Comparison of long-chain fatty acyl-CoA synthetases from rabbit heart and liver: substrate preferences and effects of Mg2+

Rabbit heart has a single, non-specific, fatty acyl-CoA synthetase (HP1) which is dependent on Mg2+, apart from the requirement for MgATP2-. Two long-chain fatty acyl-CoA synthetase activities (LP1 and LP2) can be resolved by hydroxyapatite chromatography of liver preparations; the Mg2+ requirement for these enzymes is undefined. These experiments were done to define the Mg2+ requirements of the liver enzymes and to compare them with the heart enzyme. For all three sources of enzyme and for arachidonic, oleic and palmitic acid substrates, the overall velocity of the reaction increased as [Mg2+] increased. Depending on the substrate and the source of enzyme, the increase in overall velocity could be attributed to changes in affinity or maximal velocity or both. The substrate preference of the HP1 enzyme for arachidonic acid (AA) was fifth or sixth of eight substrates regardless of the concentration of Mg2+. In contrast, increasing [Mg2+] shifted the relative substrate preference of both liver enzymes for AA. At low [Mg2+], AA was ranked seventh or eighth (least preferred) of eight substrates, whereas at high [Mg2+], AA was ranked as fifth or sixth. Hill plots of competition studies were consistent with Mg2+-induced positive co-operativity in LP1, but not in HP1 or LP2. Although enzymes from the three sources exhibit substantial kinetic differences, it is uncertain whether they are three different enzymes.