Influence of enzymatic phospholipid cleavage on the permeability of the erythrocyte membrane: III. Discrimination between the causal role of split products and of lecithin removal.

Cleavage of 55% of the lecithin in intact human erythrocytes by phospholipase A2 (bee venom) markedly inhibits the mediated transport of L-lactate (via the monocarboxylate carrier) and of L-arabinose (via the monosaccharide carrier), while the major anion exchange system (probed by oxalate) and diffusion via the lipid domain (probed by erythritol) remain essentially unaltered. the causal role of the split products, unsaturated fatty acids and saturated lysolecithin, and of lecithin removal were now studied by sequential extraction of split products with serum albumin and by their controlled insertion into normal membranes. Careful choice of the albumin-to-cell ration allowed the extraction of more than 95% or the fatty acids and up to 80% of the lysolecithin without hemolysis. Extraction of fatty acids abolished inhibition of lactate and arabinose transfer, but induced inhibition of anion exchange and translipid permeation. Subsequent extraction of lysolecithin produced no further effects except on lactate transfer, which was inhibited. Exogenous oleic and linoleic acid, at intramembrane concentrations equal to those produced by phospholipase A2, inhibit lactate and arabinose transfer, while accelerating oxalate and erythritol movements, in agreement with effects of endogenous fatty acids. Exogenous lysolecithin inhibits all mediated transfer processes but does not alter translipid permeation. This pattern differs from that obtained for endogenous lysolecithin. The action of exogenous lysolecithin can be suppressed by loading of the cells with cholesterol. Insertion of exogenous lysolecithin into cells depleted of endogenous lysolecithin does not restore the functional state before depletion, indicating that exogenous and endogenous lysolecithin may act differently.