Prominent Roles and Conflicted Attitudes of Eumelanin in the Living World

Abstract

Eumelanin, a macromolecule widespread in all the living world and long appreciated for its protective action against harmful UV radiation, is considered the beneficial component of the melanin family (ευ means good in ancient Greek). This initially limited picture has been rather recently extended and now includes a variety of key functions performed by eumelanin in order to support life also under extreme conditions. A lot of still unexplained aspects characterize this molecule that, in an evolutionary context, survived natural selection. This paper aims to emphasize the unique characteristics and the consequent unusual behaviors of a molecule that still holds the main chemical/physical features detected in fossils dating to the late Carboniferous. In this context, attention is drawn to the duality of roles played by eumelanin, which occasionally reverses its functional processes, switching from an anti-oxidant to a pro-oxidant behavior and implementing therefore harmful effects.

Keywords: anti/pro-oxidant behaviour; electron transfer; eumelanin; fossil record; melanomas; metal ion interactions; radioprotection; radiotropism; stress responses.

Figure 1

A simplified version of the synthetic pathway leading to the DHI and DHICA oligomers. (Adapted from Ref. [1]. Copyright© 2017 by the authors. Licensee MDPI, Basel, Switzerland).

Figure 2

Summary of current understanding of the location of melanosomes in different body tissues and organs and putative functions in major vertebrate classes (reproduced from Ref. [53]. Copyright© 2021 The Authors. Licensee Elsevier Ltd.).

Figure 3

Reversibility of redox processes and radical scavenging properties of EU revealed through mediated electrochemical probing (Adapted with permission from Ref. [73]. Copyright© 2019 The Authors. Published by Elsevier Ltd.).