Incorporation of Heme to Microsomal Cytochrome P-450 in the Absence of Protein Biosynthesis

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.