Oxidation of the carbanion intermediate of transaldolase by hexacyanoferrate (III)

The transaldolase-dihydroxyacetone carbanion intermediate formed in the reaction of transaldolase with its donor substrates fructose-6-P or sedoheptulose-7-P is susceptible to oxidation by hexacyanoferrate(III). The dihydroxyacetone moiety is oxidized to the corresponding 2-ketoaldehyde, i.e. hydroxy-pyruvaldehyde (CH2OH-CO-CHO). This oxidation product is, in contrast to dihydroxyacetone, readily released from the enzyme. In the presence of hexacyanoferrate(III) transaldolase thus functions as an efficient catalyst of the oxidative cleavage of its donor substrates fructose-6-P or sedoheptulose-7-P into hydroxypyruvaldehyde and glyceraldehyde-3-P or erythrose-4-P, respectively. Two moles of hexacyanoferrate(III) are reduced per mole of oxidatively cleaved donor substrate. The molecular activity for oxidative cleavage of fructose-6-P at a hexacyanoferrate(III) concentration of 0.5 mM is 0.65% of that for the normal transfer reaction with erythrose-4-P as the acceptor substrate. The present data emphasize the applicability of certain oxidants as trapping agents for enzymatic carbanion intermediates as proposed previously (Healy, M.J.,, and Christen, P. (1973) Biochemistry 12, 35-41).