Distribution of Hepatic Sulfite Oxidase among Subcellular Organelles and Its Intraorganelle Localization

Abstract

1. The subcellular distribution of sulfite oxidase was investigated in homogenates of rat liver. The activity, as measured by sulfite-dependent cytochrome c reduction, exhibited a structure-linked latency. The distribution pattern of the activity unmasked by hypotonic treatment was quite similar to that of a mitochondrial marker enzyme, succinate-cytochrome c reductase. The activity after the hypotonic treatment was further activated by low concentrations of detergents such as sodium deoxycholate, and this detergent-unmasked activity was mainly localized in microsomes. 80% of the fully unmasked activity was localized in mitochondria and 10% each in microsomes and soluble fraction.
2. Sulfite oxidases present in mitochondria, microsomes, and soluble fraction showed the same behavior on DEAE-Sephadex column chromatography and gel filtration with Sephadex G-200, and gave identical K_m values for sulfite and cytochrome c.
By using a rabbit antiserum against purified mitochondrial sulfite oxidase, immunochemical comparison was also carried out. Ouchterlony double diffusion tests in agar gel, quantitative immunoprecipitation tests, and inhibition studies of enzyme activity indicated that the antigenic properties of these three sulfite oxidase preparations were identical.
3. The sulfite-cytochrome c reductase activity in microsomes was in a latent state, and its activity was greatly stimulated by low concentrations of detergents or by sonication; the activation was accompanied by solubilization of the enzyme.
Antibody and trypsin failed to attack the enzyme in intact microsomes, whereas the solu-bilized enzyme was susceptible to inhibition by the antibody and to digestion by trypsin.
These findings suggest an internal localization of the sulfite oxidase in microsomal vesicles.