Abstract
In vivo nuclear magnetic resonance spectroscopy, in vitro gas chromatography-mass spectrometry, and automated (15)N/(13)C mass spectrometry have been used to demonstrate that glutamate dehydrogenase is active in the oxidation of glutamate, but not in the reductive amination of 2-oxogiutarate. In cell suspension cultures of carrot (Daucus carota L. cv Chantenay), primary assimilation of ammonium occurs via the glutamate synthase pathway. Glutamate dehydrogenase is derepressed in carbonlimited cells and in such cells the function of glutamate dehydrogenase appears to be the oxidation of glutamate, thus ensuring sufficient carbon skeletons for effective functioning of the tricarboxylic acid cycle. This catabolic role for glutamate dehydrogenase implies an important regulatory function in carbon and nitrogen metabolism.