Date of Award
Campus Access Dissertation
Doctor of Philosophy (PhD)
Biology/Molecular, Cellular, and Organismal Biology
Melanin is a phenolic biopolymer widely distributed as skin, eye, and hair color pigment in all animals. Two kinds of melanin, the brown to black eumelanin and the yellow to red pheomelanin are known to be present in higher animals. The occurrence of eumelanin in insects has been known for decades, but the presence of pheomelanin has only been reported recently, although the chemical nature of either type of melanin has not yet been characterized from any insect species. For that reason, I undertook a comprehensive study on the chemical nature of melanin pigment in insect cuticle.
The first goal of my research is to unravel the chemical nature of insect’s melanin and to develop a comprehensive model for insect cuticular pigmentation. I first conducted a study on how different sclerotizing precursors that are metabolized by the same set of reactions can lead to different cuticle colors. Then, I analyzed melanin obtained from different insect samples and demonstrated that insect eumelanin is mostly made up of 5,6-dihydroxyindole (DHI) originating predominantly from dopamine and not from dopa. More importantly, I showed that insects make significant amounts of pheomelanin, again originating primarily from dopamine. This result pointed out a number of similarities between insect melanogenesis and mammalian neuromelanin formation. These are: the usage of dopamine as the specific precursor of melanin component, lack of participation of signaling pathway, the nature of genes involved, and the lack of melanocyte participation. Consequently, I questioned whether insects have the potential to make neuromelanin or not. To address this question, I analyzed the presence of enzymes that can catalyze the oxidation of dopamine as well as dopachrome tautomerase in the brain tissue homogenates. My biochemical analysis revealed the presence of both phenoloxidase and dopachrome decarboxylase/tautomerase in the brain extracts of Drosophila melanogaster. Thus Drosophila has the enzymatic machinery to make the melanic component of neuromelanin.
Based on my findings on how important dopamine metabolites in the contest of insects melanogenesis and sclerotization processes, I hypothesized the existence of a new enzyme that would participate in the conversion of dopaminechrome to DHI. My search for such an enzyme in Drosophila melanogaster culminated in the characterization of a novel protein, dopaminechrome tautomerase, capable to perform the isomerization of dopaminechrome to DHI with no apparent activity towards dopachrome. Finally I was also able to demonstrate that the yellow-h gene, a member of the yellow gene family in Drosophila melanogaster, encodes dopaminechrome tautomerase. Therefore, my study presents a step forward towards understanding the overall physiological functions of the Drosophila yellow gene family.
Barek, Hanine S., "Dopaminechrome Tautomerase: A New Enzyme in the Eumelanin Biosynthetic Pathway" (2018). Graduate Doctoral Dissertations. 441.