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Human erythrocytes contain a calpain activator protein with a molecular mass of approx. 40 kDa. The activator is present in association with the plasma membrane and promotes expression of calpain activity at a concentration of Ca2+ close to physiological values. The initial step of the activating mechanism involves association of the activator with calpain, followed by autoproteolytic activation of the proteinase in the presence of 1 microM Ca2+, at a rate identical to that induced by 1 mM Ca2+. In a reconstituted system, the activator binds to erythrocyte membranes, but not to phospholipid vesicles, suggesting the participation of an intrinsic membrane protein(s). In its membrane-associated form the activator selectively binds calpain, thus favouring interaction of the proteinase with the inner surface of plasma membranes. These results further confirm the importance of a natural activator protein in promoting intracellular activation of calpain under physiological conditions through a site-directed mechanism, which explains the high specificity of the proteinase for membrane of cytoskeletal proteins.

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The production of superoxide anion in human and rat neutrophils is directly correlated to the level of protein kinase C. Such correlation has been established on a comparative basis by analysis of neutrophils from normal and hypertensive subjects, characterized by an increased amount of protein kinase C, and of neutrophils from normal and genetically hypertensive rats characterized by low amounts of the kinase.Protein kinase C activity in all these different populations of neutrophils is modulated by specific inhibitors in an identical dose-dependent fashion which results in a linearly correlated decrease in 0; production. Taken together, these results provide a direct demonstration that in neutrophils the intracellular level of protein kinase C represents one of the determinants of the rate and extent of 0; production.

No abstract