Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes

Abstract

This review presents a general view of all types of melanin in all types of organisms. Melanin is frequently considered just an animal cutaneous pigment and is treated separately from similar fungal or bacterial pigments. Similarities concerning the phenol precursors and common patterns in the formation routes are discussed. All melanins are formed in a first enzymatically-controlled phase, generally a phenolase, and a second phase characterized by an uncontrolled polymerization of the oxidized intermediates. In that second phase, quinones derived from phenol oxidation play a crucial role. Concerning functions, all melanins show a common feature, a protective role, but they are not merely photoprotective pigments against UV sunlight. In pathogenic microorganisms, melanization becomes a virulence factor since melanin protects microbial cells from defense mechanisms in the infected host. In turn, some melanins are formed in tissues where sunlight radiation is not a potential threat. Then, their redox, metal chelating, or free radical scavenging properties are more important than light absorption capacity. These pigments sometimes behave as a doubleedged sword, and inhibition of melanogenesis is desirable in different cells. Melanin biochemistry is an active field of research from dermatological, biomedical, cosmetical, and microbiological points of view, as well as fruit technology.

Activation of Melanin Synthesis in Alternaria infectoria by Antifungal Drugs

“…We performed an extensive study, using elemental analysis, UV/infrared absorption spectrophotometry, measurement of the zeta potential, and solid-state NMR analysis (43)(44)(45)(46), in order to identify the chemical characteristics of A. infectoria melanin (see the supplemental material) and found that A. infectoria melanin-derived pigments exhibited substantial absorption in the UV spectrum and none in the visible light spectrum, as well as diverse spectroscopic characteristics shared with other melanins described to date (47), including A. alternata DHN-melanin (see the supplemental material). To ascertain the chemical nature of the melanin, we used specific inhibitors of the DHN-melanin (pyroquilon) and of the DOPA-melanin (glyphosate) synthetic pathways.…”

Structural characterization of allomelanin from black oat

“…Melanins in fungi and other microorganisms are derived from tyrosine via 3,4-dihydroxyphenylalanine (DOPA), γ-glutaminyl-3,4-dihydroxybenzene (GDHB) or catechol, and 1,8-dihydroxynaphthalene (DHN) (Bell and Wheeler, 1986). It is believed that the precursor in plants is catechol, caffeic, chlorogenic, protocatechuic, or gallic acid (Solano, 2014), but there is not yet evidence. The structures of melanins have rarely been studied due to the difficulties in the isolation of melanins from natural sources and the poor solubility of the pigments.…”

Genetic background of coat colour in sheep

“…Han et al (2015) analysed the presumed relation and interaction between the ovine KIT and MITF gene expression and whether it affects the colour of sheep wool. The MITF gene codes for a protein microphthalmia-associated transcription factor, the most obvious function of which is the development of melanocytes; the MITF expression is correlated with the pigment accumulation in melanocytes (Steingrímsson et al, 2004). Causative mutations in MITF are mostly recognized to be involved in metabolic disorders in melanocytes, affecting the pigment content of the skin, hair, and eyes.…”

“…These cells are specialized in producing melanins -pigments protecting organisms from ultraviolet radiation (Solano, 2014). Melanins are divided into eumelanins (black/brown, pigmented phenotype) and pheomelanins (red/yellow, non-pigmented phenotype).…”