INHIBITION AND INDUCTION OF CYTOCHROME-P450 ISOENZYMES IN RAT LUNG

The O-dealkylation of pentoxyresorufin, a substrate for P450 2B1, was decreased in lung microsomes from rats dosed with O,O,S-trimethylphosphorodithioate, 0,0,0-trimethylphosphorothioate, bromophos, fenitrothion, p-xylene and 2,4-dichloro-(6-phenylphonoxy)ethylamine. This activity was decreased by antibodies to P450 2B1 but unaffected by antibodies to P450 1A1 or 4B1. This reduction reflected both inactivation and destruction of P450 2B1; destruction of this protein was particularly marked after bromophos and fenitrothion. Pyrazole was the only compound in this study to induce the 0-dealkylation of pentoxyresorufin. None of these compounds altered the rate of ethoxyresorufin O-dealkylation, an indicator of P450 1A1 activity, but this activity was induced greatly by both Aroclor and beta-naphthoflavone. p-Xylene was the only compound to decrease P450 4B1 activity, as determined by the N-hydroxylation of 2-aminofluorene. In the liver, bromophos, fenitrothion, p-xylene and 2,4-dichloro-(6-phenylphonoxy)ethylamine all had marked effects on the 0-dealkylation of ethoxyresorufin and pentoxyresorufin but, at the dose used, O,O,O-trimethylphosphorothioate and O,O,S-trimethylphosphorodithioate had minimal effects in this tissue. Thus, both O,O,O-trimethylphosphorothioate and O,O,S-trimethylphosphorodithioate are exquisitely selective inhibitors of pulmonary P450 2B1 activity. Their use, together with pyrazole, will facilitate future studies of the pulmonary activation of toxins by P450 2B1.