up EIA for C. Diff toxins A and B. Cofactors NAD+ (or NADP+) is a cofactor for the glutamate dehydrogenase reaction, producing α-ketoglutarate and ammonium as a byproduct. Based on which cofactor is used, glutamate dehydrogenase enzymes are divided into the following three classes: EC 1.4.1.2: L-glutamate + H2O + NAD+ 2-oxoglutarate + NH3 + NADH + H+ EC 1.4.1.3: L-glutamate + H2O + NAD(P)+ 2-oxoglutarate + NH3 + NAD(P)H + H+ EC 1.4.1.4: L-glutamate + H2O + NADP+ 2-oxoglutarate + NH3 + NADPH + H+ Role in flow of nitrogen Ammonia incorporation in animals and microbes occurs through the actions of glutamate dehydrogenase and glutamine synthetase. Glutamate plays the central role in mammalian and microbe nitrogen flow, serving as both a nitrogen donor and a nitrogen acceptor. Regulation of glutamate dehydrogenase In humans, the activity of glutamate dehydrogenase is controlled through ADP-ribosylation, a covalent modification carried out by the gene sirt4. This regulation is relaxed in response to caloric restriction and low blood glucose. Under these circumstances, glutamate dehydrogenase activity is raised in order to increase the amount of α-ketoglutarate produced, which can be used to provide energy by being used in the citric acid cycle to ultimately produce ATP. In microbes, the activity is controlled by the concentration of ammonium and or the like-sized rubidium ion, which binds to an allosteric site on GLDH and changes the Km (Michaelis constant) of the enzyme. The control of GLDH through ADP-ribosylation is particularly important in insulin-producing β cells. Beta cells secrete insulin in response to an increase in the ATP:ADP ratio, and, as amino acids are broken down by GLDH into α-ketoglutarate, this ratio rises and more insulin is secreted. SIRT4 is necessary to regulate the metabolism of amino acids as a method of controlling insulin secretion and regulating blood glucose levels. Bovine liver glutamate dehydrogenase was found to be regulated by nucleotides in the late 1950s and early 1960s by Carl Frieden. In addition to describing the effects of nucleotides like ADP, ATP and GTP he described in detail the different kinetic behavior of NADH and NADPH. As such it was one of the earliest enzymes to show what was later described as allosteric behavior. The activation of mammalian GDH by L-leucine and some other hydrophobic amino acids has also been long known, however localization of the binding site was not clear. Only recently the new allosteric binding site