L-ribulose-5-phosphate 4-epimerase from Aerobacter aerogenes.

The mechanism of L-ribulose 5-phosphate 4-epimerase of Aerobacfer aerogenes has been investigated by determining the kinetic isotope effect (KT/KH) for the epimerization of D-[4-T]xylulose 5-phosphate in the presence of D-[l-14C]xylulose 5-phosphate. The ratio of T:14C was determined for the isolated L-ribulose derived from aliquots of an epimerization reaction mixture during its progress toward equilibrium. From a plot of the ratio of the observed isotope ratio at any time to the initial or final ratio versus percentage of attainment of equilibrium, it was determined that there was essentially no isotope effect. This result contrasts with that obtained with uridine diphosphate glucose 4-epimerase where a normal isotope effect was observed and is consistent with recent observations that L-ribulose 5-phosphate 4-epimerase functions by a mechanism not involving oxidation-reduction at carbon 4.