AMMONIUM ASSIMILATION AND THE ROLE OF GAMMA-AMINOBUTYRIC-ACID IN PH HOMEOSTASIS IN CARROT CELL-SUSPENSIONS

In vivo N-15 NMR spectroscopy was used to monitor the assimilation of ammonium by cell-suspension cultures of carrot (Daucus carota L. cv Ch antenay). The cell suspensions were supplied with oxygen in the form o f either pure oxygen (''oxygenated cells'') or air (''aerated cells'') . In contrast to oxygenated cells, in which ammonium assimilation had no effect on cytoplasmic pH, ammonium assimilation by aerated cells ca used a decrease in cytoplasmic pH of almost 0.2 pH unit. This led to a change in nitrogen metabolism resulting in the accumulation of gamma- aminobutyric acid. The metabolic effect of the reduced oxygen supply u nder aerated conditions could be mimicked by artificially decreasing t he cytoplasmic pH of oxygenated cells and was abolished by increasing the cytoplasmic pH of aerated cells. The activity of glutamate decarbo xylase increased as the cytoplasmic pH declined and decreased as the p H recovered. These findings are consistent with a role for the decarbo xylation of glutamate, a proton-consuming reaction, in the short-term regulation of cytoplasmic pH, and they demonstrate that cytoplasmic pH influences the pathways of intermediary nitrogen metabolism.