Reactivity of old yellow enzyme with alpha-NADPH and other pyridine nucleotide derivatives

The reaction of Old Yellow Enzyme (OYE) with pyridine nucleotides has been examined using steady state kinetics, rapid reaction kinetics, and equilibrium binding. alpha-NADPH, beta-NADPH, and the acid breakdown products of NADPH all bind to oxidized OYE with dissociation constants below 1 microM. These complexes produce characteristic red shifts in the absorption spectrum of OYE. A similar red shift which occurs after multiple turnovers of OYE with NADPH has been found to be due to an impurity in the NADPH preparation, possibly an acid breakdown product. Anions such as chloride, acetate, azide, and phenolates compete with the pyridine nucleotides for binding to a common site in oxidized OYE. Anaerobic reduction of OYE by NADPH proceeds via two intermediates to establish a readily reversible equilibrium. In contrast to most other NADPH-dependent enzymes, both alpha- and beta-NADPH are capable of reducing OYE, and alpha-NADPH is more effective. Using beta-[4(R)-2H]NADPH, a primary deuterium isotope effect was observed in the reduction reaction. Results from rapid reaction and steady state studies showed that reduction of OYE was rate limiting in turnover. Consistent with this, the turnover number with alpha-NADPH was significantly higher than that with beta-NADPH.