Ca2+-dependent Neutral Proteinase from Human Erythrocytes: Activation by Ca2+ Ions and Substrate and Regulation by the Endogenous Inhibitor

Ca2+-dependent neutral proteinase purifies from human erythrocytes as an inactive proenzyme, that can be converted in an active low Ca2+ requiring form either by high concentrations of Ca2+ (0.1-1 mM) in the absence of the substrate, or by low concentrations of Ca2+ (1-5 microM) in the presence of digestible substrates. Activation requires dissociation to constituent inactive proenzyme subunits which are then converted to the active proteinase species still retaining their monomeric structure. The activation process produced by high Ca2+ concentrations is controlled by the endogenous inhibitor which also dissociates into constituent subunits in order to exert its inhibitory effect. An additional regulation of the activated proteinase involves an autoproteolytic process, Ca2+ and substrate dependent, producing enzyme inactivation.