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The mechanisms of reactions underlying the fluorophore formation from indolylethylamines in the Falck-Hillarp histochemical formaldehyde method were investigated with the aid of thin layer chromatography and mass spectrometry of the fluorescent products formed in protein models and freeze-dried tissue. In the reaction of formaldehyde with tryptamine and 5-hydroxytryptamine, the main fluorophores formed were 3,4-dihydro-β-carboline and the 2-methyl-3,4-dihydro-β-carbolinium compound (from tryptamine), and 6-hydroxy-3,4-dihydro-β-carboline and the 2-methyl-6-hydroxy-3,4-dihydro-β-carbolinium compound (from 5-hydroxytryptamine). From these findings, it is concluded that the fluorophore formation in the Falck-Hillarp method proceeds as follows: In the first step of the reaction, the indolylethylamines react with formaldehyde to form low fluorescent 1,2,3,4-tetrahydro-β-carbolines. In a subsequent step, these products are converted to fluorophores in either of two ways: through an autoxidation to 3,4-dihydro-β-carbolines, or through a second, acid-catalyzed reaction with formaldehyde to yield 2-methyl-3,4-dihydro-β-carbolinium compounds. Experiments with radioactive tryptamine indicated that the two alternative fluorophore-forming pathways are of fairly equal importance. The latter of these fluorophore-forming reactions was not previously known, and the interesting properties and implications of this formaldehyde-induced and acid-catalyzed reaction are discussed.

NEW ASPECTS ON REACTION MECHANISMS IN THE FORMALDEHYDE HISTOFLUORESCENCE METHOD FOR MONOAMINES