title: Incorporation of heme to microsomal cytochrome P-450 in the absence of protein biosynthesis.
abstract: Determination of the heme and protein portions of phenobarbital (PB)-inducible and 3-methylcholanthrene inducible forms of cytochrome P-450, P-450(PB-1), and P-450(MC-1), in the liver microsomes of drug-treated animals indicated the presence of 20-30% of apo-cytochrome P-450 in both cases. Inhibition of protein synthesis by cycloheximide injection to the rats did not significantly inhibit the incorporation of delta-amino[14C]levulinic acid (ALA) into the heme of P-450(PB-1) or P-450(MC-1) in the liver, indicating that the heme incorporation into microsomal cytochrome P-450 is not tightly coupled with the synthesis of the apo-cytochrome. When heme-labeled cytosol prepared from [14C]ALA-injected rats was incubated with non-radioactive microsomes in vitro, a significant amount of labeled heme was incorporated into microsomal P-450(PB-1), whereas the incorporation into P-450(MC-1) was much less. The in vitro transfer of heme from cytosol to microsome-bound cytochrome P-450 was stimulated by the addition of an NADPH-generating system to the incubation mixtures, and inhibited when the microsomes were solubilized with sodium cholate and Emulgen-913. Although the in vitro incubation of heme-labeled microsomes with non-radioactive cytosol resulted in some release of labeled heme from the microsomes, no reversible transfer of heme between cytochrome P-450 molecules bound to separate microsomal vesicles was detected when heme-labeled microsomes were incubated with non-radioactive microsomes in the presence and absence of cytosol.
mesh_terms: D000622:Aminolevulinic Acid / Q000378:metabolism; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003573:Cytochrome b Group / Q000032:analysis; D015786:Cytochromes b5; D003600:Cytosol / Q000201:enzymology; D006418:Heme / Q000096:biosynthesis / Q000378:metabolism*; D066298:In Vitro Techniques; D007930:Leucine / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D010634:Phenobarbital / Q000494:pharmacology; D011233:Precipitin Tests; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D003573:Cytochrome b Group; D006418:Heme; D000622:Aminolevulinic Acid; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D007930:Leucine; D010634:Phenobarbital