22-Carbon polyenoic acids. Incorporation into platelet phospholipids and the synthesis of these acids from 20-carbon polyenoic acid precursors by intact platelets.

The types of unsaturated fatty acids found in platelet phospholipids must be regulated by a series of controls which include specificity for activation and acylation as well as modification of circulating fatty acids by platelets prior to incubation into phospholipids. In this study we show that washed human platelets not only incorporate [1-14C]6,9,12-18:3, [1-14C]6,9,12,15-18:4, [1-14C]5,8,11-20:3, [1-14C]5,8,11,14-20:4, and [1-14C]5,8,11,14,17-20:5 into their phospholipids but also chain elongate each of these acids with subsequent acylation of the chain elongated products into phospholipids. Platelets incubated alone with 1-14C-labeled 5,8,11-20:3, 5,8,11,14-20:4, 5,8,11,14,17-20:5, 7,10,13,16,19-22:5, or 4,7,10,13,16,19-22:6 incorporated each of these acids into individual phosphoglycerides with phosphatidylinositol having the highest specific activity followed by phosphatidylcholine with phosphatidylserine approximately equal to phosphatidylethanolamine. The incorporation specificity of 4,7,10,13,16,19-22:6 was atypical since it was a relatively poor substrate for acylation into all phospholipids except phosphatidylethanolamine. The 20-carbon acids were better substrates for incorporation into phospholipids than were the 22-carbon compounds. Simultaneous incubation of 10 microM [1-14C]5,8,11,14-20:4 with increasing levels (5 to 15 microM) of each of the above five other 1-14C-labeled acids showed a concentration-dependent increase in the amount of the second fatty acid incorporated into platelet phospholipids. Dietary fat modification thus has the potential of increasing the plasma pool of 22-carbon acids for incorporation into platelets. In addition the activation of 20-carbon eicosanoid precursors by the high affinity platelet activating enzyme (Wilson, D. B., Prescott, S. M. and Majerus, P. W. (1982) J. Biol. Chem. 257, 3510-3515) will yield an acyl-CoA for both acylation and chain elongation followed by subsequent incorporation of 22-carbon acids into phosphoglycerides.