Labeling of human erythrocyte membrane proteins by photoactivatable radioiodinated phosphatidylcholine and phosphatidylserine. A search for the aminophospholipid translocase.

We have synthesized radioiodinated photoactivatable phosphatidylcholine (125I-N3-PC) and phosphatidylserine (125I-N3-PS). After incubation with red blood cells in the dark, the labeled PC could be extracted but not the corresponding PS molecule, indicating that the latter was transported by the aminophospholipid translocase, but not the former. When irradiated immediately after incorporation, N3-PS, but not N3-PC, partially blocked subsequent translocation of spin-labeled aminophospholipids. Analysis of probe distribution by SDS-polyacrylamide gel electrophoresis revealed that 125I-N3-PS labeled seven membrane bound components with molecular masses between 140 and 27 kDa: one (or several) of these components should correspond to the aminophospholipid translocase.