Serotonin acts as a radical scavenger and is oxidized to a dimer during the respiratory burst of human mononuclear and polymorphonuclear phagocytes*

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Abstract
Isolated human mononuclear and polymorphonuclear phagocytes were stimulated in the presence and absence of serotonin (5‐HT), the major secretory product of activated platelets, and the release of reactive oxygen metabolites during the respiratory burst was assessed by luminol‐enhanced chemilumi‐nescence. In the presence of 5‐HT, a dose‐dependent suppression of the chemiluminescence signal occurred, irrespective of the stimulus used to elicit the respiratory burst. A similar suppression of luminol‐enhanced chemiluminescence was also seen in a radical generating cell free system. 5‐HT was found to be oxidized by the reactive oxygen species released by stimulated phagocytes. This oxidation is prevented in the presence of other antioxidants. The major 5‐HT oxidation product was isolated by gel chromatography and identified by mass‐spectrometry as a 5‐HT dimer, probably 5,5′‐dihydroxy‐4,4′‐bitryptamine. It is concluded that the 5‐HT released from activated thrombocytes at sites of inflammation and endothelial cell damage acts as a true scavenger of reactive oxygen species generated during the respiratory burst of stimulated phagocytes and may thus modulate various aspects of cell‐mediated defence reactions.