Melanization in living organisms: a perspective of species evolution

Abstract

Eumelanin is a heteropolymer that is generally composed of hydroxylated indole residues and plays diverse protective functions in various species. Melanin is derived from the amino acid tyrosine and production of melanin is a highly complex oxidative process with a number of steps that can either proceed enzymatically or non-enzymatically. Although melanin plays important protective roles in many species, during melanization, particularly in steps that can proceed non-enzymatically, many toxic intermediates are produced, including semiquinones, dopaquinone, indole-quinones and moreover, the production of many reactive oxygen species. To mitigate the production of reactive species, a number of proteins that regulate the biochemical process of melanization have evolved in various living species, which is closely related to adaptation and physiological requirements. In this communication, we discuss differences between non-enzymatic and enzymatic processes of melanization and the enzymatic regulation of melanization in difference species with an emphasis on differences between mammals and insects. Comparison between melanization and insect sclerotization is also emphasized which raises some interesting questions about the current models of these pathways.

Lipase-Catalyzed Synthesis of Kojic Acid Derivative in Bioreactors and the Analysis of Its Depigmenting and Antioxidant Activities

“…In this process, oxidation of L-dopa by oxygen molecules produces a dopa-semiquinone radical and superoxide. The dopa-semiquinone might be oxidized by superoxide, resulting in the formation of dopaquinone and hydrogen peroxide, respectively [33].…”

Melanocytes and Skin Immunity

“…Melanization has important protective roles in many species as many toxic intermediates may be produced, including semiquinones, dopaquinone, indolequinones, and many reactive oxygen species (Vavricka et al, 2010). These intermediate compounds are believed to exert strong antimicrobial activities, and melanin, the end-product of melanization, may have the capacity to trap, inhibit, and even kill invading bacteria and other microorganisms (Mackintosh, 2001;Burkhart and Burkhart, 2005;Fuentes et al, 2014).…”

A Venom Serpin Splicing Isoform of the Endoparasitoid Wasp Pteromalus puparum Suppresses Host Prophenoloxidase Cascade by Forming Complexes with Host Hemolymph Proteinases

“…Melanization is a conserved part of humoral immunity in insects (3). The melanization pathway is initiated in response to infection and wounding (4) and is followed by a sequential activation of serine proteinases (5).…”

“…Active PO oxidizes tyrosine and o-diphenols to quinones, which further polymerize to form melanin. Melanin is then deposited on the surface of invaders and seals them from spreading within the host or, in the case of parasitoid eggs, from hatching and completing development (3,6).…”