Effects of 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine on hepatic cytochrome P-450 heme, apoproteins, and catalytic activities following in vivo administration to rats.

Various 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC) cause mechanism-based inactivation of cytochrome P-450 (P-450) via heme destruction. We have examined the time course of effects of DDC analogues on the catalytic activities and apoproteins of the major beta-naphthoflavone-, dexamethasone-, and phenobarbital-inducible isozymes of rat liver P-450 following in vivo administration. In beta-naphthoflavone-treated rats, all DDC analogues examined caused loss of the P-450 chromophore and dramatic loss of 7-ethoxyresorufin O-deethylase activity, a catalytic marker for P-450c. The isopropyl, hexyl, and isobutyl analogues caused the most pronounced loss/alteration of P-450c apoprotein levels, as revealed by two monoclonal antibodies (MAbs), 1-31-2 and 1-7-1. The apoprotein of P-450d was not altered. In dexamethasone-treated rats, all analogues except 4-hexyl-DDC caused loss of the P-450 chromophore and erythromycin N-demethylase activity, a catalytic marker for P-450p-related isozymes. Only 4-isopropyl-DDC caused significant loss/alteration of the apoprotein of P-450p-related forms, as revealed by MAb 2-13-1. In phenobarbital-treated rats, all analogues reduced the level of the P-450 chromophore, whereas only 4-hexyl-DDC and 4-isopropyl-DDC lowered 7-pentoxyresorufin O-dealkylase activity, a catalytic marker for P-450b. MAbs 2-66-3 and 2-8-1 revealed no change in the level of phenobarbital-inducible apoproteins recognized by these probes. In agreement with our previous in vitro studies [Mol. Pharmacol. 35;626-634 (1989)], P-450 c and p are targets for mechanism-based inactivation by DDC analogues. However, unlike the situation in vitro, loss of enzyme activity in vivo is, at least in some instances, accompanied by loss/alteration of the corresponding P-450 apoprotein.