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Glutamate decarboxylase has been purified from potato tubers. The final preparation was homogeneous as judged from native and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Gel filtration on Sephadex (3-200 gave a relative molecular mass M,, of 91 000 for the native enzyme. Sodium dodecyl sulfate polyacrylamide gel electrophoresis gave a subunit M , of 43000. Thus the enzyme appears to be a dimer of identical subunits. It has 2 mol pyridoxal 5'-phosphate/mol protein, which could not be removed by exhaustive dialysis o r gel filtration on Sephadex (3-25. The enzyme has an absorpion maximum at 370 nm in sodium phosphate buffer, pH 5.8. Reduction of the enzyme with sodium borohydride abolished the absorption maximum at 370 nm with attendant loss of catalytic activity. The enzyme exhibited pH-dependent spectral changes. The enzyme was specific for L-glutamate and could not decarboxylate other amino acids tested. The enzyme was maximally active at pH 5.8 and a temperature of 37°C. Isoelectric focussing gave a pl of 4.7 K , values for L-glutamate and pyridoxal 5'-phosphate were 5.6 mM and 2 pM respectively. Thiol-directed reagents and heavy metal ions inhibited the enzyme, indicating that an -SH group is required for activity. The nature of the functional groups at the active site of the enzyme was inferred from competitive inhibition studies. L-Glutamate promoted inactivation of the enzyme caused by decarboxylation-dependent transamination was demonstrated. The characteristics of potato enzyme were compared with enzyme from other sources.Glutamate decarboxylase (GAD, L-glutamate 1-carboxylase) catalyses the conversion of L-glutamate to 4-aminobutyrate (Abu) by a-decarboxylation. It is the key enzyme in the Abu shunt pathway [I -61, which provides an alternative route for linking glutamate to tricarboxylic acid cycle. GAD is widely distributed in nature and occurs in bacteria [7], fungi [S], higher plants [9] and vertebrate neural tissue [lo]. The enzyme has been purified from bacteria [ l l , 121 and mammalian brain [13 -161 and characterized in detail. Although Abu is present in considerable amounts and this enzyme activity has been detected in crude extracts prepared from a number of higher plant tissues [17, 181, there are only very few reports on the purification of GAD from higher plants [19-211. Potato tuber is known to contain good amount of Abu in its free amino acid pool [22] but the metabolic function of Abu in these tubers is not known. We have undertaken a study on the formation of Abu and its further metabolism in potatos and we report here a simple and rapid method for purification of GAD from potato tubers and some of the characteristics of this enzyme. MATERIALS AND METHODSPotato tubers (Solanum tuherosum, Kufri Chandramukhi cultivar) were obtained from the local market within one month after harvest and were stored at room temperature during the studies.

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