ROLE OF CHOLESTEROL IN THE STRUCTURAL ORDER OF LENS MEMBRANE-LIPIDS

Cholesterol may order or disorder phospholipids. The physiological con tribution of cholesterol to the structural order of lens membrane lipi ds was determined. Cholesterol and phospholipid from bovine lens nucle ar and cortical tissue were separated by thin layer chromatography. Th e effect of cholesterol upon the trans to gauche transition of the hyd rocarbon chains was assessed by measuring CH2 infrared stretching band frequencies as cholesterol was added back to the phospholipids. Altho ugh the relative cholesterol level of nuclear lipid was much higher th an that of the cortex (59 vs. 36 mol%, respectively), the structural o rder of unfractionated nuclear and cortical lipids were similar at phy siological temperature. Cholesterol added to lipids devoid of choleste rol produced a sharp biphasic effect on the structural order of nuclea r lipids, increasing the trans conformation from 56% of 0 mol% cholest erol to 74% at 18% cholesterol to 41% trans at 59 mol% cholesterol. Ch olesterol addition produced a shallow biphasic change in the percentag e trans conformation of cortical lipids. Maximum order (about 40% tran s conformation) was seen at a cholesterol level equal to that of intac t cortical lipid (36 mol%). The physiological role of cholesterol is t o increase the structural order of cortical membrane lipid and decreas e order in nuclear lipid. The net result is a similarity in the struct ural order of cortical and nuclear membrane. We suggest that the diffe rent response of cortical and nuclear lipids to added cholesterol is l inked to differences in the phospholipid composition between these two lens regions. In the absence of cholesterol, nuclear phospholipids ar e much more highly ordered than those of the cortex. (C) 1996 Academic Press Limited