Plasma membranes: Structural analyses of neutral lipids and phospholipids of rat liver

Abstract

Plasma membranes of rat liver were prepared by rate-isopycnic zonal centrifugation for structural studies of the membrane lipids. The distribution of fatty acids esterified at the 1- and 2-positions of phosphatidyl choline (PC), phosphatidyl ethanolamine (PE), phosphatidyl serine (PS), phosphatidyl inositol (PI), and diphosphatidyl glycerol (DPG), and the fatty acid composition of cholesterol ester, triglyceride, free fatty acid, sphingomyelin, and free ceramide lipid classes were determined. The carbon number distribution of triglycerides and the diglycerides, derived from PC and PE, were also determined. The plasma membranes were composed of approximately 25% lipid. Cholesterol was the major neutral lipid and PC and PE were the major phospholipids. Cholesterol esters contained a very high percentage of 16:1 acid relative to whole rat liver. Triglyceride carbon number distribution and fatty acid composition were similar to whole liver percentages. Ceramide and sphingomyelin fatty acid percentages were similar and resembled values derived from total rat liver. The 1-positions of the phosphoglycerides, except DPG, were esterified with primarily saturated acids, and the 2-positions were esterified with mainly polyunsaturated acids. Positional distribution of PS and DPG fatty acids was similar to that found in whole rat liver, whereas the distribution of fatty acids at the 2-position of PI was dissimilar. The proportion of 18:0 and 16:0 acids at the 1-position of membrane PC and PE was different from the percentage found in whole rat liver, but the sum of the two acids was the same in both. Likewise, the proportion of 18:2 and 20:4 acids at the 2-position of PC and PE was different for two separate membrane preparations, and different from total rat liver PC and PE percentages, but the sum of the two acids was nearly identical in each. The carbon number distribution of PC diglycerides was similar to whole liver PC, whereas the distribution of membrane PE diglycerides differed. Similarities in the structure of lipids derived from liver membranes and whole liver suggest that the molecular species of most lipid classes of the same tissue are randomly distributed among cellular organelles, although the class composition may differ significantly between organelles. The selective incorporation of certain molecular species into some lipid classes may give rise to some of the properties of liver plasma membranes.