The reaction catalyzed by glutamate dehydrogenase is:
The glutamine synthetase reaction is also important in several respects. First it produces glutamine, one of the 20 major amino acids. Second, in animals, glutamine is the major amino acid found in the circulatory system. Its role there is to carry ammonia to and from various tissues but principally from peripheral tissues to the kidney, where the amide nitrogen is hydrolyzed by the enzyme glutaminase (reaction below); this process regenerates glutamate and free ammonium ion, which is excreted in the urine.
Note that, in this function, ammonia arising in peripheral tissue is carried in a non-ionizable form which has none of the neurotoxic or alkalosis-generating properties of free ammonia.
Liver contains both glutamine synthetase and glutaminase but the enzymes are localized in different cellular segments. This ensures that the liver is neither a net producer nor consumer of glutamine. The differences in cellular location of these two enzymes allows the liver to scavenge ammonia that has not been incorporated into urea. The enzymes of the urea cycle are located in the same cells as those that contain glutaminase. The result of the differential distribution of these two hepatic enzymes makes it possible to control ammonia incorporation into either urea or glutamine, the latter leads to excretion of ammonia by the kidney.
When acidosis occurs the body will divert more glutamine from the liver to the kidney. This allows for the conservation of bicarbonate ion since the incorporation of ammonia into urea requires bicarbonate (see below). When glutamine enters the kidney, glutaminase releases one mole of ammonia generating glutamate and then glutamate dehydrogenase releases another mole of ammonia generating α-KG. The ammonia will ionizes to ammonium ion (NH4+) which is excreted. The net effect is a reduction in the concentration of hydrogen ion, [H+], and thus an increase in the pH (see also Kidneys and Acid-Base Balance).
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