γ-Aminobutyric Acid Metabolism in Plants

Abstract

The metabolism of γ-aminobutyric acid (γAB) has been studied in higher plants, particularly in peas and peanuts. Transamination appeared to form the first step in γAB degradation although transaminase activities were very low. The relatively active γAB transaminase associated with whole pea plants possessing nodulated roots appears to reside almost entirely within the nodules. γAB transamination was demonstrated conclusively in extracts of mitochondria from cotyledons of peanut seedlings; pyruvic acid acted as a better amino-group acceptor than α-ketoglutaric acid (αKG). γAB transaminase activity present in the microsomal and soluble cytoplasmic fractions of the cells was very low γAB was not metabolized perceptibly by intact mitochondria from peanut, but when various organic acids were supplied simultaneously, an extra uptake of oxygen occurred and was associated with γAB disappearance. Aspartate, alanine, and ammonia were formed using the nitrogen atom of γAB. The metabolic pathway followed by the carbon skeleton of γAB was traced by supplying C¹⁴-labelled material to leaf discs of peas and to mitochondria from peanut cotyledons. Radioactivity was incorporated into organic acids, amino-acids, and respiratory carbon dioxide in a manner suggesting that γAB was converted into succinate which was then metabolized by the enzymes of the Krebs cycle present in the plant mitochondria. Glutamic decarboxylase was shown to be present largely in the non-particulate (soluble) cytoplasm of cells. The enzymes responsible for γAB synthesis and degradation, glutamic decarboxylase, and γAB transaminase, respectively, therefore largely reside in different sub-cellular fractions.