Polyunsaturated fatty acids of the retina

Abstract

Polyunsaturated acyl chains provide the lipid matrix of membranes with a high degree of fluidity, facilitating translational, rotational and vibrational forms of motion to protein constituents. One of the most highly fluid membranes found in nature is that from the outer segments of photoreceptors, within which movements of rhodopsin have been measured, t* Studies on retinal fatty acids have shown that these structures! '2'3A5 are enriched in highly unsaturated acyl chains derived from the essential fatty acids of the linoleic and linolenic families. An important proportion of retinal lipid metabolism is committed to the maintenance of these special structures; the rest of the tissue, the neural retina, is an integral part of the central nervous system. We have been studying the lipid composition and metabolism in the retina as a whole. We began surveying small lipid pools like free fatty acids, diacylglycerols and phosphatidic acids *'s't9 bearing in mind their possible involvement in the biosynthetic and catabolic processes of phospholipids. Since the retina synthesizes phosphoglycerides and triacylglycerols at high rates as shown by the efficient labeling from radioactive glycerol, 2° an obvious requirement is the availability of endogenous polyenoic fatty acids which are introduced into lipids after being esterified to coenzyme A. This chapter summarizes some studies from this laboratory on polyenoic fatty acids in retinal membranes, showing that they conform a highly dynamic pool because: (a) they are actively released from retinal membranes in vitro, and (b) they may be actively taken up into retinal lipids under the same conditions. Moreover, it is shown that phospholipid molecular species containing these fatty acids are actively synthesized in retina.