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Transfer of mitochondrial protoheme to apocytochrome b5 in vitro was accomplished in a reconstitution system consisting of isolated mitochondria (donor) and apocytochrome b5 (acceptor), which required the existence of cytosol. Properties of formed cytochrome b5 were confirmed by its absorption spectra and the function as NADH-cytochrome c reductase. The content of formed cytochrome b5 was dependent on reaction time and the concentration of mitochondrial protoheme, apocytochrome b5, and cytosolic protein. This heme transfer protein was purified to homogeneity and identified with glutathione S-transferases (GSTs), by their same elution patterns in column chromatographies and the same degree of inhibited activities on the immunotitration study. Double immunodiffusion analysis revealed this protein to be GST-C2 (Yb' Yb'). These observations lead to the conclusion that Yb' subunit of GST located in the cytosol of rat liver stimulates the transfer of mitochondrial protoheme to apocytochrome b5, which indicates that GST has an unrecognized function as yet, involving on the biosynthesis of microsomal cytochrome b5. |
