Transbilayer movement of various phosphatidylcholine species in intact human erythrocytes

Phosphatidylcholine specific phospholipid exchange protein was used to introduce (14C)-labeled phosphatidylcholine of different fatty acyl compositions into the intact human erythrocyte. Hydrolysis by a combination of phospholipase A2 and sphingomyelinase was applied to prove that originally all newly introduced phosphatidylcholine resided in the outer monolayer. Subsequently the erythrocytes were reincubated at 37°C, and redistribution of the introduced (14C)phosphatidylcholine was monitored by applying the combination of phospholipases after different times of incubation. In the situation where 20% of the native erythrocyte phosphatidylcholine had been replaced by phosphatidylcholine from (14C)choline-labeled rat liver microsomal membranes, a slow translocation of the (14C)microsomal phosphatidylcholine was found, with a half-time of transbilayer equilibration of 10.8 hr. Furthermore, the transbilayer movement of probe amounts of (14C)dipalmitoyl-phosphatidylcholine, (14C)egg phosphatidylcholine and (14C)soybean phosphatidylcholine was studied under conditions whereby the fatty acyl composition of the bulk erythrocyte phosphatidylcholine remained unchanged. In correlation to the increasing unsaturation of the probe, half-times for the transbilayer equilibration were calculated to be 26.9, 12.8, and 8.1 hr, respectively.