Direct transfer of reduced nicotinamide adenine dinucleotide from glyceraldehyde-3-phosphate dehydrogenase to liver alcohol dehydrogenase.

The reduction of benzaldehyde and p-nitrobenzaldehyde by NADH, catalyzed by horse liver alcohol dehydrogenase (LADH), has been found to be faster when NADH is bound to glyceraldehyde-3-phosphate dehydrogenase (GPDH) than with free NADH. The rate of reduction of aldehyde substrate with GPDH-NADH follows a Michaelian concentration dependence on GPDH-NADH. The reaction velocity is independent of GPDH concentration when [GPDH] greater than [NADH]total. The Km for GPDH-NADH is higher than that for free NADH. The reaction velocities in the presence of excess GPDH over NADH cannot be accounted for on the basis of the free NADH concentration arising from dissociation of the GPDH-NADH complex. These observations suggest that transfer of NADH from GPDH to LADH proceeds through the initial formation of a GPDH-NADH-LADH complex. Arguments for a direct enzyme-coenzyme-enzyme transfer mechanism are substantiated and quantitated both by steady-state kinetic studies and by determinations of all of the appropriate enzyme-coenzyme equilibrium dissociation constants. In contrast, over a similar concentration range, the complex lactate dehydrogenase (LDH)-NADH is not a substrate for the LADH-catalyzed reductions. Likewise, the LADH-NADH complex is not a substrate for the LDH-catalyzed reduction of pyruvate.