The IFPCS presidential lecture: a chemist's view of melanogenesis

Interaction of Hermansky-Pudlak Syndrome genes in the regulation of lysosome-related organelles

Hermansky-Pudlak Syndrome (HPS) is a genetically heterogeneous disease caused by abnormalities in the synthesis and/or trafficking of lysosome-related organelles (LROs) including melanosomes, lamellar bodies of lung type II cells and platelet dense granules. At least 15 genes cause HPS in mice, with a significant number specifying novel subunits of protein complexes termed BLOCs (Biogenesis of Lysosome-related Organelles Complexes). To ascertain whether BLOC complexes functionally interact in vivo, mutant mice doubly or triply deficient in protein subunits of the various BLOC complexes and/or the AP-3 adaptor complex were constructed and tested for viability and for abnormalities of melanosomes, lung lamellar bodies and lysosomes. All mutants, including those deficient in all three BLOC complexes, were viable though the breeding efficiencies of multiple mutants involving AP-3 were severely compromised. Interactions of BLOC protein complexes with each other and with AP-3 to affect most LROs were apparent. However, these interactions were tissue and organelle dependent. These studies document novel biological interactions of BLOC and AP-3 complexes in the biosynthesis of LROs and assess the role(s) of HPS protein complexes in general health and physiology in mammals. Double and triple mutant HPS mice provide unique and practical experimental advantages in the study of LROs.

A coiled-coil domain of melanophilin is essential for Myosin Va recruitment and melanosome transport in melanocytes

Melanophilin (Mlph) regulates retention of melanosomes at the peripheral actin cytoskeleton of melanocytes, a process essential for normal mammalian pigmentation. Mlph is proposed to be a modular protein binding the melanosome-associated protein Rab27a, Myosin Va (MyoVa), actin, and microtubule end-binding protein (EB1), via distinct N-terminal Rab27a-binding domain (R27BD), medial MyoVa-binding domain (MBD), and C-terminal actin-binding domain (ABD), respectively. We developed a novel melanosome transport assay using a Mlph-null cell line to study formation of the active Rab27a:Mlph:MyoVa complex. Recruitment of MyoVa to melanosomes correlated with rescue of melanosome transport and required intact R27BD together with MBD exon F-binding region (EFBD) and unexpectedly a potential coiled-coil forming sequence within ABD. In vitro binding studies indicate that the coiled-coil region enhances binding of MyoVa by Mlph MBD. Other regions of Mlph reported to interact with MyoVa globular tail, actin, or EB1 are not essential for melanosome transport rescue. The strict correlation between melanosomal MyoVa recruitment and rescue of melanosome distribution suggests that stable interaction with Mlph and MyoVa activation are nondissociable events. Our results highlight the importance of the coiled-coil region together with R27BD and EFBD regions of Mlph in the formation of the active melanosomal Rab27a-Mlph-MyoVa complex.

Hepatotoxin-induced hypertyrosinemia and its toxicological significance

A (1)H Nuclear Magnetic Resonance (NMR) spectroscopic investigation of the effects of single doses of four model hepatotoxins on male Sprague-Dawley rats showed that hypertyrosinemia was induced by three of the treatments (ethionine 300 mg/kg, galactosamine hydrochloride 800 mg/kg and isoniazid 400 mg/kg) but not by the fourth (thioacetamide 200 mg/kg). Concomitant histopathological and clinical chemistry analyses showed that hypertyrosinemia could occur with or without substantial hepatic damage and that substantial hepatic damage could occur without hypertyrosinemia. However, in the rats dosed with galactosamine hydrochloride, which showed highly variable amounts of liver damage at ca. 24 h after dosing, a clear relationship was found between the degree of hypertyrosinemia and the extent of the hepatic necrosis induced. In line with the cause of clinically observed Type II Tyrosinemia, we consider that the critical event in the onset of hepatotoxin-induced hypertyrosinemia is likely to be a reduction in hepatic tyrosine aminotransferase (TAT) activity. We discuss mechanisms by which TAT activity could be lost with special consideration given to pyridoxal 5'-phosphate (P5P) depletion and to the inhibition of protein synthesis. This analysis may have implications for the interpretation of clinical measures of liver status such as Fischer's ratio and the branched-chain tyrosine ratio (BTR).

