Flavin adenine dinucleotide is a major endogenous fluorophore in the inner ear.

When illuminated with visible light, hair cells can exhibit autofluorescence (Lewis et al. [1982] Science 215, 1641-1643) concentrated in the basal pole near the synapses (Sento and Furukawa [1987] J. Comp. Neurol. 258, 352-367). The autofluorescence is enhanced by formaldehyde. The level of fluorescence is high enough to interfere with fluorescence microscopy of hair cells and to suggest that the fluorescent substance might have a particular role in hair-cell function. To identify this substance, we extracted a substance with formaldehyde-enhanced fluorescence from the inner ears of goldfish and purified it chromatographically. The substance copurified with FAD and had the same fluorescence emission spectrum. Two further results supported the identity of the endogenous fluorescent substance with FAD. First, as is the case with flavins, the autofluorescence in inner ear tissue examined within a few hours after fixation was reduced by addition of dithionite. Second, as is the case with the formaldehyde-enhanced fluorophore, the fluorescence of FAD was enhanced by formaldehyde. FAD accounted for 90% of flavins in goldfish inner ears; its concentration in the sensory epithelium was estimated to be about 30 nmol/g tissue weight, one of the highest tissue concentrations known. The FAD is probably associated with an unidentified flavoprotein concentrated in the basal, synaptic region of the hair cell.