Regulation of energy metabolism in Trypanosoma (Schizotrypanum) cruzi epimastigotes. II. NAD+-dependent glutamate dehydrogenase.

Trypanosoma (Schizotrypanum) cruzi epimastigotes (EP stock) grown in complex LIT medium rapidly consume the glucose present but, under aerobic conditions, continue growth in its absence with the concomitant excretion of ammonia, suggesting the utilization of amino acids for energy production. A search for metabolic pathways responsible for amino acid oxidation led to the detection of a NAD+-dependent glutamate dehydrogenase (L-glutamate:NAD+ oxidoreductase, E.C.1.4.1.2) which is different from an NADP+-dependent enzyme previously reported. The enzyme has been partially purified and its kinetic and regulatory properties studied in both directions of the reaction. Km values were 3.6 mM for alpha-ketoglutarate, 0.170 mM for NADH and 16 mM for NH+4, Vmax = 0.67 mumol min-1/mg-1 protein for aminative reduction; Km values were 23.5 mM for L-glutamate and 2.9 mM for NAD+, Vmax = 0.02 mumol min-1 mg-1 protein for deaminative oxidation, Tris buffer, pH 7.6. The enzyme is strongly inhibited by ATP, GTP, ADP and GDP (50% inhibition at 0.75 mM ATP, 3 mM MgCl2). S-Acetyl-CoA is also a potent inhibitor of the enzyme. The results demonstrate the presence of a specific pathway for the oxidation of amino acids, which is tightly regulated by the energy charge and the Krebs cycle activity in T. cruzi epimastigotes.