Amino acid catabolism and malic enzyme in differentiating Dictyostelium discoideum

Amino acids produced from protein degradation are the major energy source for differentiation and aging in Dictyostelium discoideum. Considering the reactions involved in the conversion of amino acids from an average protein into tricarboxylic acid cycle intermediates, a route from a cycle intermediate (probably malate) to acetyl coenzyme A is required for the complete utilization of amino acids. Citrate was isolated from cells pulse-labeled with (14)C-labeled amino acids and was cleaved with citrate lyase. When cells were pulse-labeled with [U-(14)C]-glutamate the specific radioactivity of the acetate and oxaloacetate portions of citrate were consistent with the conclusion that one-third of the carbon flowing through the tricarboxylic acid cycle is removed for the synthesis of acetyl coenzyme A. The data were also consistent with the patterns of carbon flux required to maintain steady-state levels of cycle intermediates in cells catabolizing amino acids. It is suggested that the malic enzyme (EC 1.1.1.40) catalyzes the synthesis of acetyl coenzyme A from malate and is responsible for the observed citrate labeling pattern. In cell extracts the activity of this enzyme increased markedly with the onset of differentiation. The properties of partially purified (40-fold) malic enzyme isolated at culmination indicated that the enzyme was allosteric and was positively affected by aspartate and glutamate. Thus, amino acid production from protein degradation would stimulate a reaction essential for the efficient utilization of these amino acids for energy.