A specific fluorogenic reaction for tryptophan residues using isatin in organic solvents.

The development of new fluorogenic reactions for specific chemical groups is of increasing interest in cytochemistry. We describe the application of the nonfluorescent compound isatin on methanol-fixed blood smears. When treated with 0.1% isatin in absolute ethanol or acetone for 30 min and observed under violet-blue exciting light, eosinophil leucocyte granules show a bright green fluorescence. This fluorogenic reaction is abolished after blocking of tryptophan residues by performic acid oxidation. Spectrofluorometric studies with amino acids in vitro reveal that isatin forms a unique and specific fluorescent product with tryptophan.