Influence of enzymatic phospholipid cleavage on the permeability of the erythrocyte membrane. I. Transport of non-electrolytes via the lipid domain.

In order to further elucidate the influence of membrane lipids on transport via the lipid domain of the erythrocyte membrane, simple non-electrolyte diffusion was investigated by tracer flux measurements in whole cells after cleavage of up to 65% of phosphatidylcholine or sphingomyelin by phospholipase A2 from Naja naja, or by sphingomyelinase. A new type of labelled model non-electrolyte was used in this study, readily available by reacting a non-labelled thiol with a labelled alkylating SH-reagent. In spite of the marked enzymatic alterations of the membrane, which lead to the occurrence of large quantities of lysophosphatidylcholine and long chain fatty acids, or of ceramide, the permeability of the lipid domain remained unaffected. This finding is very surprising, since the physical properties of the lipid phase (microviscosity, structure of the membrane interface) are likely to be perturbed in the enzyme-treated membranes. Sphingomyelinase-treated cells undergo stomatocytic shape changes followed by deep invaginations of the membrane and finally endocytosis, while phospholipase A2-treated cells essentially maintain their normal shape.