BOROHYDRIDE REDUCTION OF L-GLUTAMATE DECARBOXYLASE

Inhibitors of crayfish glutamic acid decarboxylase

Crayfish glutamic acid decarboxylase (GAD), like the homologous enzymes from other species, is inhibited by carbonyl-trapping agents (e.g. aminooxyacetic acid; AOAA) and sulfhydryl reagents (e.g. 5,5'-dithiobis-(2-nitrobenzoic acid); DTNB). It also is inhibited by the product GABA, many anions (e.g. SCN- and Cl-), and some cations (e.g. Zn+2). The inhibition by AOAA, but not that by DTNB, was prevented by increasing the concentration of the pyridoxal phosphate (PLP) coenzyme. GABA blocked the effects of PLP on enzyme activity. The inhibition by AOAA, DTNB, GABA, and chloride all were competitive with substrate. The effect of GABA occurs at physiological concentrations and may contribute to the regulation of GAD activity in vivo. The quantitative effect of anions is dependent on the cation with which they are administered. ATP stimulated GAD activity in homogenates prepared with potassium phosphate or Tris-acetate buffer, even when no exogenous PLP was provided.

Assay and properties of glutamic acid decarboxylase in homogenates of crayfish nervous tissue

The activity of glutamic acid decarboxylase (GAD) was measured in homogenates of crayfish nervous tissue. Radioactive GABA and CO2 were formed from radioactive glutamic acid in approximately equimolar amounts. Product formation was linear for 9.5 hr at 11-32 degrees C with about 1-30 micrograms homogenate protein. Enzyme activity remained high at pH 7-10 but declined steeply above pH 10.5 and below pH 7. Enzyme activity was stimulated by pyridoxal phosphate, 2-mercaptoethanol, and potassium phosphate; at higher than optimal concentrations of each the activity was reduced. Sodium phosphate altered the stimulatory effect of potassium phosphate. Crayfish GAD behaves like a typical neural GAD but is distinguishable biochemically from GAD of other species.

Purification and general properties of glutamate decarboxylase from Clostridium perfringens

1. A seven-step procedure for preparing highly purified glutamate decarboxylase from Clostridium perfringens is described. 2. The homogeneity of the pure enzyme was established by sucrose-density-gradient centrifugation and starch-gel electrophoresis. 3. The isoelectric point of the pure enzyme is about pH4.5 and the molecular weight is 290000. 4. The pH optimum for activity is 4.7. The pure enzyme is specific for l-glutamate; beta-hydroxyglutamate is decarboxylated at a lower rate. 5. Evidence is presented that each mol of enzyme contains 2mol of firmly bound pyridoxal 5-phosphate. 6. Resolution does not occur at acid pH; by dilution with neutral or alkaline buffers the enzyme is inactivated and the coenzyme is released. 7. Reconstitution of active enzyme was obtained by protecting the apoenzyme with thiol compounds.