Replacement of molecular species of phosphatidylcholine: influence on erythrocyte Na transport

Abstract

The phosphatidylcholine-specific transfer protein (PC-Tp) from bovine liver was used to replace endogeneous erythrocyte phosphatidylcholine (PC) with various amounts of five different molecular species of PC. Furosemide-sensitive (FS) Rb uptake, Na-Li exchange, and Na-K pump rates were considered in relation to the nature and extent of those replacements. Changes in fatty acid contents of PC after incorporation of different molecular species fell within a variation range (10-30%) similar to that found in large populations of healthy individuals. Di16:0-PC accelerated Na-Li exchange and FS Rb uptake by approximately 40 and 25%, respectively. Some reduction (20%) in FS Rb uptake was seen in 16:0/18:2-PC-enriched erythrocytes. Incorporation of 16:0/22:6-PC accelerated Na-Li exchange and FS Rb uptake by greater than 40 and 20%, respectively. Apart from inhibitory effects of 16:0/18:1-PC and di16:0-PC (24 and 19%, respectively) the Na-K pump rate was virtually unchanged by incorporation of different PC molecular species. Exogeneous PC molecules are exclusively inserted in the outer membrane leaflet and, particularly in the case of di16:0-PC, migrate slowly to the cytoplasmic leaflet. Prolonged incubation of cells (up to 21 h) after replacement with di16:0-PC showed that both Na-Li exchange and FS Rb uptake rates responded differently to redistribution of newly inserted molecules over both bilayer halves. Compared with cells exhibiting a selective incorporation of di16:0-PC in the outer monolayer, additional enrichment with disaturated species in the inner monolayer accelerated FS Rb uptake, whereas Na-Li exchange rate reverted to control values. It is concluded that small changes in fatty acid composition of PC induced by limited replacement of phospholipid molecular species can cause considerable changes in Na-Li exchange rate and FS Rb uptake. Differences in phospholipid molecular species composition could contribute to known interindividual variability of both Na-Li exchange and Na-K cotransport.