Lysolipids and membrane damage: lysolecithin and its interaction with myelin.

Contemporary interest in the enzyme phospholipase A2 (PLA2) understandingly is centred around its role in the release of unsaturated fatty acids as the precursors for other biologically active molecules such as the prostaglandins. However, the other products of the reaction, the lysophosphatides, and in particular lysophosphatidylcholine (LPC), are potent detergents which at high concentrations will solubilize most membranes with ease and which at lower concentrations will perturb membrane properties. Thus the reacylation of LPC is an important and significant aspect of phospholipid metabolism. Early interest in the 1-acyl phosphatides was in terms of their cytolytic activity (Gottfried & Rapport, 1963), most studies being performed on the erythrocyte (see the review by Weltzien, 1979). However, a wide range of membrane and hence cell properties, are disturbed on exposure of low concentrations of LPC, e.g. increases in ionic permeability (Condrea et al., 1967; Lawrence et al., 1974; Gallo et al., 1984), activation of membrane-bound enzymes (Anttinen, 1976; Hochman, 198 1) and the alteration of cell behaviour, e.g. the activation of macrophages (Munder et al., 1977; Ngwenya & Yamamoto, 1985). LPC was thought at one time to be involved in complement-mediated cytolysis, but this proved to be untenable (Kinsky, 1972). With such divers potential for interaction with membranes, it is worth examining in detail the interaction of LPC with a specific membrane system, myelin, both in terms of its activity in vitro in solubilizing the purified membrane, and in vivo in producing demyelination. Following the report by Webster (1957) that LPC would completely clear homogenates of rat brain, we investigated the solubilization of purified rat brain myelin by LPC prepared from egg lecithin (Gent et al., 1964). From these studies it was concluded that (i) a stoichiometric amount of LPC was required to completely solubilize the myelin (empirically defined as W,,,,,,, the weight of LPC required to solubilize completely the equivalent of 100 mg dry weight of myelin; approx. 40 mg/100 mg in the case of rat myelin), and (ii) the solubilization process was one of steady attrition of the membrane particles by the release of a lipid-protein-LPC ternary complex. The solubilization process is extremely rapid, being around 90% complete in 5 min at 37°C. It was concluded that the soluble complex represented a modular myelin lipoprotein complex with a molecular mass of 109 kDa. However, further studies revealed that the solubilization products were much more heterogeneous (Gent & Gregson, 1965; Gent et al., 1971a, b), and that the process of solubilization was biphasic (Gent et al., 197 1 c). The ternary complexes were best resolved by isopycnic centrifugation on a sucrose gradient. Two groups of complexes were resolved (Fig. 1 a): a high density group (in rat myelin covering the density range from 1.034 to 1.054 g ~ r n ~ ) showing strong light scattering and