Regulation of cytochrome P-450 monooxygenases in the mouse

Abstract

The mechanism of induction of cytochrome P-450 monooxygenasees by phenobarbital and a variety of structurally diverse xenobiotics (e.g., polyhalogenated biphenyls, organochlorine pesticides) is not well understood. Recently, the compound 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) has been identified as a highly potent phenobarbital-like agonist in mice. This finding has led to the suggestion that a receptor-mediated process may govern the induction of cytochrome P-450 monooxygenases by phenobarbital and phenobarbital-like agonists. The work presented in the manuscript has addressed this suggestion by examining: (1) the effect of structural alteration of the TCPOBAP molecule on enzyme induction activity, (2) the induction response to phenobarbital and TCPOBAP among inbred mouse strains, (3) the spectrum of monooxygenase activities induced by phenobarbital and TCPOBOP compared to 3-methylcholantherene, isosafrole and pregnenolone 16α-carbonitrile (PCN) and (4) the binding of [3H] TCPOBOP in hepatic cytosol. The results of the structure-activity experiments illustrate that changes in the structure of the pyridyloxy or benzene ring markedly affect enzyme induction activity and provide additional indirect evidence for a receptor-mediated response. An evaluation of monooxygenase induction by TCPOBOP for 27 inbred mouse strains and by phenobarbital for 15 inbred mouse strains failed to identify a strain which was completely nonresponsive to these compounds, although several strains exhibited decreased responsiveness for select monooxygenase reaction. TCPOCOP, PCN and phenobarbital were al found to significantly increase the rate of hydroxylation of testosterone at the 2α-, 6β- and 15β positions buy only TCPOBOP and phenobarbital dramatically increased the rate of phentoxyresorufin O-dealkylation. This result demonstrates that TCPOBOP most closely resembles phenobarbital in its mode of monooxygenase induction in mice. Sucrose density gradient analysis of [3H] TCPOBOP-hepatic cytosol incubation failed to identify specific, saturable binding of [3H] TCPOBOP to cytosolic macromolecular elements...