Oxidative interactions of cholesterol in the milk fat globule membrane.

The effects of oxidative interactions between cholesterol and milk fat globule membrane (MFGM) components, i.e., nonlipid fraction, total lipid, nonpolar lipid and polar lipid, on cholesterol oxidation were studied in the presence and absence of water. In the dry state, cholesterol natively present in MFGM appeared to be protected at 135 degrees C. The nonpolar lipid and nonlipid fraction contributed to the protective effect of MFGM. Added cholesterol accelerated the oxidation of membrane lipid fractions. At 75 degrees C, pure cholesterol and membrane lipid fractions did not show significant interaction. However, cholesterol and other lipids in MFGM were less stable than when these were heated separately. When cholesterol and membrane lipids were mixed in an aqueous medium at 75 degrees C, each accelerated the oxidation of the other. The MFGM exhibited a high protective effect on cholesterol oxidation in an aqueous environment. The nonlipid fraction protected cholesterol against oxidation, whereas the lipid fraction was destructive. In the absence of water, the net balance between these two opposing factors was destructive. The presence of water reversed the balance in favor of protection.