Proteolytic activity in erythrocyte precursors.

Thalassemia is characterized by unequal rates of synthesis of the alpha and beta globin chains that are part of the hemoglobin tetramer. In the type of thalassemia due to a defect in beta-chain synthesis (beta-thalassemia), this imbalance results in a relative exoess of alpha-chains. We have studied the susceptibility of excess free alpha-chains to proteolysis. Incubation of isotopically labeled peripheral blood lysates from individuals with beta-thalassemia trait in the presence of bone marrow or normoblast lysates from thalassemic or hematologically normal individuals resulted in a decrease in the alpha/beta ratio and a loss of free alpha-chain radioactivity. Neither contamination with leukocytes nor higher ATP contents in young erythrocytes appeared to be responsible for this activity in normoblasts and bone marrow. We propose that erythroid precursor cells possess proteolytic activity that is markedly diminished in mature cells. This activity serves an important control function in the regulation of hemoglobin synthesis. It accounts at least in part for the more balanced synthesis of alpha- and beta-chains observed in bone marrow than in peripheral blood in heterozygous beta-thalassemia. It also plays a fine-tuning role in maintaining balanced synthesis in non-thalassemic erythrocytes.