PHOSPHOLIPIDS WITH A SHORT ACYL-CHAIN MODULATE PHOSPHOLIPASE AND ACYLTRANSFERASE ACTIVITIES

1-Acyl lysophosphatidylcholine prepared from egg yolk has been chemica lly reacylated to form decanoyl, dodecanoyl, myristoyl and palmitoyl d erivatives of phosphatidylcholine. The liposomes formed by these semi- synthetic phospholipids have been characterized by calorimetry, X-ray diffraction and fluorescence probe methods. Asymmetric phosphatidylcho lines tend to promote formation of excimers of a codispersed fluoresce nt phospholipid itoyl-sn-2-(1-pyrenedecanoyl)-L-alpha-phosphatidic aci d) (2 mol%). Excimer formation is correlated with the rate of hydrolys is of the fluorescent anionic phospholipid by Crotalus venom phospholi pase A(2). Codispersion with the semi-synthetic phosphatidylcholine of cholesterol or unsaturated fluid lecithin modulated both excimer form ation and the susceptibility of the fluorescent probe to hydrolysis by venom phospholipase A(2) at 22 degrees C. Similar results were obtain ed with hydrolysis of a radiolabelled substrate, oyl-sn-2-[1-C-14]inol eoylphosphatidylethanolamine, codispersed with the semi-synthetic phos phatidylcholine. Enrichment of rat hepatocyte plasma membranes with se mi-synthetic asymmetric phosphatidylcholines was mediated by incubatio n of membranes with phospholipid dispersions in the presence of a phos pholipid exchange protein. Enrichment of the membranes with semi-synth etic phosphatidylcholines of between 30 and 60% of the membrane phosph atidylcholine was achieved. The resulting alteration of the biomembran e is associated with a decreased activity of endogenous membrane phosp holipase A(2) acting on extramembranous radiolabelled substrate vesicl es. By contrast, the activity of acyl-CoA:lysophospholipid acyltransfe rase is increased in membranes enriched with highly asymmetric phospho lipids.