Identification of 3,4-dihydroxyphenylalanine, 5,6-dihydroxyindole, and N-acetylarterenone during eumelanin formation in immune reactive larvae of Drosophila melanogaster.

3,4-Dihydroxyphenylalanine, 5-6-dihydroxyindole, and N-acetylarterenone were detected by electrochemical methods in the hemolymph of immune reactive larvae of Drosophila melanogaster following parasitization by the wasp Leptopilina boulardi. Determinations of the catechols were made after separation by reverse phase, ion-pairing high pressure liquid chromatography with electrochemical detection. The presence of 5,6-dihydroxyindole unequivocally establishes the eumelanin pathway in the defense response of Drosophila, and confirms previous investigations which have implicated certain catecholamine metabolizing enzymes in insect immunity. The occurrence of N-acetylarterenone, a derivative of the principal sclerotizing agent N-acetyldopamine, verifies the existence and proposed involvement of quinone methide isomerase in the regulation of catecholamine metabolism, and suggests that the cellular capsule formed by Drosophila in immune reactions against parasites is most likely a composite of both eumelanin and sclerotin. The absence of 3,4-dihydroxyphenylacetic acid in hemolymph samples from immune reactive hosts suggests that during parasitization certain catecholamines and metabolic precursors may be re-employed in alternate pathways, some of which may be used in defense reactions.