Activation by hemoglobin of the Ca2+-requiring neutral proteinase of human erythrocytes: structural requirements.

The proenzyme form of the Ca2+-requiring neutral proteinase of human erythrocytes (procalpain) is converted to the active proteinase (calpain) by low concentrations of Ca2+ in the presence of appropriate substrates such as beta-hemoglobin or heme-free beta-globin chains. Modification of these substrates by limited proteolysis with calpain abolishes their ability to promote the conversion of procalpain. A similar requirement for the presence of unmodified beta-hemoglobin or heme-free beta-globin chains is observed for the autocatalytic inactivation of calpain. The conversion of procalpain to calpain is accompanied by a small decrease in the molecular mass of the catalytic subunit, from 80 kDa to 75 kDa; however, the activation is not accelerated by the addition of a small quantity of calpain. The autocatalytic inactivation of active CANP is related to the disappearance of the 75 kDa subunit and the formation of smaller peptide fragments.