L-Glutamic acid decarboxylase (GAD) is an enzyme necessary for the synthesis of the inhibitory neurotransmitter GABA in GABAergic nerve endings. In the presence of pyridoxal phosphate as cofactor, the enzyme decarboxylates glutamate to yield GABA and CO2. GAD exists in two isoforms encoded by two different genes. These isoforms are GAD65 and GAD67 with molecular weights of 65 and 67 kDa, respectively. Interestingly, GAD is only present in GABAergic neurons and so can be used as a marker.
In experimental animals, administration of drugs to inhibit GAD invariably leads to epileptic seizures.
