ABSTRACT
Aldehyde dehydrogenase 2 (ALDH2) catalyzes oxidation of toxic aldehydes to carboxylic acids. Physiologic levels of Mg(2+) ions influence ALDH2 activity in part by increasing NADH binding affinity. Traditional fluorescence measurements monitor the blue shift of the NADH fluorescence spectrum to study ALDH2-NADH interactions. By using time-resolved fluorescence spectroscopy, we have resolved the fluorescent lifetimes (τ) of free NADH (τ=0.4 ns) and bound NADH (τ=6.0 ns). We used this technique to investigate the effects of Mg(2+) on the ALDH2-NADH binding characteristics and enzyme catalysis. From the resolved free and bound NADH fluorescence signatures, the K(D) for NADH with ALDH2 ranged from 468 µM to 12 µM for Mg(2+) ion concentrations of 20 to 6000 µM, respectively. The rate constant for dissociation of the enzyme-NADH complex ranged from 0.4s(-1) (6000 µM Mg(2+)) to 8.3s(-1) (0 µM Mg(2+)) as determined by addition of excess NAD(+) to prevent re-association of NADH and resolving the real-time NADH fluorescence signal. The apparent NADH association/re-association rate constants were approximately 0.04 µM(-1)s(-1) over the entire Mg(2+) ion concentration range and demonstrate that Mg(2+) ions slow the release of NADH from the enzyme rather than promoting its re-association. We applied NADH fluorescence lifetime analysis to the study of NADH binding during enzyme catalysis. Our fluorescence lifetime analysis confirmed complex behavior of the enzyme activity as a function of Mg(2+) concentration. Importantly, we observed no pre-steady state burst of NADH formation. Furthermore, we observed distinct fluorescence signatures from multiple ALDH2-NADH complexes corresponding to free NADH, enzyme-bound NADH, and, potentially, an abortive NADH-enzyme-propanal complex (τ=11.2 ns).
