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Granule‐associated flavin adenine dinucleotide (FAD) is responsible for eosinophil autofluorescence

Unstained human eosinophils exhibit marked autofluorescence in comparison to other leukocytes due to a granule‐associated fluorescent substance. Fluorescence spectroscopy of granule extracts reveals excitation maxima at ~380 and ~450 nm with a single emission at ~520, characteristic of flavins. The fluorescent material from eosinophil granule extracts was characterized by fluorescence, high‐performance liquid chromatographic, and enzymatic analyses. First, acidification to pH 2.6 resulted in increased fluorescence, indicative of flavin adenine dinucleotide (FAD). Second, because flavin mononucleotide (FMN) and riboflavin cannot be distinguished by acidification, high‐performance liquid chromatography was performed and revealed a predominance of FAD and smaller amounts (<15%) of both FMN and riboflavin. Third, the presence of FAD was clearly demonstrated by reconstitution of the activity of d‐amino acid oxidase, a FAD‐dependent enzyme, when granule extracts were added to the apoenzyme. Thus, we have identified FAD as the predominant fluorophore in eosinophil granules. The small amounts of FMN and riboflavin detected may result from the hydrolysis of FAD under the acidic conditions of granule extraction. Because fluorescent material is deposited onto target cells by eosinophils, it is possible that granule‐associated flavoproteins may act as a source of hydrogen peroxide and/or superoxide, which, in conjunction with eosinophil peroxidase, could yield potent cytotoxic agents.