Effect of the mitogenic lectin concanavalin A on the thermotropic behavior of glycosyl-free cationic lipids and their mixtures with zwitterionic lipids

The effect of concanavalin A (Con A) on the thermotropic behavior of positively charged, glycosyl-free lipids and their mixtures with zwitterionic lipids was investigated by differential scanning calorimetry. The gel to liquid-crystal phase transition enthalpy of pure dipalmitoylcholine (DPC) was found to be significantly increased in the presence of Con A (delta H = 31.2 and 42.5 KJ mol-1 lipid in the presence and in absence of Con A, respectively). Addition of the lectin to DPC liposomes, furthermore, induces the appearance of a new phase transition centered at 320 K. These results are interpretable by a partial hydrophobic interdigitation of the lectin molecule into the liposomal bilayer. The effect of Con A on the phase behavior of three 2:1 mixtures of zwitterionic and of positively charged lipids was also investigated. Phase diagrams of the systems dipalmitoyl-phosphatidylcholine-dihydrosphingosine (DPPC-DHS), sphingomyelin-dipalmitoylcholine (SPM-DPC), and dimyristoylphosphatidylcholine-dipalmitoylcholine (DMPC-DPC) are presented. In lipid mixtures of limited miscibility (DPPC-DHS and SPM-DPC), Con A induces pronounced phase-separation effects. These effects are attributable to a direct hydrophobic interaction of the lectin with the liposomal bilayer and do not require the presence of specific receptor groups. The possible relationship between lectin-induced phase separations in the lipid matrix of biomembranes, and the observed changes in membrane permeability, membranal enzymatic activities, etc., is briefly discussed.