Protease and peptidase activities during reticulocyte maturation.

Rabbit reticulocytes were fractionated according to buoyant density on albumin gradients. The properties of fractionated cells were consistent with the thesis that the procedure separated the reticulocytes according to their degree of maturation. Catabolism of aminoethylcysteine-induced abnormal protein and puromycin-peptides declined with increasing cellular maturity. Cell-free extracts showed a similar decrease in degradative activity: addition of extracts of young cells stimulated proteolysis in extracts from old cells. Peptidase activity also declined during maturation. In the more mature cells abnormal protein was located increasingly in the cell-debris fraction, analogous to Heinz bodies.