Determination of eumelanin in human urine

Normal and malignant melanocytes produce melanins and melanin-related metabolites, most of which are retained in the cells but some are secreted into the blood and then excreted in the urine. In this study, we developed a method to measure levels of eumelanin in urine samples and evaluated its clinical significance in comparison with the melanin-related metabolites 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MI2C) and 5-S-cysteinyldopa (5-S-CD), and with pheomelanin, measured after degradation as 4-amino-3-hydroxyphenylalanine (4-AHP). The method is based on the production of pyrrole-2,3,5-tricarboxylic acid (PTCA) on permanganate oxidation of eumelanin, followed by quantification by liquid chromatography. For 118 urine samples from 10 control subjects, mean urinary excretions of PTCA, 6H5MI2C, 5-S-CD and 4-AHP were 19, 67, 37 and 59 micromol/mol creatinine respectively. In melanoma patients (n = 45), the mean urinary excretions of PTCA, 6H5MI2C, 5-S-CD, and 4-AHP were 91, 926, 4070 and 3530 micromol/mol creatinine respectively. Median level of PTCA in melanoma patients was elevated 2.1-fold compared with control subjects. The degrees of elevation for 6H5MI2C, 5-S-CD, and 4-AHP were 1.8-, 22- and 6.2-fold respectively. Thus, although urinary PTCA is of little clinical value in following the progression of melanoma, urinary 4-AHP appears to be of considerable value in this respect.

The slaty mutation affects eumelanin and pheomelanin synthesis in mouse melanocytes

The slaty (Dct(slt)) mutation is known to reduce the activity of dopachrome tautomerase (DCT) in melanocytes. However, it is unknown whether the reduced DCT activity leads to a defect in the proliferation and differentiation of mouse melanocytes. To address this point, the proliferation and differentiation of neonatal melanocytes from Dct(slt)/Dct(slt) congenic mice in serum-free primary culture were investigated in detail. The proliferation of slaty epidermal melanoblasts/melanocytes in culture did not differ from that of wild-type mice. However, the differentiation was greatly inhibited. Tyrosinase (TYR) activity detected by dopa reaction as well as staining of DCT in slaty melanocytes was greatly reduced. The content of eumelanin in cultured slaty melanocytes was reduced, whereas the content of pheomelanin in media derived from cultured 7.5-day-old slaty melanocytes was greatly increased. The contents of eumelanin and pheomelanin in the neonatal slaty epidermis and dermis were reduced, except that the pheomelanin content in 3.5-day-old dermis was increased. These results suggest that the slaty mutation affects both eumelanin and pheomelanin synthesis in developmental stage-specific and skin site-specific manners, and, in addition, the gene controls the differentiation of melanocytes via the regulation of activity of TYR in addition to its own DCT.

Ca++-switch induction of RPE differentiation

Cultured retinal pigment epithelial (RPE) cells are commonly used as a model of the tissue to study their involvement in visual diseases. Unfortunately, cultured RPE often lose their differentiated phenotype reducing their usefulness as a model of the RPE in vivo. In this study, we used a Ca++-switch protocol to initiate the patterned expression of several phenotypic and functional markers of RPE differentiation. Cultured RPE cells from adult donors were maintained through at least six serial passages prior to assay to minimize their differentiated properties. The cells were then subjected to the Ca++-switch protocol and maintained at confluence for up to 4 months. Paired control and Ca++-switch cells were examined for phenotype, pigmentation, and the expression of tyrosinase, CRABP, myocilin, and bestrophin by western blot analysis. The Ca++-switch protocol led to a rapid restriction of N-cadherin to lateral cell borders, and to expression of tyrosinase by day 4. After 8 weeks, the experimental RPE monolayers began to accumulate visible pigment, and after 12 weeks CRABP expression was observed. Myocilin was observed at 4 months after the Ca++-switch but bestrophin was not detected at any time point. Our results suggest this protocol may drive epithelial morphogenesis in RPE cells. We note two specific differences in cells plated in low Ca++, reduced spreading on the substrate and coordinated development of cadherin adhesion when the Ca++-concentration is returned to normal. Thus, we suggest that this method produces phenotypic changes through multiple cell signalling pathways.

