The tricarboxylic acid cycle in Dictyostelium discoideum. III. Analysis of steady state and dynamic behavior.

The examination of model robustness in the previous paper (Shiraishi, F., and Savageau, M. A. (1992) J. Biol. Chem. 267, 22919-22925 led to the suggestion that the current model for the tricarboxylic acid cycle in Dictyostelium discoideum is ill-determined with respect to one or more of the features reflecting pyruvate metabolism. This conclusion is further supported here by results of steady state and dynamic analyses. The tricarboxylic acid cycle, according to the current model, is poised on a knife's edge with its behavior rigidly determined; any alteration of the system's components leads to nonviable behavior, as exemplified by explosive accumulation of pyruvate and loss of steady state in response to a minute change in the level of malate dehydrogenase. With the additional results in this paper, we are able to refine the diagnosis of the problem and suggest three different areas of the current model that might profitably be re-examined by experiment. These include the kinetics of the reactions at the malate branch point, the turnover times for the alanine, glutamate, and aspartate pools in vivo, and the dynamic mass balances for the cofactor NAD. We also suggest a minimal modification in the current model that could alleviate or circumvent some of these problems.