EVIDENCE FOR A SINGLE NON-ARACHIDONIC ACID-SPECIFIC FATTY ACYL-COA SYNTHETASE IN HEART WHICH IS REGULATED BY MG2+

Previous reports indicated that arachidonic acid is incorporated into the isolated perfused rabbit heart in preference to other fatty acids, and that incorporation of arachidonic acid, but not other fatty acids , is inhibited during Mg2+ depletion. In this study, we have not been able to demonstrate an arachidonic acid-specific fatty acyl-CoA synthe tase in rat or rabbit heart by hydroxyapatite chromatography. Kinetic evidence was consistent with a single enzyme, as the slopes of pseudo- Hill plots were not significantly different from -1. The single fatty acyl-CoA synthetase present appears to prefer C-18:0 unsaturated fatty acids to arachidonate, and had about the same affinity for C-10:0:-C- 14:0 saturated fatty acids as for arachidonate. At 35 mu M arachidonat e, enzyme velocity increased as the total Mg2+ was increased from 3 to 80 mM. Calculated [MgATP] indicated that the MgATP complex was not ra te-limiting. At low concentrations, Mn2+ and Ni2+ supported activity, but CU2+ and Zn2+ did not. Low Ca2+ concentrations activated only olei c acid conversion. Kinetic analysis indicated that the V-max of the en zyme was increased with increasing concentrations of ionized Mg2+ for both oleic acid and arachidonic acid. The data are consistent with the hypothesis that Mg2+ has a direct effect on fatty acyl-CoA synthetase activity, and suggest that preference for oleic acid and arachidonic acid can be influenced by the ionic milieu.