Synthesis of glutamate by mitochondria – An anaplerotic function for glutamate dehydrogenase

The photorespiratory nitrogen cycle was initially thought to be a closed cyclic process. If this were true the loss of glutamate, glutamine, serine or glycine to other processes, such as protein synthesis or export from the leaves, would not be possible in a stoichiometric sense. However, recent studies with ¹⁵N]-labeled amino acids show that there are alternative sources of nitrogen for photorespiration, indicating that the nitrogen cycle is not a closed cyclic system. In addition recent work with ¹⁵NH₄Cl and ¹⁵N]-glycine and a metabolically competent mitochondria system has shown that glutamate is synthesized in the mitochondria. Hence the glutamate dehydrogenase (GDH, EC 1.4.1.2) in mitochondria could also be active in the reassimilation of NH₄. We would like to propose that one function of mitochondrial GDH is to synthesize glutamate from some of the NH₄ released by photorespiration and that this glutamate represents a reserve for use in biosynthetic reactions.