Differences and similarities among three acidic endopeptidases associated with human erythrocyte membranes. Molecular...

The three acidic proteinases (designated I, II and III, respectively) associated with human erythrocyte membranes were solubilized and purified to an electrophoretically homogeneous state by conventional procedures. Comparative analysis of chemical properties, including amino acid composition and fragmentation by cyanogen bromide cleavage, revealed significant differences among proteinases I, II and III. On the other hand, complete identity among the three proteolytic enzymes was observed on the basis of the peptide bonds specifically hydrolyzed in both glucagon- and phenylalanine-deprived oxidized B chain of insulin. In fact, each of the three proteinases produced splitting of the glucagon molecule between phenylalanine-22 and valine-23, while the susceptible bonds in the oxidized B chain of insulin proved to be those between leucine-15 and tyrosine-16 and between phenylalanine-25 and tyrosine-26, respectively.