The transfer of heme from cytochrome P-450 to human serum albumin and rabbit apohermopexin was investigated. These studies were conducted using both membrane-bound and solubilized cytochrome P-450 preparations, as well as the modified form of time hemoprotein, cytochrome P-420. Data indicate that the plasma heme-binding proteins used in this study were unable to remove heme from either membrane-bound or solubilized preparations of cytochrome P-450; in contrast, the heme of cytochrome P-420 present in the above preparations was readily transferred to both serum albumin and apohemopexin. These findings suggest that a barrier exists for heme transfer from cytochrome P-450 to the heme-binding proteins. This barrier is attributed to the shielding effect exerted on the prosthetic group by the tertiary structure of the approtein. This shielding effect in turn imposes selectivity on the substances which may come in contact with the heme moiety of the cytochrome. Furthermore, hemopexin was found, in all of the preparations studied, to have a greater affinity for for the prosthetic group of cytochrome P-420 than serum albumin. Accordingly, hemopexin is indicated as the possible intra- and intercellular heme transfer protein for the microsomal hemoprotein, cytochrome P-420.
ACKNOWLEDGMENTS The authors thank Jenny Soechting for her technical assistance.
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