Compartmentation of Organic Acids in Corn Roots I. Differential Labeling of 2 Malate Pools

Bicarbonate-(14)C and acetate-(3)H were simultaneously provided to corn roots to give 2 isotopic forms of malate in the tissue, malate-(14)C produced by dark fixation reactions and malate-(3)H produced by reactions of the tricarboxylic acid cycle. Following a short pulse of exposure to the isotopes, the dissimilation of both isotopic forms of malic acid was followed. The rate of utilization of malate-(3)H was much faster than that of malate-(14)C.These results are interpreted as showing that the malate produced from (14)CO(2) is in a pool physically separated from that in the tricarboxylic acid cycle. The introduction of the 2 isotopes through distinct metabolic pathways produced the differential labeling of 2 distinct pools of malate.