Diversity of pigmentation in cultured human melanocytes is due to differences in the type as well as quantity of melanin

Cultured human melanocytes differ tremendously in visual pigmentation, and recapitulate the pigmentary phenotype of the donor's skin. This diversity arises from variation in type as well as quantity of melanin produced. Here, we measured contents of eumelanin (EM) and pheomelanin (PM) in 60 primary human melanocyte cultures (51 neonatal and nine adults), and correlated some of these values with the respective activity and protein levels of tyrosinase, and the melanocortin-1 receptor (MC1R) genotype. Melanocytes were classified into four phenotypes (L, L+, D, D+) as depicted by visual pigmentation using light microscopy, and by the pigmentary phenotype of the donor's skin. There were large differences in total melanin (TM) and EM, which increased progressively for L, L+, D and D+ melanocytes. TM content, the sum of EM and PM, showed a good correlation with TM measured spectrophotometrically, and with the activity and protein levels of tyrosinase. Log EM/PM ratio did not correlate with MC1R genotype. We conclude that: (i) EM consistently correlates with the visual phenotype; (ii) lighter melanocytes tend to be more pheomelanic in composition than darker melanocytes; (iii) in adult melanocyte cultures, EM correlates with the ethnic background of the donors (African-American > Indian > Caucasian); and (iv) MC1R loss-of-function mutations do not necessarily alter the phenotype of cultured melanocytes.

Identification of novel functional variants of the melanocortin 1 receptor gene originated from Asians

Human melanocortin 1 receptor (MC1R) is a seven transmembrane G-coupled protein receptor that upregulates the cAMP pathway. Several functional variants of MC1R that show an impaired ability to activate the cAMP pathway are strongly associated with fair skin and red hair in Europeans and European descendants. The sequence variations of the MC1R gene were repeatedly investigated against worldwide populations; however, there was no evidence that functional variant of MC1R exists in non-European descendants. We report the presence of novel functional variants of MC1R with Asian origins. Three novel variants of MC1R, Phe147Delta, Thr157Ile, and Pro159Thr, were identified in our screening for the sequence variations of the MC1R gene against 995 individuals from 30 Asian and Oceanian populations; there was a single case for the Pro159Thr variant allele and two instances of Phe147Delta and Thr157Ile variant alleles. Our pharmacological assay revealed that Phe147Delta, Thr157Ile, and Pro159Thr variant showed similar or more dramatically impaired activities in comparison with Arg151Cys, which is a major functional variant of MC1R in Europeans. These functional variant alleles were geographically localized in relatively high latitudes, which suggest that the adaptation to ambient UV light intensity may play an important role in shaping the geographical distribution of MC1R alleles in Asia and Oceania.

Melanocortin-1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin-1 receptor (MC1R). MC1R is a member of the G-protein-coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or alpha-melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist-independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.

Quantitative fluorescence spectra and quantum yield map of synthetic pheomelanin

Spectroscopic studies of pheomelanin and its constituents have been sparse. These data present what is by far the most complete description of the fluorescence characteristics of synthetic pheomelanin. Emission spectra between 260 and 600 nm were acquired for excitation wavelengths between 250 and 500 nm at 1-nm intervals. A quantum yield map is also presented, correcting the fluorescence intensities for differences in species concentration and molar absorptivity. These fluorescence features exhibit interesting similarities and differences to eumelanin, and these data are interpreted with respect to possible chemical structures. Overall, these data suggest that pheomelanin oligomers may be more tightly coupled than those of eumelanin. Finally, the quantum yield is shown to be on the order of 10(-4) and exhibit a complex dependence on excitation energy, varying by a factor of 4 across the energies employed here.

The relation between melanocortin 1 receptor (MC1R) variation and the generation of phenotypic diversity in the cutaneous response to ultraviolet radiation

The melanocortin 1 receptor (MC1R) is known to play an important role in determining physiological variation in human pigmentation, and consequently human susceptibility to ultraviolet radiation. A reason for wider interest is that the considerable phenotypic diversity has been in part generated by the effects of gene dosage, and the presence of a large number of mutations at this G-protein coupled receptor that are not functionally equivalent. Thus, a range of mutations at a single receptor locus can lead to a complex range of graded phenotypes.

