The transport of de novo synthesized protoheme into the conventional microsomal fraction and endoplasmic reticulum associated with mitochondria (MAER) was studied by injecting amino[14C]levulinic acid (ALA) into phenobarbital-treated rats to evaluate the role of MAER in the trafficking of heme between mitochondria and endoplasmic reticulum. In mitochondria, the specific radioactivity of the radiolabeled heme reached a maximum level at 4 min after the injection of 14C-ALA. The specific radioactivity in cytosol was about 2-fold lower than that in microsomes, suggesting that the cytosolic pathway of the heme transport from mitochondria to endoplasmic reticulum is not predominant, because the specific radioactivity of heme in cytosol should be higher than that in microsomes if heme is transported mainly through cytosol. MAER showed higher specific radioactivity than the conventional microsomal fraction up to 4 min and thereafter the specific radioactivities in MAER and the conventional microsomal fraction became nearly the same. The extents of decrease in cytochrome P-450 and the radioactivity in microsomes by the treatment with allylisopropylacetamide which destroyed cytochrome P-450 but not cytochrome b5, were essentially the same, suggesting that most of the radiolabeled heme in microsomes was incorporated into cytochrome P-450. These results suggest that MAER is a preferential site for the protoheme transport from mitochondria to endoplasmic reticulum.
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