CO(2) Metabolism in Corn Roots. I. Kinetics of Carboxylation and Decarboxylation.

The kinetics of (14)CO(2) carboxylation and decarboxylation in corn root tips were determined to ascertain the sequence of product formation and subsequent utilization, and to obtain further evidence to predict the enzymes mediating the carboxylation and decarboxylations. The carboxylation data indicated that the first product was oxaloacetate followed by malate and aspartate. Malate was the first stable product which could be detected. Decarboxylation data indicated that a large fraction of the (14)CO(2) release and turnover of (14)C was accountable for by a decrease in malate: however, essentially all labeled amino acids turned over rapidly and at a greater rate than organic acids. The data generally support the hypothesis that CO(2) fixation in corn root tips is via P-enolpyruvate carboxylase and malic dehydrogenase and that subsequent malate metabolism is for the most part by direct decarboxylation, possibly by the malic enzyme.