CALCIUM-CALMODULIN ACTIVATION OF SOYBEAN GLUTAMATE-DECARBOXYLASE

Recently, we provided preliminary evidence for calcium (Ca2+)/ calmodu lin (CaM) stimulation of plant glutamate decarboxylase (GAD; EC 4.1.1. 15). In the present study, a detailed characterization of the phenomen on is described. CAD was partially purified from various soybean (Glyc ine max L. Merr.) tissues (developing seed coat and cotyledons, leaf, and root) in the presence of EDTA by a combination of ammonium sulfate precipitation and anion-exchange fast protein liquid chromatography. GAD activity showed a sharp optimum at pH 5.8, with about 12% of maxim al activity at pH 7. It was stimulated 2- to 8-fold (depending on the tissue source) in the presence of Ca2+/CaM at pH 7 but not at pH 5.8. Furthermore, when the protease inhibitor phenylmethylsulfonyl fluoride was omitted from the purification procedure, GAD activity was insensi tive to Ca2+/CaM but was similar in magnitude to CaM-stimulated activi ty. The stimulation by Ca2+/CaM was fully inhibited by the CaM antagon ists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide and trifluoper azine. With saturating CaM or Ca2+, the concentrations of Ca2+ and CaM required for half-maximal stimulation were about 7 to 11 mu M and 25 nM, respectively. The effect of Ca2+ and CaM appeared to be through a 2.4-fold stimulation of V-max and a 55% reduction in K-m. The results suggested that CAD is activated via Ca2+ signal transduction.