GLUTAMIC CARBOXYLASE OF THE MATURE WHEAT LEAF

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Abstract
A study of the distribution of glutamic carboxylase within the developing wheat plant revealed that it was absent in young plants, and was present only in traces in mature roots, and that it accumulated in mature leaves. Glutamic carboxylase was particularly abundant in the mature and senescent third leaf. Extracts of leaves of other cereals showed only weak activity, while extracts of roots, other than barley, were inactive. The high enzyme activity of the barley root extracts was exceeded only by that of mature wheat leaf extracts. A convenient method is described for enzyme storage at −40 °C. and a purification procedure was developed which effected a 500-fold concentration (nitrogen basis). The glutamic carboxylase activity of crude extracts was enhanced by preparatory exposure to phosphate buffer; after selective salt precipitation and lengthy dialysis, activity was reduced, but could be restored by the addition of pyridoxal phosphate. A heat-stable inhibitor of glutamic carboxylase at its pH optimum was found in the ether-soluble organic acid fraction of the cell sap of Kalanchoe leaves. Similar inhibiting effects were shown by malate, tartrate, and citrate, but not by succinate, fumarate, aspartate, and alanine. Kinetic studies indicated that the inhibition of plant glutamic carboxylase by cyanide is noncompetitive.