Abstract
We have examined the fatty acid substrate specificity of arachidonoyl-CoA synthetase from human platelet membranes. A variety of positional isomers and chain-length analogs of arachidonic acid [20:4(5, 8, 11, 14)] were synthesized, and assayed for their ability to inhibit arachidonoyl-CoA formation or to serve as substrates for the synthetase. The chain-length specificity of the synthetase for delta 8,11,14 trienoic fatty acids was C19 greater than C18 = C20 much greater than C21 greater C22. Inhibition activity by positional isomers of arachidonate was 20:4(5, 8, 11, 14) approximately equal to 20:4(6, 9, 12, 15) = 20:4(7, 10, 13, 16) much greater than 20:4(4, 7, 10, 13), however, Vmax for arachidonate was greater than that for 20:4(6, 9, 12, 15). The enzyme apparently "counts" double bonds from the carboxyl terminus. As counted from the methyl terminus we found that several n-6,-9,-12 fatty acids were ineffective as inhibitors [18:3(6, 9, 12); 19:4)4, 7, 10, 13); 21:3(9, 12, 15)], whereas all methylene-interrupted tri- and tetraenoic fatty acids which contained delta 8 and delta 11 double bonds were potent inhibitors. The delta 11 double bond was best associated with optimal inhibition: 20:3(5, 11, 14) had a lower Ki than 20:3(5, 8, 14). 13-Methyl-20:3(8, 11, 14) did not inhibit the enzyme. Partially purified enzyme from calf brain, depleted of nonspecific long-chain acyl-CoA synthetase, exhibited the same fatty acid specificity as crude platelet enzyme.
Highlights
T o distinguish these possibilities,we have examined theability of a variety of polyunsaturated fatty acids differing in chain length 20:3(5, 11, 14) had a lower K i than 20:3(5, 8, 14). 13-Methyl- and degree and position of unsaturation to serve as in
Prior to the present experiments, it was suggested that arachidonoyl-CoA synthetase is required for eicosanoid precursor homeostasis in celltshat produce these mediators (1-4), but there was not enough information to determine whether this was the primary function of the synthetase
Among commonfatty acids that occur in mammals, onlyarachidonate, dihomo-y-linolenate,eicosapentaenoate, and docosahexaenoatecan serve assubstrates
Summary
Platelets and other cells that synthesize eicosanoids (prostaglandins,leukotrienes, andrelatedcompounds) esterify arachidonic acid [20:4(5, 8, 11, 14)] into their membrane phospholipids. This specificity is conferred by an arachidonate-specific, long-chain acylCoA synthetase that prefers eicosanoid precursor fatty acids as substrates to palmitate, stearate, oleate, and linoleate (1). T h e arachidonoyl-CoA synthetase is responsible for high affinity uptake and esterification of eicosanoid precursorsby human platelets (2)and by HSDMICl murine fibrosarcoma cells. Linoleic, arachidonic, and dihomoy-linolenic acids wereobtainedfrom Nu-Chek-Prep, Inc. All other labeled and unlabeled fatty acids were synthesized as described (5,6). Liquid chromatography on C-18 reverse phase columns as described(3,6). 2-Mercaptoethanolwas obtained from Eastman (Rochester, NY)
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