EVIDENCE FOR SULFITE OXIDASE ACTIVITY IN SPINACH LEAVES

The present paper provides evidence that spinach chloroplasts possess a sulphite oxidase activity coupled with oxygen consumption and reduct ion of ferricyanide. This activity is associated with thylakoids and s olubilized by non-ionic biological detergents. The pH and temperature dependencies of sulphite oxidase activity solubilized by Triton X-100 from spinach thylakoids were consistent with those of an intrinsic mem brane protein. This isolated activity was insensitive towards radical scavengers (mannitol, mannose and fructose) and catalase, and was inhi bited only with very high concentrations of superoxide dismutase. Thus , observed sulphite oxidation was not induced through the photosynthet ic electron transport system, but achieved via a thylakoid membrane en zymic system showing a sulphite oxidase activity. Kinetic parameters o f thylakoid sulphite oxidase were measured and compared with those of other sulphite oxidases.