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 synthetase 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 C18:0 unsaturated fatty acids to arachidonate, and had about the same affinity for C10:0 -C14:0 saturated fatty acids as for arachidonate. At 35 microM arachidonate, enzyme velocity increased as the total Mg2+ was increased from 3 to 80 mM. Calculated [MgATP] indicated that the MgATP complex was not rate-limiting. At low concentrations, Mn2+ and Ni2+ supported activity, but Cu2+ and Zn2+ did not. Low Ca2+ concentrations activated only oleic acid conversion. Kinetic analysis indicated that the Vmax of the enzyme was increased with increasing concentrations of ionized Mg2+ for both oleic acid and arachidonic acid. The data are constant 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.