Marco d'Ischia

Chemical and structural diversity in eumelanins : unexplored bio-optoelectronic materials

Eumelanins, the characteristic black, insoluble, and heterogeneous biopolymers of human skin, hair, and eyes, have intrigued and challenged generations of chemists, physicists, and biologists because of their unique structural and optoelectronic properties. Recently, the methods of organic chemistry have been combined with advanced spectroscopic and imaging techniques, theoretical calculations, and methods of condensed-matter physics to gradually force these materials to reveal their secrets. Herein we review the latest advances in the field with a view to showing how the emerging knowledge is not only helping to explain eumelanin functionality, but may also be translated into effective strategies for exploiting their properties to create a new class of biologically inspired high-tech materials.

Melanins and melanogenesis: methods, standards, protocols

Despite considerable advances in the past decade, melanin research still suffers from the lack of universally accepted and shared nomenclature, methodologies, and structural models. This paper stems from the joint efforts of chemists, biochemists, physicists, biologists, and physicians with recognized and consolidated expertise in the field of melanins and melanogenesis, who critically reviewed and experimentally revisited methods, standards, and protocols to provide for the first time a consensus set of recommended procedures to be adopted and shared by researchers involved in pigment cell research. The aim of the paper was to define an unprecedented frame of reference built on cutting‐edge knowledge and state‐of‐the‐art methodology, to enable reliable comparison of results among laboratories and new progress in the field based on standardized methods and shared information.

Mechanism of inhibition of melanogenesis by hydroquinone

Hydroquinone (HQ) is one of the most effective inhibitors of melanogenesis in vitro and in vivo, and is widely used for the treatment of melanosis and other hyperpigmentary disorders. In an attempt to get some insight into the molecular mechanism of the depigmenting action, which is still very poorly understood, we have investigated the effect of HQ on the tyrosinase catalysed conversion of tyrosine to melanin. Incubation of 0.5 mM tyrosine with 0.07 U/ml tyrosinase in phosphate buffer at pH 6.8 in the presence of 0.5 mM HQ led to no detectable melanin formation, due to the preferential oxidation of HQ with respect to tyrosine (HPLC evidence). Kinetic investigations showed that HQ is a poorer substrate of tyrosinase than tyrosine; yet, it may be effectively oxidised in the presence of tyrosine owing to the generation of catalytic amounts of dopa acting as cofactor of tyrosinase. Product analysis of HQ oxidation with tyrosinase in the presence of dopa showed the predominant formation in the early stages of hydroxybenzoquinone (HBQ), arising from enzymic hydroxylation and subsequent oxidation of HQ, along with lower amounts of benzoquinone (BQ). These results suggest that the depigmenting activity of HQ may partly be related to the ability of the compound to act as an alternate substrate of tyrosinase, thereby competing for tyrosine oxidation in active melanocytes.

Effect of metal ions on the rearrangement of dopachrome

In vitro experiments are reported showing that a number of transition metal ions exert a profound influence on both the kinetics and chemical course of the rearrangement of dopachrome, a key step in the biosynthesis of melanins. HPLC analysis shows that Cu2+, Ni2+ and Co2+ are particularly effective in inducing the non-decarboxylative rearrangement of dopachrome at physiological pH values, leading mainly to 5,6-dihydroxyindole-2-carboxylic acid, whereas in the absence of metal ions the reaction proceeds with concomitant loss of carbon dioxide to give almost exclusively 5,6-dihydroxyindole. Kinetic experiments provide evidence that the rate of the metal-promoted rearrangement is first order with respect to both aminochrome and metal concentration and decreases in the presence of increasing concentrations of EDTA, consistent with a mechanism involving a direct 1:1 dopachrome-metal ion interaction in the transition state. When considered in the light of the known metal accumulation in pigmented tissues, the results of this study provide a new entry into the regulatory mechanisms involved in the biosynthesis of melanins.

Melanins and melanogenesis: from pigment cells to human health and technological applications

During the past decade, melanins and melanogenesis have attracted growing interest for a broad range of biomedical and technological applications. The burst of polydopamine‐based multifunctional coatings in materials science is just one example, and the list may be expanded to include melanin thin films for organic electronics and bioelectronics, drug delivery systems, functional nanoparticles and biointerfaces, sunscreens, environmental remediation devices. Despite considerable advances, applied research on melanins and melanogenesis is still far from being mature. A closer intersectoral interaction between research centers is essential to raise the interests and increase the awareness of the biomedical, biomaterials science and hi‐tech sectors of the manifold opportunities offered by pigment cells and related metabolic pathways. Starting from a survey of biological roles and functions, the present review aims at providing an interdisciplinary perspective of melanin pigments and related pathway with a view to showing how it is possible to translate current knowledge about physical and chemical properties and control mechanisms into new bioinspired solutions for biomedical, dermocosmetic, and technological applications.

