Compartmentation of Organic Acids in Corn Roots. III. Utilization of Exogenously Supplied Acids

The rates of utilization of exogenously supplied ¹⁴C labeled acids by corn roots was compared to the utilization of these acids generated endogenously in the mitochondria from acetate-³H. ¹⁴C-labeled citrate, pyruvate, succinate, glutamate or aspartate were supplied with acetate-³H in a 15 minute pulse and the ¹⁴C and ³H contents of extracted acids were measured over a 4 hour period. It was found, in contrast to previous experiments with malate, that these exogenously added acids were used as rapidly as the endogenous forms. Apparently, therefore, these acids penetrate readily into the mitochondria and do not enter cytoplasmic pools which are not in ready equilibrium with those in the mitochondria. Small amounts of labeled glutamate were produced from succinate-2,3-³H by corn root tissue. Since glutamate would not be expected to be labeled by reactions of the tricarboxylic acid cycle it was concluded that it was produced rather directly from succinate. The minor pool of glutamate generated in this way retained its radioactivity while that generated in the cycle was rapidly lost. An extra-mitochondrial location of this pool of glutamate is therefore suggested.