Biosynthesis of sulfite oxidase in rat liver - presence of a large precursor form of the enzyme in cytosol.

Abstract

The time courses of in vivo incorporation of [3H]leucine into mitochondrial, microsomal, and cytosolic sulfite oxidase were examined and it was found that the cytosol contained a precursor pool of newly synthesized sulfite oxidase which was rapidly transport into mitochondria. At 10 min after the injection of the radioactive leucine, most of the radioactivity of cytosolic sulfite oxidase was associated with the precursor whose molecular size was about 3,000 daltons larger than the authentic sulfite oxidase subunit. We concluded that sulfite oxidase was synthesized as a larger precursor in the cytosol, and transported into mitochondria to be converted to the mature enzyme. The turnover rate of the sulfite oxidase in rat liver was also determined by measuring the decay of enzyme radioactivity after the injection of [14C]leucine. Mitochondrial, microsomal, and cytosolic sulfite oxidase had an almost identical half-life of 3-4 days.