In the Reuber (H35) hepatoma cell strain, microsomal aryl hydrocarbon (benzo[a]pyrene) hydroxylase is induced 25-fold by the polycyclic hydrocarbon benz[a]anthracene but is not induced by the steroid hormone dexamethasone. Soluble tyrosine aminotransferase is induced sixfold by dexamethasone and twofold by benz[a]anthracene. Each enzyme requires similar inducer concentrations for induction, and their induction kinetics are similar. The induction of each enzyme requires RNA and protein synthesis; in each case the transcriptional and translational steps can occur independently. The two induction systems are differentially sensitive to inhibitors of macromolecular synthesis. Simultaneous exposure to both inducers produces increases in both enzyme activities that are greater than those produced by either inducer alone. Each inducer acts at a pretranslational level to produce this synergistic effect. The results suggest that the requirements for macromolecular synthesis are similar for the induction of each enzyme, but that the turnover of enzyme-specific macromolecules may differ for each.

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