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Enzymatic fractionation of carbon isotopes by phosphoenolpyruvate carboxylase from c(4) plants.

The carbon atoms of glucose and malate in C(4) plants are 2 to 3 per thousand enriched in (12)C with respect to atmospheric CO(2); whereas these intermediates in C(3) plants are 15 to 18 per thousand enriched with (12)C with respect to atmospheric CO(2). The enzymatic synthesis of malate from phosphoenolpyruvate and bicarbonate in preparations of leaves of Sorghum bicolor, Haygrazer result in a carbon isotope fractionation of about 3 per thousand. The enzymatic synthesis of phosphoglyceric acid from ribulose 1,5-diP and CO(2) in these preparations (contaminated with carbonic anhydrase) at 24 C and 37 C result in a carbon isotope fractionation of 33.7 per thousand and 18.3 per thousand, respectively. These data are consistent with the conclusion that the small enrichment of (12)C in the carbon atoms of malate and glucose (with respect to atmospheric CO(2)) in leaves of Sorghum bicolor, Haygrazer occurs at the phosphoenolpyruvate carboxylase step.