The exchange of erythrocyte membrane phospholipids with rat liver extracts in vitro

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Abstract
Intact rat or human erythrocytes and their isolated (ghost) membranes were incubated with the high speed supernatant fraction of homogenates derived from 32P-labeled rat livers. Phospholipid molecules were transferred between the red cell membranes and the liver extracts, as reflected by the convergence of their specific radioactivities with time. Whereas ghosts usually approached isotopic equilibrium with the liver supernatant fraction during a few hours of incubation at 37° C, the exchange of phospholipids by intact cells was no more than one-half, even after 18 hr. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin were all exchanged in both intact cells and ghosts, albeit to different extents. (A control experiment, incubating 32P-labeled rat erythrocytes or ghosts with unlabeled rat liver extracts, also demonstrated the exchange of all four major phospholipids.) These data may signify that the phospholipids on the cytoplasmic side of the membrane of intact erythrocytes do not exchange with the phospholipids in exogenous liver extracts. If so, all four major phospholipid classes would appear to be present to some extent at both membrane surfaces. The first inference is in agreement with several other studies on this membrane, while the second inference is not.