L-glutamate 1-carboxylase and 4-aminobutyrate: 2-oxoglutarate aminotransferase in succulent plants

Abstract

The results are given which have been obtained in the prospective study of the glutamate decarboxylase in 23 plant species belonging to 6 botanical familes of succu­ lent plants. L-glutamate 1-carboxylyase is found in all the species of Cactaceae, Eufor- biaceae, Liliaceae and Amarilidaceae tested. On the other hand, it has not been found in the species of Crasulaceae and Aizoaceae which have been studied. The optimum pH (5.7-6.O) and the Michaelis-Menten constant (1.6 and 4.7 X 10-3 M for Cercus tor- tuosus and Opuntia vulgaris respectively) agree with the data published for this enzyme in plants. In the experimental conditions, the coenzyme molecule (pyridoxal-5-phos- phate) remains firmly bound to the protein. By means of «salting out» a 20- fold purified fraction has been obtained from Opuntia vulgaris stem.

Aminotransferase activities have been investigated in Opuntia vulgaris (stem and fruit), Cereus tortuosus (stem) and Agave americana (flowers, leaves and scape). Trans­ aminating activity has been found between 4-aminobutyrate and 2-oxoglutarate in all those materials. Pyruvate and oxaloacetate could not replace 2-oxoglutarate as keto­ acid acceptors. The best results have been obtained —according to its apparent intra- mitochondrial location — with preparations of ultrasonically disrupted mitochondria. The inverse transaminating reaction —glutamate: succinic semialdehyde aminotrans­ ferase— has also been clearly shown.

These results seem to confirm that the alternative 4-aminobutyrate by-pass system exists in most of the succulent plants studied, whose well-known hypoxic metabolism gives especial interest to the knowledge of its physiological role.