An isotopically labeled cross-linking reagent, succinimido 3-(3-[125I]iodo-4-azidophenyl)propionate, has been synthesized and coupled to 1-acyl-2-(aminocaproyl)phosphatidylcholine according to previously described procedures [Schroit, A. J., & Madsen, J. (1983) Biochemistry 22, 3617–3623]. 125I- and N3-labeled phosphatidylserine (125I-N3-PS) was produced from the phosphatidylcholine (PC) analogue by phospholipase D catalyzed base exchange in the presence of L-serine. These phospholipid analogues are photoactivatable, are labeled with 125I at high specific activity, completely incorporate into synthetic vesicles, and spontaneously transfer between membranes. When an excess of acceptor vesicles or red blood cells (RBC) was mixed with a population of donor vesicles containing the 125I-N3-phospholipids, approximately 40% of the analogues transferred to the acceptor population. After transfer in the dark to RBC, all of the 125I-N3-PC incorporated into the cells could be removed by washing with serum, whereas the 125I-N3-PS could not. After photolabeling of intact RBC, ~50% of the PC and 20% of the PS cross-linked to membrane proteins as determined by their insolubility in CHCl3/MeOH. Analysis of probe distribution by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that 125I-N3-PS preferentially labeled a Mr 30000 peptide which contained ~30% of the protein-bound label.
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