PHOSPHOLIPID-SYNTHESIS BY CHICK RETINAL MICROSOMES - FATTY-ACID PREFERENCE AND EFFECT OF FATTY-ACID BINDING-PROTEIN

The acylation of 1-palmitoyl-sn-glycerophosphocholine (1-16:0-GPC) or 1-palmitoyl-sn-glycerophosphoethanolamine (1-16:0-GPE) was measured using the microsomal fraction prepared from retinas of 14-15-day-old chick embryos. Rates of incorporation of exogenously supplied fatty acids into diacyl-GPC were generally 5-7 times greater than into diacyl-GPE. Substrate preferences for incorporation into diacyl-GPC and diacyl-GPE were, respectively, 18:2 > 18:3 = 20:5 > 20:4 > 18:1 > 22:6 = 18:0 and 18:2 > 22:6 greater-than-or-equal-to 18:3 = 18:0 greater-than-or-equal-to 20:4 = 18:1 > 20:5. The apparent selectivities were not consistent with the reported fatty acid compositions of these lipid classes. The addition of partially purified fatty acid binding protein (FABP) to the reaction had no effect either on overall rates of incorporation or on the substrate preference. When fatty acyl-CoA substrates were used, rates of incorporation of the 18:0 derivative were much higher than with the fatty acid, while rates with other fatty acyl-CoA were similar to those with the respective fatty acid. Substrate preferences for CoA derivatives incorporated into diacyl-GPC were: 18:0 > 20:4 > 18:2 greater-than-or-equal-to 22:6, and into diacyl-GPE: 20:4 = 22:6 > 18:0 > 18:2. Polyunsaturated fatty acyl CoA (PUFA-CoA) were thus favored for incorporation into diacyl-GPE, and to a lesser extent into diacyl-GPC, a result that is consistent with composition data. When purified FABP was added to the reactions, there was an increase in the incorporation of 18:0-CoA and a decrease or no change in the incorporation of PUFA-CoA. The deacylation/reacylation cycle thus appears to play a role in the modification of phospholipid composition. The data are not consistent, however, with a role for FABP in directing PUFA toward membrane lipid synthesis.