Tissue, subcellular, and submitochondrial distributions of semidehydroascorbate reductase: possible role of semidehydroascorbate reductase in cofactor regeneration.

The immediate product of ascorbate oxidation coupled to dopamine-beta-hydroxylation is not dehydroascorbate, as previously thought, but rather semidehydroascorbate. For this reason, the possible participation of the enzyme semidehydroascorbate reductase (SDR) in cofactor regeneration was investigated. In the adrenal medulla, the primary subcellular localization of this reductase was shown to be in the mitochondria. Submitochondrial fractionation studies indicated that SDR is an outer membrane protein. Thus, although dopamine-beta-hydroxylase and SDR have different subcellular localizations, a physiological role for SDR in beta-hydroxylation still appears plausible through reduction of cytosolic semidehydroascorbate. The specific activities of SDR in various rat and guinea pig tissues appear to parallel their ascorbate contents, suggesting a similar participation of SDR in ascorbate metabolism in other tissues.