Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis

Abstract Glutamate decarboxylase (GAD) exists as two isoforms, GAD 65 and GAD 67 . GAD activity is regulated by a cycle of activation and inactivation determined by the binding and release of its co‐factor, pyridoxal 5′‐phosphate. Holoenzyme (GAD with bound co‐factor) decarboxylates glutamate to form GABA, but it also catalyzes a slower transamination reaction that produces inactive apoGAD (without bound co‐factor). Apoenzyme can reassociate with pyridoxal phosphate to form holoGAD, thus completing the cycle. Within cells, GAD 65 is largely apoenzyme (∼93%) while GAD 67 is mainly holoenzyme (∼72%). We found striking kinetic differences between the GAD isoforms that appear to account for this difference in co‐factor saturation. The glutamate dependent conversion of holoGAD 65 to apoGAD was about 15 times faster than that of holoGAD 67 at saturating glutamate. Aspartate and GABA also converted holoGAD 65 to apoGAD at higher rates than they did holoGAD 67 . Nucleoside triphosphates (such as ATP) are known to affect the activation reactions of the cycle. ATP slowed the activation of GAD 65 and markedly reduced its steady‐state activity, but had little affect on the activation of GAD 67 or its steady‐state activity. Inorganic phosphate opposed the effect of ATP; it increased the rate of apoGAD 65 activation but had little effect on apoGAD 67 activation. We conclude that the apo‐/holoenzyme cycle of inactivation and reactivation is more important in regulating the activity of GAD 65 than of GAD 67.