Quantitative comparison between aminophospholipid translocase activity in human erythrocytes and in K562 cells

Spin-labeled phospholipids were used to determine the transbilayer movement of phospholipids in human erythrocytes, in K562 cells and in human neonatal red cells. The erythroleukemia cell line, K562, as well as human neonatal red cells, which are rich in reticulocytes, were considered as representative of human erythrocyte precursor cells. In the nucleated cells, the difference between outside-inside movement of aminophospholipids and that of phosphatidylcholine or sphingomyelin analogues allowed us to discriminate between lipid internalization due to aminophospholipid translocase activity and to endocytosis. From the initial rates of aminophospholipid inward movement, we inferred that the activity of the aminophospholipid translocase is higher in the precursor cells than in mature erythrocytes.