Trichomonas vaginalis: characterization of its glutamate dehydrogenase

An NADP-linked glutamate dehydrogenase (EC 1.4.1.4) was found in the soluble fraction of Trichomonas vaginalis. Its molecular weight was about 230,000 (gel filtration). The enzyme, partially purified by diafiltration and hydroxyapatite column chromatography, was heat stable (1 hr at 57 C). It catalyzed both the amination of alpha-ketoglutarate (mean Km 0.6 mM) and the deamination of glutamate (mean Km 1.2 mM) The optimum pH of the amination reaction was 6.7, and that of the deamination reaction was 8. Glutamate was a competitive inhibitor of the amination reaction (mean Ki 5.6 mM) and alpha-ketoglutarate a partially competitive inhibitor of the deamination reaction (mean Ki 0.45 mM). Both guanosine and inosine diphosphates (1 mM) increased the Km alpha-ketoglutarate fivefold (mean Ki's 0.3 and 0.4 mM, respectively). Guanosine diphosphate reduced the Km glutamate 40%. Adenosine di- and triphosphate (1 mM) were ineffective. Because the amination reaction displayed substrate inhibition, guanosine and inosine diphosphates were potent natural inhibitors, and ammonia released by deamination reactions would tend to raise pH (amination operative at acid pH), we hypothesize that the deamination reaction may predominate in the living organism.