Superoxide dismutase is preferentially degraded by a proteolytic system from red blood cells following oxidative modification by hydrogen peroxide

Abstract

The cupro-zinc enzyme superoxide dismutase (SOD) undergoes an irreversible (oxidative) inactivation when exposed to its product, hydrogen peroxide (H 2O 2). Recent studies have shown that several oxidatively modified proteins (e.g., hemoglobin, albumin, catalase, etc.) are preferentially degraded by a novel proteolytic pathway in the red blood cell. We report that bovine SOD is oxidatively inactivated by exposure to H 2O 2, and that the inactivated enzyme is selectively degraded by proteolytic enzymes in cell-free extracts of bovine erythrocytes. For example, 95% inactivation of SOD by 1.5 mM H 2O 2 was accompanied by a 106 fold increase in the proteolytic susceptibility of the enzyme during (a subsequent) incubation with red cell extract. Both SOD inactivation and proteolytic susceptibility increased with H 2O 2 concentration and/or time of exposure to H 2O 2. Pre-incubation of red cell extracts with metal chelators, serine reagents, or sulfhydryl reagents inhibited the (subsequent) preferential degradation of H 2O 2-modified SOD. Furthermore, a slight inhibition of degradation was observed with the addition of ATP. We suggest that H 2O 2-inactivated SOD is recognized and preferentially degraded by the same. ATP-independent, metallo- serine- and sulfyhydryl- proteinase pathway which degrades other oxidatively denatured red cell proteins. Related work in this laboratory suggests that this novel proteolytic pathway may actually consist of a 700 kDa enzyme complex of proteolytic activities. Mature red cells have no capacity for de novo proteins synthesis but do have extremely high concentrations of SOD. Red cell SOD generates (and is, therefore, exposed to) H 2O 2 on a continuous basis, by dismutation of superoxide (from hemoglobin autooxidation and the interaction of hemoglobin with numerous xenobiotics). We suggest that preferential degradation of H 2O 2-modified SOD may protect erythrocytes against the accumulation of oxidatively denatured SOD aggregates, and potentially toxic protein fragments.