Glutamate dehydrogenase
Glutamate dehydrogenase (GLDH) is an enzyme, present in most microbes and the of eukaryotes, as are some of the other enzymes required for urea synthesis, that converts glutamate to α-ketoglutarate, and vice versa. In animals, the produced ammonia is usually used as a substrate in the urea cycle. Typically, the α-ketoglutarate to glutamate reaction does not occur in mammals, as glutamate dehydrogenase equilibrium favours the production of ammonia and α-ketoglutarate. Glutamate dehydrogenase also has a very low affinity for ammonia (high Michaelis constant 
of about 1 mM), and therefore toxic levels of ammonia would have to be present in the body for the reverse reaction to proceed (that is, α-ketoglutarate and ammonia to glutamate and NAD(P)+). In bacteria, the ammonia is assimilated to amino acids via glutamate and aminotransferases. In plants, the enzyme can work in either direction depending on environment and stress. Transgenic plants expressing microbial GLDHs are improved in tolerance to herbicide, water deficit, and pathogen infections. They are more nutritionally valuable.
The enzyme represents a key link between catabolic and anabolic pathways, and is, therefore, ubiquitous in eukaryotes. In humans the relevant genes are called GLUD1 (glutamate dehydrogenase 1) and GLUD2 (glutamate dehydrogenase 2), and there are also at least 8 GLDH pseudogenes in the human genome as well, probably reflecting microbial influences on eukaryote evolution.
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