Chemistry of photoreceptor membrane preparations from squid retinas

Photoreceptor membrane preparations were made from retinas of the squid Loligo (Doryteuthis) plei for protein and lipid analysis. Lipid analysis was also completed on a single membrane preparation from Loligo pealei. (1) The membranes contain 75 wt. % protein and 25 wt. % lipid. Neutral lipids make up 26 mol % of the total lipid, the remaining 74% being phospholipid. No glycolipids were observed. (2) Free fatty acids and cholesterol comprise 8.6 and 17 mol %, respectively of the total lipid. No other neutral lipids were found. (3) Phosphatidylethanolamine and phosphatidylcholine are the major phospholipids. Lysophosphatidylcholine, lysophosphatidylethanolamine, sphingomyelin, and phosphatidylserine are present in small quantities. Phosphatidylinositol was not detected in the membranes. (4) The levels of polyunsaturated fatty acids, principally 20:4 omega 6, 20:5 omega 3, and 22:6 omega 3 are higher in the squid membranes than in any othr vertebrate or invertebrate retina that has been examined thus far. These acids account for 58 mol % of the fatty acids in phosphatidylcholine and phosphatidylserine, 75 mol % of the free fatty acids, and nearly 90% of the fatty acids of lyso- and phosphatidylethanolamine. The results from L. plei and L. pealei were indistinguishable. (5) Rhodopsin is the major protein of the membrane preparations and has a molecular weight of 50 500 +/- 850 determined by sodium dodecyl sulfate polyacrylamide gel disc electrophoresis.