The reversible activation by Mn2+ ions of the Ca2+-requiring neutral proteinase of human erythrocytes

Mn2+ (50 microM) satisfies the requirement for activity of the purified Ca2+-dependent neutral proteinase from human erythrocytes. Unlike the activation by Ca2+ [E. Melloni et al. (1984) Biochem. Int. 8, 477-489], the effect of Mn2+ is fully reversible and does not involve autodigestion of the native 80-kDa catalytic subunit. However, the native dimeric proenzyme (procalpain), which contains both the 80-kDa subunit and a smaller 30-kDa subunit, is not activated by Mn2+ alone but also requires the presence of micromolar concentrations of Ca2+. Under these conditions, 40% of the maximum activity is expressed without dissociation of the 80- and 30-kDa subunits. Mn2+, but not micromolar Ca2+, can also partially satisfy the metal requirement of the native 80-kDa subunit isolated after dissociation of the heterodimer. This activity is further enhanced by the addition of 5 microM Ca2+, which is ineffective in the absence of Mn2+. After procalpain is converted to active calpain by incubation with Ca2+ and substrate [S. Pontremoli et al. (1984) Biochem. Biophys. Res. Commun. 123, 331-337] full activity is observed with 5 microM Mn2+, which now substitutes completely for Ca2+. Activation of procalpain by Mn2+ represents a new mechanism for modulation of the Ca2+-dependent proteinase activity.