The Onset of Tricarboxylic Acid Cycle Activity with Aging in Potato Slices

Abstract

Fresh potato slices released no radioactive CO2 from organic acids of the tricarboxylic acid cycle labeled with C14 in such a way that a full turn of the cycle was required for label evolution. In marked contrast, aged potato disks evolved copious quantities of C14O2 from the same organic acids. The labeled acids provided were pyruvate, citrate, [alpha] -ketoglutarate, succinate, fumarate, malate, acetate, and glycolate. Glucose, acetaldehyde, and malonate were presented as well. A comparison of label distribution in fresh and aged tissue following administration of the above-listed substrates indicates a block in the tricarboxylic acid cycle in fresh tissue between the steps of citrate and [alpha]-ketoglutarate oxidation. Pyrrolidone-2-carboxylic acid, which appears to be derived from glutamine during hot extraction, shared the labeling characteristics of the organic acids both with respect to specific activity and the enhancement of labeling in aged disks. Gluta-mate labeling was much more extensive, and relatively indifferent to tissue age. Compartmentation is indicated, with the prospect that glutamate, and perhaps 1 or more of the 4-carbon dicarboxylic acids, may be synthesized extramitochondrially.