Dehydrogenase Binding Sites Abolish the "Dark" Fraction of NADH: Implication for Metabolic Sensing via FLIM

The fluorescence of dinucleotide NADH has been exploited for decades to determine the redox state of cells and tissues in vivo and in vitro. Particularly, nanosecond (ns) fluorescence lifetime imaging microscopy (FLIM) of NADH (in free vs bound forms) has recently offered a label-free readout of mitochondrial function and allowed the different "pools" of NADH to be distinguished in living cells. In this study, the ultrafast fluorescence dynamics of NADH-dehydrogenase (MDH/LDH) complexes have been investigated by using both a femtosecond (fs) upconversion spectrophotofluorometer and a picosecond (ps) time-correlated single photon counting (TCSPC) apparatus. With these enhanced time-resolved tools, a few-picosecond decay process with a signatory spectrum was indeed found for bound NADH, and it can best be ascribed to the solvent relaxation originating in "bulk water". However, it is quite unlike our previously discovered ultrafast "dark" component (∼26 ps) that is prominent in free NADH (Chemical Physics Letters 2019, 726, 18-21). For these two critical protein-bound NADH exemplars, the decay transients lack the ultrafast quenching that creates the "dark" subpopulation of free NADH. Therefore, we infer that the apparent ratio of free to bound NADH recovered by ordinary (>50 ps) FLIM methods may be low, since the "dark" molecule subpopulation (lifetime too short for conventional FLIM), which effectively hides about a quarter of free molecules, is not present in the dehydrogenase-bound state.