Transbilayer distribution of phospholipids in photoreceptor membrane studied with various phospholipases

The distribution of the three major phospholipids of bovine rod outer segment disk membranes over the two faces of the membrane has been studied by means of treatment with phospholipase C, phospholipase A2 and phospholipase D. Two different preparations of rod outer segment disk membranes have been used, which are called 'stacked disks' and 'disk vesicles' on account of their morphological appearance. The hydrolysis patterns obtained by phospholipase treatment of these preparations have been compared to those of a retinal lipid suspension or detergent-solubilized disk membranes, which serve as control preparations with a similar phospholipid composition but a random availability of the phospholipids. Special attention is given to the early phase of enzyme treatment in order to eliminate secondary effects on the molecular organization of the membrane due to appreciable phospholipid hydrolysis. Analysis of the hydrolysis patterns for all three phospholipases in stacked disks, as compared to those in randomized control preparations, suggests a slightly asymmetrical distribution of phosphatidylcholine (40--45% at the outer face) and phosphatidylethanolamine (55--60% at the outer face) and a symmetrical distribution of phosphatidylserine in rod outer segment disk membranes. Extensive treatment with phospholipases C and A2 leads ultimately to nearly complete hydrolysis of all phospholipids, but with phospholipase D a final level of 40% phospholipid hydrolysis is observed in stacked disk preparations. This suggests that in the latter case the inner face of the membrane is inaccessible to the enzyme. Further work will be necessary in order to substantiate these conclusions.