Metabolism of glutamate and T'-aminobutyrate in cultured tobacco cells

In cultured tobacco cells glutamate-U-14C administrated was readily converted to j-aminobutyrate (GABA) by decarboxylation, however, GABA-1-14C remained un-changed. Glutamate decarboxylase was found in tobacco cells and reached its maximum activity in the rapidly growing stage during culture. Enzyme activity paral-leled formation of GABA from glutamate-U-uC. A high content of GABA in tobacco cells seems to be due to the rapid decarboxylation of glutamate by glutamate decarboxy-lase and a slow turn over of GABA. Glutamate plays a central role in the biosynthesis of amino acids. There have been many reports on the fate of this amino acid in plants {1-3). Naylor and Tolbert (4) observed that in leaves of barley seedling fed glutamate-^C; glutamine, malate, and aspartate were major products in darkness. In an anaer-obic state, GABA was the major product. Recently, in studies on acetate meta-bolism in cell suspension cultures of Paul's Scarlet rose, Fletcher and Beevers (5) examined the distribution of radioactivity recovered after feeding glutamate-14C. Malate was the most heavily labeled compound. GABA is generally accepted as the decarboxylation product of glutamate (6, 7) and a wide distribution of glutamate decarboxylase in plants has been found by Okunuki (6"), and Schales et al. (9). The further metabolism of GABA involves transamination to give succinic semialdehyde, which is then converted to succinate to undergo final oxidation through the tricarboxylic acid cycle, as suggested by Naylor and Tolbert (4) and later demonstrated by Dixon and Fowden (6). As previously reported (10), cultured tobacco cells accumulate glutamine and GABA. This prompted us to examined the metabolism of glutamate and GABA in cultured tobacco cells, the results of which is reported here. Materials and methods Glutamate-U-14C (165 mCi/mmole) and GABA-1-14C (16 mCi/mmole) wer