Ion-exchange and adsorption of Fe(III) by Sepia melanin

Comparisons of the Structural and Chemical Properties of Melanosomes Isolated from Retinal Pigment Epithelium, Iris and Choroid of Newborn and Mature Bovine Eyes¶

Melanosomes were isolated from the retinal pigment epithelium (RPE), iris and choroid of mature (age >2 years) and newborn (age <1 week) bovine eyes. Scanning electron microscopy was utilized to analyze the morphology of the melanosomes, which were found to vary among different tissues and different ages. While the total content of amino acids differs slightly (ranging from 9% to 15% by mass), the distributions of the amino acids are similar. The pheomelanin content is low in the choroid and the RPE (0.1-0.5%), and moderate in the iris (<2%); therefore, the major melanin component of bovine eye melanosomes is eumelanin, independent of the shape of the melanosomes. The yields of pyrrole-2,3,5-tricarboxylic acid from melanosomes decrease in the following order: choroid > iris > RPE, and exhibit decreasing yields with age. 13C solid-state nuclear magnetic resonance (NMR) spectroscopic analysis of iris and choroid melanosomes indicates the same trends. These observations suggest that the 5,6-dihydroxyindole-2-carboxylic acid contents decrease in the following order: choroid > iris > RPE, and decrease with age. Moreover, the 13C solid-state NMR spectra show (1) for the same age samples, the CH:Cq ratio for choroid is larger than that for iris melanosomes; and (2) an increase in the concentration of carbonyl groups with age within each type of melanosome.

The Neuromelanin of Human Substantia Nigra: Physiological and Pathogenic Aspects

Neuromelanin (NM) accumulates as a function of age in normal human substantia nigra (SN) but is relatively depleted in the SN of patients with Parkinson disease (PD). Several studies have been performed to further our understanding of the role of NM in neuronal aging and neurodegenerative mechanisms of PD. To this purpose, NM from human SN was isolated and its structure and molecular interactions were investigated. Cysteinyl-dopamine was shown to be one precursor of NM synthesis. A striking affinity of NM for specific metals, lipids, drugs and pesticides was found in vitro, and in animal and human brain postmortem studies. Because of these affinities, NM seems to play a protective role in the human brain by blocking toxic molecules. On the other hand, experiments in cell culture indicate that NM can activate microglia, eliciting the release of cytotoxic factors that can induce neurodegeneration.

Binding of Metal Ions to Melanin and Their Effects on the Aerobic Reactivity¶

The binding of Mg(II), Ca(II), Zn(II), Cu(II) and Fe(III) to ETDA-washed Sepia melanin is quantified by inductively coupled plasma mass spectrometry. By monitoring the solution pH change associated with metal uptake, it is concluded that Mg(II), Ca(II) and Zn(II) bind to carboxylic acid groups in melanin, Cu(II) binds to hydroxyl (OH) groups and Fe(III) binds to OH or amine groups. The aerobic reactivity of melanins with different metal contents is analyzed by examining their ability to cause strand breaks in supercoiled pUC18 DNA. Cu(II)- and Fe(III)-enriched melanins induce the most damage. Hydroxyl radical, *OH, is proposed to be one of the reactive oxygen species responsible.

Characterization of the Fe(III)-binding Site in Sepia Eumelanin by Resonance Raman Confocal Microspectroscopy¶

The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)-like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm(-1); the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm(-1) bands are assigned to Fe-OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin-binding site in melanin is composed of CAT-like structural units.