Transverse distribution of phospholipids in the vertebrate photoreceptor membrane

A study has been made of the distribution of the three major phospholipids of the bovine photoreceptor membrane over the two faces of the lipid bilayer. Three different approaches have been used: a . Treatment of isolated intact outer segments and disk vesicles (lysed outer segments) with three different phospholipases and determination of the degradation pattern. As randomized control preparations a retinal lipid suspension and detergent solubilized disks were used. b . Treatment of intact and lysed outer segments with the amino group reagent trinitrobenzenesulfonic acid (with and without treatment by phospholipase D) and determination of modification of phosphatidylethanolamine and phosphatidylserine. c . Determination of the fatty acid composition of the membrane phospholipids during treatment by phospholipase D or TNBS. The results consistently indicate a nearly symmetric distribution of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine and their fatty acids over the two membrane faces, with a slight preponderance of the first phospholipid on the inner side and possibly a slight excess of the second one on the outer side.