Insect melanogenesis. II. Inability of Manduca phenoloxidase to act on 5,6-dihydroxyindole-2-carboxylic acid.

Eumelanins in animals are biosynthesized by the combined action of tyrosinase, 3,4-dihydroxyphenylalanine (DOPA)chrome isomerase, and other factors. Two kinds of eumelanins were characterized from mammalian systems; these are 5,6-dihydroxyindole (DHI)-melanin and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin. In insects, melanin biosynthesis is initiated by phenoloxidase and supported by DOPAchrome isomerase (decarboxylating). Based on the facts that DOPA is a poor substrate for insect phenoloxidases and DHI is the sole product of insect DOPAchrome isomerase reaction, it is proposed that insects lack DHICA-melanin. Accordingly, the phenoloxidase isolated from the hemolymph of Manduca sexta failed to oxidize DHICA. Control experiments reveal that mushroom tyrosinase, as well as laccase, which is a contaminant in the commercial preparations of mushroom tyrosinase, are capable of oxidizing DHICA. Neither the whole hemolymph nor the cuticular extracts of M. sexta possessed any detectable oxidase activity towards this substrate. Thus, insects do not seem to produce DHICA-eumelanin. A useful staining procedure to localize DHICA oxidase activity on gels is also presented.