Biochemistry of Amino Acids - A. Meister 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Decarboxylation of Amino Acids
Plant Amino Acid Decarboxylases

Okunuki [217] first discovered glutamate decarboxylase in certain higher plants in 1937; it was later shown that this enzyme is widely distributed throughout the plant kingdom. $\gamma$-Aminobutyric acid turned out to be equally widespread. Plant glutamate decarboxylase has been thoroughly investigated by Schales and coworkers [218, 219]. They obtained the partially purified enzyme from carrots, with an optimum pH around 6. To ensure that the decarboxylation of glutamic acid follows first-order kinetics, The addition of Pyridoxal phosphate is required [219].

Fowden and Done [220, 221] recently discovered another amino acid decarboxylase in plants. Shortly before that, these authors isolated $\gamma$-methylglutamic and $\gamma$-amino-$\alpha$-methylenebutyric acids from peanuts. The structural relationships among these Amino Acids suggested, by analogy with the decarboxylation of glutamic acid, that $\gamma$-amino-$\alpha$-methylenebutyric acid originates from $\gamma$-methylglutamic acid. It was experimentally demonstrated that extracts from peanuts, red pepper, and barley roots catalyze the following reaction:

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This decarboxylase has not yet been purified; whether it is identical to glutamate decarboxylase remains unknown at present.