Lipid composition, membrane structure relationships in lens and muscle sarcoplasmic reticulum membranes

Membrane lipid composition varies in different tissues and species. Since a defined lipid composition is essential to the function of many membranes, the relationship between membrane lipid composition and structure was determined using infrared and Raman spectroscopy in four membranes containing a calcium pump: rabbit fast and slow twitch muscle sarcoplasmic reticulum and human and bovine lens fiber cell membranes. We found that membrane sphingolipid and phosphatidylcholine content were correlated to a decrease and increase, respectively, in the infrared lipid CH2 symmetric stretching band frequency. We interpret the change in frequency as a change in lipid hydrocarbon chain structural order. This was confirmed by Raman order parameters. The high degree of hydrocarbon chain saturation found in the variable amide chains of sphingolipids is likely to account for this correlation. Lipid phase transition temperature and cooperativity also correlated to sphingolipid and phosphatidylcholine content, and are the forces defining the order in at physiological temperature in the samples studied. Ca(2+)-ATPase caused an increase in the CH2 symmetric stretching frequency in fast twitch muscle sarcoplasmic reticulum (interpreted as an increase in hydrocarbon chain disorder), but had no effect on slow twitch muscle sarcoplasmic reticulum lipid hydrocarbon chain structure. In the natural systems studied, we find that it is the lipid hydrocarbon chain saturation that defines lipid hydrocarbon chain order.