Comparison of the Interaction of Tomatine with Mixed Monolayers Containing Phospholipid, Egg Sphingomyelin, and Sterols

The interaction of the glycoalkaloid tomatine with monolayers of a phospholipid (dimyristoylphosphatidylcholine, DMPC), and sphingolipid (egg sphingomyelin), and cholesterol is compared. Using measurements of the surface pressure response as a function of the subphase concentration of tomatine, interfacial binding constants are estimated for mixed monolayers of DMPC and cholesterol and for those of egg sphingomyelin and cholesterol of mole ratio 7:3. The binding constants obtained suggest a stronger interaction of tomatine with DMPC and cholesterol mixed monolayers, reflecting easier displacement of cholesterol from its interaction with DMPC than from its interaction with egg sphingomyelin. Mixtures of tomatine and cholesterol are found to spread directly at the water–air interface and form stable monolayers, suggesting that cholesterol holds tomatine at the interface despite the absence of observed monolayer behavior for tomatine alone. The interaction of tomatine with DMPC and cholesterol monolayers is found to exhibit a pH dependence in agreement with previously reported results for its interaction with liposomes; in particular, the interaction is much less at pH 5 than at pH 7 or pH 9. It is found that while tomatine interacts strongly with monolayers containing sitosterol, it does not interact with monolayers containing sitosterol glucoside. The response of monolayers of varying composition of DMPC and cholesterol to tomatine is also examined. Brewster angle microscopy (BAM) reveals further evidence for formation of suspected islands of tomatine + cholesterol complexes upon interaction with mixed monolayers of lipid and sterol.