Transbilayer distribution and mobility of phosphatidylcholine in intact erythrocyte membranes. A study with phosphatidylcholine exchange protein.

1. The exchange of phosphatidylcholine between intact human or rat erythrocytes and rat liver microsomes was greatly stimulated by phosphatidylcholine-specific exchange proteins from rat liver and beef liver. It was found, however, that compared to the exchange reaction between phospholipid vesicles and rat liver microsomes, much higher concentrations of exchange protein were required in the case of intact red blood cells and microsomes. 2. In human erythrocytes, 75% of the phosphatidylcholine was available for exchange within 2 h at 37 degrees C. No additional exchange was observed during the next 2 h, indicating slow, if any, transbilayer movement of the residual phosphatidylcholine. 3. In rat erythrocytes 50--60% of the phosphatidylcholine was readily available for the exchange proteins. The residual phosphatidylcholine was exchanged at a much lower rate with a half time for equilibration of 7 h. 4. These results confirm in an independent way the asymmetric distribution of phosphatidylcholine over the membrane of human and rat erythrocytes as well as the occurrence of a slow transbilayer movement of this lipid in rat erythrocytes.