Structural and functional reconstitution of hepatic cytochrome P-450 in vivo. Reversal of allylisopropylacetamide-mediated destruction of the hemoprotein by exogenous heme.

Abstract

After the heme moiety of hepatic cytochrome P-450 in rats had been destroyed by allylisopropylacetamide (AIA), intravenously injected heme significantly increased the cytochrome P-450 content and mixed function oxidase activity of liver microsomes. (The term "heme" is used throughout this paper for iron protorphyrin IX, irrespective of its redox state). In the absence of AIA treatment, heme administration failed to elicit these effects. The heme-mediated increase of cytochrome P-450 was most pronounced in phenobarbital-pretreated rats (46%), but was detectable also in 3-methylcholanthrene-pretreated (6%) and in untreated animals (7%). Restoration of mixed function oxidase activity by heme appeared to depend on reconstitution of cytochrome P-450 and was greatest for oxidases whose activity is stimulated by phenobarbital. Under optimal experimental conditions, heme enhanced the residual activity of ethylmorphine N-demethylase 3-fold when 92% of the initial enzyme activity had been destroyed by AIA. Reconstitution of cytochrome P-450 was dependent on the relative amount of cytochrome P-450 destroyed by AIA and on the dose of heme administered. Formation of holocytochrome P-450 appeared to be independent of apoprotein synthesis as it was unaffected by cycloheximide. Thus, reconstitution of cytochrome P-450 following AIA-mediated destruction of its original prosthetic heme reflects direct incorporation of administered heme into residual apoprotein in the endoplasmic reticulum. These findings demonstrate that exogenous heme can partially restore the structure and function of hepatic cytochrome P-450 following its drug-mediated destruction.