The Conversion of Long Chain Saturated Fatty Acids to Their @Unsaturated, ,&y-Unsaturated, and @-Hydroxy Derivatives by Enzymes from the Cellular Slime Mold, Dictyostelium discoideum

A major problem in the study of fatty acid metabolism at the subcellular level has been the demonstration of the individual steps of both oxidation and synthesis (1, 2). Oxidation of long and short chain fatty acids by intact mitochondria is considered to proceed directly to acetate, without the accumulation of intermediates (3). Synthesis of fatty acids from acetyl coenzyme A and malonyl-CoA by the soluble “synthetase complex” likewise occurs as a concerted process, and significant amounts of intermediates have not been detected (4). Only in the incompletely defined synthetic system from mammary gland have such intermediates been directly demonstrated, and these are limited to compounds of chain length Cq to Ca (5). Each of the enzymes of /3 oxidation has been highly purified and studied in detail as the catalyst of a single step (6). Most of these studies have been carried out with fatty acids of chain length Cl2 or less, and a number of them have relied on spectrophotometric and enzymatic assays for characterization of the products (7-Q). The structures of the long chain fatty acid intermediates have generally been inferred from the identification of the Cq, Ce, and Cs intermediates. In the biosynthetic system, a number of separate intermediate steps have also been studied, using chemical, spectrophotometric, and exchange tracer techniques (2, 10). As in /3 oxidation, only the short chain fatty acid intermediates have been isolated and characterized. Using substrates of Ce chain length, Brady and his coworkers (11, 12) have shown the participation of synthetic c~,@unsaturated, P-keto, and D( -)-@-hydroxyacyl-CoA derivatives in the biosynthesis of fatty acid chains (11, 12). The inability to detect long chain fatty acid intermediates in these intact systems may have several causes. Until the advent of gas-liquid chromatography and thin layer chromatography, these compounds were difficult to isolate and characterize. Also, there is evidence that, at least in some systems, the intermediates may exist only bound to enzyme sulfhydryls as thiol esters (2), and, therefore, never would accumulate unless the enzymes were used in substrate concentrations. Such a phenomenon could also explain the difficulty of demonstrating the entry of synthetic intermediates into the biosynthesis of fatty acids (4). Whatever the explanation, the long chain fatty acid derivatives presumed to be the intermediates in fatty acid oxidation a,nd synthesis appear never to have been isolated and characterized. It is therefore of interest that enzymes from the cellular slime mold Dictyostelium discoideum have been found which transform saturated long chain fatty acyl-CoA derivatives to a series of long chain intermediates, each of which accumulates in considerable quantity. These compounds include long chain trans-a,P-unsaturated fatty acids which are the postulated intermediates in fatty acid oxidation and synthesis, long chain D( -)-/3-hydroxy acids which are the presumed intermediates in fatty acid synthesis, but not in p oxidation, and long chain cisand trans-p,y-unsaturated fatty acids for which a metabolic role is not clear. The same compounds are formed during the incubation of palmityl-CoA with the mitochondrial fraction of guinea pig liver,1 and, therefore, the reactions described in this paper are not unique to the cellular slime mold.