Transport of the precursor for sulfite oxidase into intermembrane space of liver mitochondria: binding of the precursor to outer mitochondrial membrane.

Rat liver mitochondria were incubated with in vitro translation products programmed by liver RNA, then disrupted by sonication and subjected to sucrose density gradient centrifugation. Pre-sulfite oxidase bound preferentially to the outer mitochondrial membrane recovered with the inner membrane. This outer membrane could not be separated from the inner membrane by recentrifugation, suggesting the tight association of both membranes. The binding was not affected by pretreatment of mitochondria with proteolytic enzymes. The mature enzyme and its precursor synthesized in isolated hepatocytes have isoelectric points of 4.2 and 5.5, respectively. The molecular size of the precursor in cytosol was estimated to be about 100,000 daltons (dimer) by gel filtration.