The asymmetric transmembrane distribution of phosphatidylethanolamine, phosphatidylserine, and fatty acids of the bovine retinal rod outer segment disk membrane

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Abstract
The transmembrane distribution of the major aminophospholipids in the bovine retinal rod outer segment disk membrane, phosphatidylethanolamine and phosphatidylserine, was determined using a novel pair of permeable and impermeable covalent modification reagents. The values for the percentages of phosphatidylethanolamine and phosphatidylserine in the outer monolayer were calculated from a simple expression which takes into account the leakage of impermeable reagent into the disk lumen as monitored by the extent of labeling of lysine entrapped in the lumen. We infer from our results that at least 73 to 87% of the disk phosphatidylethanolamine and 77 to 88% of the disk phosphatidylserine are in the outer disk membrane monolayer. The fatty acid composition of the inner aminophospholipids is slightly more saturated than the outer aminophospholipids. Calculations using the lateral surface areas occupied by the disk membrane lipids suggest that 65 to 100% of the disk phosphatidylcholine is on the inner membrane surface. Since the disk phosphatidylcholine is also somewhat more saturated than the phosphatidylethanolamine and phosphatidylserine of the outer monolayer, the total inner membrane monolayer fatty acid composition is more saturated than that of the outer monolayer fatty acid composition.