The concentration of malonyl-coenzyme A and the control of fatty acid synthesis in vivo.

An enzymatic method has been developed for the measurement of malonyl-CoA in perchioric acid extracts of liver. With this method the relative activities of acetyl-CoA carboxylase and fatty acid synthetase have been studied in vivo by comparing the diet-induced changes in malonyl-CoA concentrations with rates of 3Hz0 incorporation into fatty acids. Malonyl-CoA concentrations were low in the fat-fed or 4% hours starved animal (0.004 to 0.006 pmole per g wet weight) but approached normal-fed levels (0.013 pmole per g wet weight) within 3 hours of refeeding the starved animal. The highest concentrations of malonyl-CoA (0.025 pmole per g wet weight) were found in the meal-fed animal. The de nova rates of fatty acid synthesis in vivo varied between 0.02 pmole of CO units per mi n per g wet weight of liver in the starved or fat-fed animals t o 0.45 pmole, of CZ units per min per g in the meal-fed group. Malonyl-CoA concentrations and the rate of tritium incorporation into fatty acids were increased above starved values in all fed groups except that group fed a high fat diet. A number of intermediary metabolites which have been proposed as “short term” controllers of the rates of fatty acid synthesis have been measured in freeze-clamped liver. There was no correlation between the rate of fatty acid synthesis and the liver content of citrate, ATP, ADP, glucose, glucose 6-phosphate, or a-glycerophosphate. In addition, short term control did not appear to be exerted by free mitochondrial [NAD+]: [NADH], free cytoplasmic [NAD+]: [NADH], or [NADP+]: [NADPH], “energy charge” or phosphorylation state. Increases of the malonyl-CoA content and the rate of fatty acid synthesis above the starved or fat-fed values occurred when the tissue content of long chain acyl-CoA decreased below starved values. It was concluded that short term control of fatty acid synthesis in vivo most likely results from an inhibition of acetylCoA carboxylase by long chain acyl-CoA. The variations in the rate of fatty acid synthesis which occurred in the carbohydrate-fed groups probably result from variations in the quantity of the enzyme fatty acid synthetase.