Unraveling complex molecular transformations of N-β-alanyldopamine that account for brown coloration of insect cuticle.

N-β-Alanyldopamine (NBAD) and N-acetyldopamine (NADA) are catecholamines that are used by insects as sclerotizing precursors to harden their cuticle. They share a common pathway utilizing the same set of sclerotizing enzymes. Yet, cuticles using NBAD are brown, while cuticles using NADA are colorless. To identify the cause of this major unresolved color difference, molecular transformations of NBAD with cuticular enzymes were investigated. Reactions of NBAD and NADA with native cuticle isolated from the wandering stages of Sarcophaga bullata larvae as well as the reactions of NBAD with cuticular sclerotization enzymes - phenoloxidase, quinone isomerase and quinone methide isomerase - were investigated using UV-Vis spectroscopy, high-performance liquid chromatography (HPLC), and mass spectrometry (MS). In addition, the reactivity of enzymatically generated NBAD quinone was investigated by MS. Reactions of NBAD with sclerotizing enzymes isolated from Sarcophaga bullata larvae generate colorless products such as N-β-alanylnorepinephrine, N-β-alanylarterenone, dehydro NBAD, the benzodioxan dimers of dehydro NBAD and other minor adducts, the same kind of compounds generated by NADA reaction with cuticular enzymes. However, oxidation of NBAD produces colored quinone adducts, in addition. NADA, which lacks the amino group, did not produce these quinone adducts. LC/MS analysis of the reaction mixture of NBAD-cuticular enzyme reactions reveals the novel production of colored quinone adducts that are not possible for NADA. Therefore, our results suggest that the brown coloration of cuticle formed through NBAD crosslinking is likely due to the formation and accumulation of NBAD quinone and its adducts, while NADA quinone adducts tend not to form during NADA crosslinking, producing a nearly colorless cuticle.