The reaction of 5-hydroxyindoleacetic acid with ortho-phthalaldehyde and cysteine.

The metabolite 5-hydroxyindoleacetic acid, the major metabolic product of serotonin, can be measured by reacting it with orthophthalaldehyde to form a highly fluorescent product. The inclusion of cysteine in this reaction causes an increase in the amount of fluorescence. The rate of fluorescence formation for reaction containing cysteine is much faster than the rate for the reaction without cysteine. The differences observed for both excitation and remission maxima between reaction products formed in the presence and absence of cysteine indicate that these products are different. Furthermore, the significant difference in the regression coefficients of fluorescence of the reaction products versus HC1 concentration adds further evidence that the reaction products formed in the presence and absence of cysteine are different. HC1 appears to influence the reaction of 5-HIAA, OPT and cysteine in several ways: by causing the destruction of 5-HIAA, by affecting the fluorescence of the product formed, and by influencing the amount of reaction product formed.