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The dynamics of polarized fluorescence in reduced nicotinamide adeninedinucleotide (NADH) at 436~nm under two-photon excitation at 720~nm byfemtosecond laser pulses in alcohol dehydrogenase (ADH)-containing buffersolution has been studied experimentally and theoretically. A global fitprocedure was used for determination of the fluorescence parameters fromexperimental data. The interpretation of the experimental results obtained wassupported by \emph{ab initio} calculations of NADH structure in solutions. Atheoretical model was developed for description of the polarized fluorescencedecay in enzyme-NADH binary complexes that considered several possibleinteraction scenarios. We suggest that the origin of a significant enhancementof the nanosecond decay time value in the ADH-bounded NADH compare with thefree NADH can be attributed to the significant decrease of non-radiativerelaxation probabilities due to decrease of charges separations in thenicotinamide ring in the conditions of an apolar ADH-NADH binding siteenvironment. The existence of a single decay time in the ADH-NADH complex inthe nanosecond time-domain in comparison with two decay times observed in freeNADH can be attributed to a single NADH unfolded \emph{trans}-like conformationbounded within the ADH site. Comparison of the experimental data obtained andthe theory developed suggested the existence of an anisotropic relaxation timeof about 1 ns related most likely with interactions between excited NADH andthe ADH binding site that resulted in the rearrangement of nuclear distributionand rotation of fluorescence transition dipole moment.

Two-Photon Excited Fluorescence Dynamics in Enzyme-Bound NADH: the Heterogeneity of Fluorescence Decay Times and Anisotropic Relaxation