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

DS Riddick, SS Park, HV Gelboin and GS Marks

Department of Pharmacology and Toxicology, Queen's University, Kingston, Ontario, Canada.

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.

Volume 37, Issue 1, pp. 130-136, 01/01/1990
Copyright © 1990 by American Society for Pharmacology and Experimental Therapeutics

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