Two Forms of the γ‐Aminobutyric Acid Synthetic Enzyme Glutamate Decarboxylase Have Distinct Intraneuronal Distributions and Cofactor Interactions

TLDR

Glutamate decarboxylase (GAD) produces the inhibitory neurotransmitter GABA and exists in two mammalian isoforms, GAD67 and GAD65. Using isoform‑specific antibodies, the authors mapped GAD67 to be broadly distributed in neurons while GAD65 localizes mainly to axon terminals. GAD67 is largely present as active holoenzyme, whereas only about half of GAD65 is holoenzyme, implying that the balance of apo‑ and holo‑GAD65 in terminals may regulate GABA synthesis in response to activity.

Abstract

Abstract: Glutamate decarboxylase (GAD) catalyzes the production of γ‐aminobutyric acid (GABA), a major inhibitory neurotransmitter. The mammalian brain contains two forms of GAD, with Ms of 67,000 and 65,000 (GAD 67 and GAD 65 ). Using a new antiserum specific for GAD 67 and a monoclonal antibody specific for GAD 65 , we show that the two forms of GAD differ in their intraneuronal distributions: GAD 67 is widely distributed throughout the neuron, whereas GAD 65 lies primarily in axon terminals. In brain extracts, almost all GAD 67 is in an active holoenzyme form, saturated with its cofactor, pyridoxal phosphate. In contrast, only about half of GAD 65 (which is found in synaptic terminals) exists as active holoenzyme. We suggest that the relative levels of apo‐GAD 65 and holo‐GAD 65 in synaptic terminals may couple GABA production to neuronal activity.