Transbilayer distribution of phospholipid fatty acyl chains in photoreceptor membrane

The transverse distribution of the fatty acyl chains of the major phospholipids over the two faces of the photoreceptor membranes has been determined in bovine rod outer segment (stacked disk) preparations. For this purpose, the fatty acid composition of the phospholipids has been analyzed before and after treatment with trinitrobenzenesulfonic acid and phospholipase D. The latter agents are used under conditions in which they have been demonstrated to attack only the outer (cytoplasmic) face of the membrane. After treatment with trinitrobenzenesulfonic acid or phospholipase D, the fatty acid composition of the unreacted phospholipids is the same as that before treatment, regardless of the extent of modification or hydrolysis attained. The fatty acid composition of phosphatidic acid, resulting from phospholipase D action, also remains unchanged during progressive hydrolysis. These results indicate that the fatty acyl chains of the major phospholipids have the same composition on either side of the disk membrane. Together with our previously published evidence for the distribution of the major phospholipids in rod outer segment disk membranes, this means that both the phospholipids and their fatty acyl chains have a remarkably symmetrical distribution over the two membrane faces. On the basis of literature data it is concluded that this approximate symmetry reflects the high mobility of the entire phospholipid pool of disk membranes, thus including appreciable transbilayer movements of the phospholipids.