Oxidation Chemistry of Catecholamines and Neuronal Degeneration: An Update

Aberrant oxidative pathways of catecholamine neurotransmitters, i.e. dopamine and norepinephrine, are an important biochemical correlate of catecholaminergic neuron loss in some disabling neurodegenerative diseases of the elderly, notably Parkinsons disease. In an oxidative stress setting, under conditions of elevated lipid peroxidation, iron accumulation, impaired mitochondrial functioning and antioxidant depletion, catecholamines are oxidatively converted to the corresponding o-quinones, which may initiate a cascade of spontaneous reactions, including intramolecular cyclization, aminoethyl side chain fission and interaction with molecular targets. The overall outcome of the competing pathways may vary depending on contingent factors and the biochemical environment, and may include formation of nitrated derivatives, neuromelanin deposition, generation of chain fission products, conjugation with L-cysteine leading eventually to cytotoxic responses and altered cellular function. In addition, catecholamines may interact with products of lipid peroxidation and other species derived from oxidative breakdown of biomolecules, notably glyoxal and other aldehydes, leading e.g. to tetrahydroisoquinolines via Pictet-Spengler chemistry. After a brief introductory remark on oxidative stress biochemistry, the bulk of this review will deal with an overview of the basic chemical pathways of catecholamine oxidation, with special emphasis on the analogies and differences between the central neurotransmitters dopamine and norepinephrine. This chemistry will form the basis for a concise discussion of the latest advances in the mechanisms of catecholamine-associated neurotoxicity in neuronal degeneration.

An integrated approach to the structure of Sepia melanin. Evidence for a high proportion of degraded 5,6-dihydroxyindole-2-carboxylic acid units in the pigment backbone

Abstract The relative proportion of 5,6-dihydroxyindole (DHI) versus 5,6-dihydroxyindole-2-carboxylic acid (DHICA) units in Sepia melanin and their degree of structural integrity were assessed by an integrated approach involving quantitative determination of pyrrole-2,3-dicarboxylic acid (PDCA) and pyrrole-2,3,5-tricarboxylic acid (PTCA), specific degradation products of DHI- and DHICA-derived units; carboxyl content; rate of ferricyanide consumption; and absorption spectrum. Analyses of both intact and surfactant-solubilised pigment samples, in comparison with synthetic DHI and DHICA melanins, revealed that Sepia melanin consists of a mixture of oligomeric structures incorporating over 75% of DHICA-derived units and only 20% of DHI-derived units, occurring for the most part in an irreversibly degraded form, possibly as pyrrolecarboxylic acids.

Structural modifications in biosynthetic melanins induced by metal ions

A number of transition metal ions with a wide distribution in biological systems, e.g., Cu2+, Co2+ and Zn2+, are shown to affect markedly the chemical properties of melanins formed by the tyrosinase-catalysed oxidation of dopa. Acid decarboxylation and permanganate degradation provide evidence that melanins prepared in the presence of metal ions contain a high content of carboxyl groups arising from the incorporation of 5,6-dihydroxyindole-2-carboxylic acid (DICA) into the pigment polymer. Naturally occurring melanins from cephalopod ink and B16 mouse melanoma were found to be much more similar to melanins prepared in the presence of metal ions than to standard melanins prepared in the absence of metal ions. These results suggest that the presence of carboxylated indole units in natural melanins is probably due to the intervention in the biochemical pathway of metal ions which, as recently shown, catalyse the formation of DICA versus 5,6-dihydroxyindole in the rearrangement of dopachrome.

Oxidative stress as a multiple effector in Fanconi anaemia clinical phenotype

Abstract:  Fanconi anaemia (FA) is a genetic disease characterised by bone marrow failure with excess risk of myelogenous leukaemia and solid tumours. A widely accepted notion in FA research invokes a deficiency of response to DNA damage as the fundamental basis of the ‘crosslinker sensitivity’ observed in this disorder. However, such an isolated defect cannot readily account for the full cellular and clinical phenotype, which includes a number of other abnormalities, such as malformations, endocrinopathies, and typical skin spots. An extensive body of evidence pointing toward an involvement of oxidative stress in the FA phenotype includes the following: (i) In vitro and ex vivo abnormalities in a number of redox status endpoints; (ii) the functions of several FA proteins in protecting cells from oxidative stress; (iii) redox‐related toxicity mechanisms of the xenobiotics evoking excess toxicity in FA cells. The clinical features in FA and the in vivo abnormalities of redox parameters are here reconsidered in view of the pleiotropic clinical phenotype and known biochemical and molecular links to an in vivo prooxidant state, which causes oxidative damage to biomolecules, resulting in an excessive number of acquired abnormalities that may overwhelm the cellular repair capacity rather than a primary deficiency in DNA repair. FA may thus represent a unique model disease in testing the integration between the acquisition of macromolecular damage as a result of oxidative stress and the ability of the mammalian cell to respond effectively to such damage.

Nitric oxide-induced nitration of catecholamine neurotransmitters: a key to neuronal degeneration?

Exposure of the neurotransmitters dopamine (1a) and norepinephrine (1b), as well as of other catechol compounds (1c-e), to nitric oxide (NO) in aerated phosphate buffer at room temperature leads to the corresponding 6-nitroderivatives (2a-e) in yields higher than 80%. Formation of nitration products depends on the presence of oxygen and is inhibited by excess ascorbic acid, whereas sulfhydryl compounds, e.g. cysteine, and scavengers of reactive oxygen species, such as catalase and superoxide dismutase, exert no significant inhibitory effect. O-Methylated catechols are poorly or not reactive toward NO. These and other observations are consistent with a mechanism involving coupling of a semiquinone radical with NO or a higher oxide, e.g. nitrogen dioxide (NO2). The observed formation of potentially toxic 6-nitrocatecholamines under physiologically relevant conditions may open new perspectives to an understanding of the biochemical processes underlying NO-induced toxicity and neuronal degeneration.