Self-Digestion ofIluman Erythrocyte Membranes

Sialic acid residues are responsible for the negative charge at the cell surface in human erythrocytes (Eylar et al., 1962), and are involved in Mand N-group specificity (Ebert & Jurgen, 1972). The sialic acid content decreases as erythrocytes age (Balduini et al., 1974; Baxter & Beeley, 1975), and its role as a determinant of the erythrocyte lifespan is described in several reports (Jancik & Schauer, 1974; Durocher et al., 1975; Gattegno et al., 1975). Human erythrocyte 'ghost' membranes can digest their own sialoglycoproteins and release into the incubation medium a glycopeptide containing sialic acid, glucosamine, galactosamine, galactose and mainly polar amino acids (Brovelli et al., 1976). If glycoprotein breakdown has a physiological role in erythrocyte aging, it must also take place in intact cells; therefore we have tested intact human erythrocytes for the ability to digest in vitro their membrane glycoproteins. Moreover, it seems reasonable that a mechanism must exist by which this process is inhibited in the young cell. On the basis of metabolic considerations, the ability of ATP and glutathione to modulate membrane self-digestion