COENZYME A-DEPENDENT MODIFICATION OF FATTY ACYL CHAINS OF RAT-LIVER MEMBRANE PHOSPHOLIPIDS - POSSIBLE INVOLVEMENT OF ATP-INDEPENDENT ACYL-COA SYNTHESIS

Certain species of fatty acyl chains of microsomal lipids from rat hep atocytes underwent desaturation when the microsomal fraction was incub ated with CoA and NADH. For instance, 18:0, 18:2(n-6), and 20:3(n-6) i ncorporated into membrane lipids were gradually converted to 18:1(n-9) , 18:3(n-6), and 20:4(n-6), respectively. Further, 20:5(n-3) and 18:3( n-6) were metabolized to 22:5(n-3) and 20:4(n-6), respectively, throug h chain elongation or chain elongation plus subsequent desaturation wh en malonyl-CoA was present. In contrast to esterified fatty acids, neg ligible change was observed for free fatty acids under the same experi mental conditions. It is apparent that ATP-independent acyl-CoA synthe sis is implicated in such enzymatic modification of fatty acyl chains. The presence of either gel-filtered cytosol or bovine serum albumin m arkedly potentiated the reaction. However, the addition of ATP and Mg2 + did not accelerate the overall reaction induced in the presence of C oA alone. These results suggest that the pathway of CoA-dependent/ATP- independent modification of fatty acyl chains is sufficiently active t o account for the desaturation and chain elongation of fatty acids est erified in membrane lipids, especially phospholipids, of living cells.