EVIDENCE FOR ORIENTATION-CONSERVED TRANSFER IN THE TCA CYCLE IN SACCHAROMYCES-CEREVISIAE - C-13 NMR-STUDIES

When[4-C-13]glutamate is incubated with yeast cells, the [C-13]asparta te formed shows a [2-C-13]/[3-C-13] ratio that is greater than 1. If t he conversion occurs through the Krebs TCA cycle with the symmetrical intermediates succinate and fumarate, the ratio is expected to be 1. T he addition of fluoroacetate to the incubation increases the ratio fur ther. When the mutants Aco1- or CS1- are used, little or no aspartate is formed. When [2-C-13]acetate is incubated with yeast, the C2-/C3-as partate ratio is 1.5 at 2 min and decreases to 1.0 after 10 min of inc ubation. These results indicate that orientation-conserved transfer oc curs at the succinate thiokinase and succinate dehydrogenase steps of the Krebs TCA cycle.