Biosynthesis of Fatty Acids I. STUDIES WITH ENZYMES OBTAINED FROM LIVER

The requirement for bicarbonate in the incubation medium for the conversion of octanoic acid to stearic acid by slices of liver tissue was reported some 10 years ago (1). Furthermore, the inclusion of bicarbonate in the homogenizing medium permitted the preparation of a consistently active enzyme system from pigeon liver which catalyzed the conversion of acetate or acetyl coenzyme A to long chain fatty acids (2). The significance of these observations was not fully realized until the complete dependence of fatty acid synthesis on the presence of HCO,-, adenosine 5’-triphosphate, and Mg++ or Mn++ (3, 4), in addition to acetyl coenzyme A, was demonstrated. The requirement for CO% for fatty acid synthesis was also demonstrated in an enzyme system obtained from avocado (5), intact yeast (6)) and cell-free preparations of yeast (7). The finding that HCQwas not incorporated into long chain fatty acids by enzyme systems which require the presence of COZ (3, 4) suggested that acetyl-CoA was first carboxylated to malonyl-CoA. Accordingly, malonyl-CoA was synthesized and was found to be a required precursor for the synthesis of long chain fatty acids in an enzyme system obtained from pigeon liver tissue (8). The enzymatic carboxylation of acetyl-CoA was demonstrated soon thereafter (7, 9, lo), and the participation of this reactant in fatty acid synthesis was fully confirmed (11-13). The original demonstrations of the conversion of malonyl-CoA to long chain fatty acids (8) were performed with crude enzyme preparations obtained from pigeon liver which catalyzed the reduction of acetyl-CoA by TPNH to acetaldehyde. This finding led to the tentative proposal that elongation of the carbon chain of fatty acids might occur by reduction of acyl-CoA derivatives to the respective aldehydes, followed by condensation between the carbonyl carbon of the aldehyde with the methylene carbon of malonyl-CoA (8). Experiments with purified enzyme preparations indicate that such a mechanism is probably incorrect. It is the purpose of the present communications to report the results obtained with purified enzyme preparations which afford a clearer insight into the nature and the sequence of reactions that occur in the biosynthesis of long chain fatty acids.