Evidence for deamination by glutamate dehydrogenase in higher plants: Commentary

Abstract

Glutamate dehydrogenase (GDH; EC 1.4.1.2) catalyses the interconversion of 2-oxoglutarate and glutamate. For many years it was assumed that GDH established a net flux in the direction of glutamate synthesis in higher plants and that this ubiquitous enzyme was responsible for the assimilation of ammonium (Davies 1968). However, this view changed with the discovery of glutamate synthase (GOGAT; EC 1.4.1.13), and it is now generally accepted that it is the glutamate synthase cycle, involving the successive operation of glutamine synthetase (GS; EC 6.3.1.2) and GOGAT, that is the major pathway for the assimilation of ammonium by higher plants (Lea and Miflin 1974; Stewart et al. 1980; Lea et al. 1990; Oaks 1994). Although it has been argued that there may be circumstances in which GDH could still provide a route for ammonium assimilation (Cammaerts and Jacobs 1985; Srivastava and Singh 1987; Loyola-Vargas et al. 1988; Magalhaes et al. 1990), recent attention has focused on the oxidative deamination of glutamate by GDH and the hypothesis that GDH performs an anaplerotic function in plants (Robinson et al. 1991, 1992). We used a combination of in vivo and in vitro methods to investigate the function of GDH in carrot (Daucus carota L.) cell suspension cultures, and the results were entirely consistent with a role for the enzyme in the deamination of glutamate. Firstly, enzyme assays of cell extracts showed that GDH activity was lowest in cells in the exponential phase of growth and reached a maximum in the stationary phase (Robinson et al. 1991). This suggests that GDH is unlikely to be important in ammonium assimilation, since the exponential phase coincides with a period of rapid nitrogen assimila-