of P-450 per minute) was lowered from 4.6 to as little as 1/10 this value. Exposure of the cultures to poly rI.rC (5 μg/ml of culture) during the second 24 hr of culture caused a 40% increase in the cytochrome P-450 content of the hepatocytes. The specific activity of cytochrome P-450 de novo relative to the N-demethylation of aminopyrine was restored to that of the cytochrome P-450 of freshly isolated hepatocytes or the cytochrome P-450 of microsomes isolated from liver homogenates. The mouse interferon preparation (1000 units/ml of culture medium) was considerably less potent as an inducer of cytochrome P-450 in cultured hepatocytes, but the specific activity of the cytochrome P-450 de novo induced by mouse interferon was as high as that of the cytochrome P-450 induced by poly rI.rC. Relative to benzo[a]pyrene hydroxylase activity, the specific activity of the cytochrome P-450 that survived the first 24 hr of culture was about the same as that for the cytochrome P-450 of microsomes isolated from liver homogenates. Both poly rI.rC and mouse interferon preparation induced the hydroxylase activity in 24-hr-old cultures of hepatocytes. Poly rI.rC did not induce the hydroxylase activity in cultured Reuber hepatoma cells. Poly rI.rC was shown to induce interferon activity in cultured hepatocytes. Methods are described for the determination of aminopyrine N-demethylase and sensitive azoreductase, which is inhibited by dicoumarol and thought to be DT diaphorase (EC 1.6.99.2). In 9000 X g rat hepatic supernatant the relative importance of the oxygen-sensitive and -insensitive mechanisms depends on the substrate. Even microsomal azoreductase can be nearly oxygen-insensitive, as in the case of butter yellow azo reduction. Previous investigations of the oxygen reactivity of the azo anion free radical make it unlikely that oxygen-insensitive azo reductases form the azo anion free radical intermediate. These azoreductases presumably reduce azo compounds by a 2-electron transfer to form the hydrazine intermediate directly on the surface of the enzyme.