Slc7a11 gene controls production of pheomelanin pigment and proliferation of cultured cells

In mammals, >100 genes regulate pigmentation by means of a wide variety of developmental, cellular, and enzymatic mechanisms. Nevertheless, genes that directly regulate pheomelanin production have not been described. Here, we demonstrate that the subtle gray (sut) mouse pigmentation mutant arose by means of a mutation in the Slc7a11 gene, encoding the plasma membrane cystine/glutamate exchanger xCT [Kanai, Y. & Endou, H. (2001) Curr. Drug Metab. 2, 339-354]. A resulting low rate of extracellular cystine transport into sut melanocytes reduces pheomelanin production. We show that Slc7a11 is a major genetic regulator of pheomelanin pigment in hair and melanocytes, with minimal or no effects on eumelanin. Furthermore, transport of cystine by xCT is critical for normal proliferation, glutathione production, and protection from oxidative stress in cultured cells. Thus, we have found that the Slc7a11 gene controls the production of pheomelanin pigment directly. Cells from sut mice provide a model for oxidative stress-related diseases and their therapies.

Altered cell surface expression of human MC1R variant receptor alleles associated with red hair and skin cancer risk

The human melanocortin-1 receptor gene (MC1R) encodes a G-protein coupled receptor that is primarily expressed on melanocytes, where it plays a key role in pigmentation regulation. Variant alleles are associated with red hair colour and fair skin, known as the RHC phenotype, as well as skin cancer risk. The R151C, R160W and D294H alleles, designated 'R', are strongly associated with the RHC phenotype and have been proposed to result in loss of function receptors due to impaired G-protein coupling. We recently provided evidence that the R151C and R160W variants can efficiently couple to G-proteins in response to alpha-melanocyte stimulating hormone. The possibility that altered cellular localization of the R151C and R160W variant receptors could underlie their association with RHC was therefore considered. Using immunofluorescence and ligand binding studies, we found that melanocytic cells exogenously or endogenously expressing MC1R show strong surface localization of the wild-type and D294H alleles but markedly reduced cell surface expression of the R151C and R160W receptors. In additional exogenous expression studies, the R variant D84E and the rare I155T variant, also demonstrated a significant reduction in plasma membrane receptor numbers. The V60L, V92M and R163Q weakly associated RHC alleles, designated 'r', were expressed with normal or intermediate cell surface receptor levels. These results indicate that reduced receptor coupling activity may not be the only contributing factor to the genetic association between the MC1R variants and the RHC phenotype, with MC1R polymorphisms now linked to a change in receptor localization.

Role of keratinocyte-derived factors involved in regulating the proliferation and differentiation of mammalian epidermal melanocytes

Melanocytes characterized by the activities of tyrosinase, tyrosinase-related protein (TRP)-1 and TRP-2 as well as by melanosomes and dendrites are located mainly in the epidermis, dermis and hair bulb of the mammalian skin. Melanocytes differentiate from melanoblasts, undifferentiated precursors, derived from embryonic neural crest cells. Because hair bulb melanocytes are derived from epidermal melanoblasts and melanocytes, the mechanism of the regulation of the proliferation and differentiation of epidermal melanocytes should be clarified. The regulation by the tissue environment, especially by keratinocytes is indispensable in addition to the regulation by genetic factors in melanocytes. Recent advances in the techniques of tissue culture and biochemistry have enabled us to clarify factors derived from keratinocytes. Alpha-melanocyte-stimulating hormone, adrenocorticotrophic hormone, basic fibroblast growth factor, nerve growth factor, endothelins, granulocyte-macrophage colony-stimulating factor, steel factor, leukemia inhibitory factor and hepatocyte growth factor have been suggested to be the keratinocyte-derived factors and to regulate the proliferation and/or differentiation of mammalian epidermal melanocytes. Numerous factors may be produced in and released from keratinocytes and be involved in regulating the proliferation and differentiation of mammalian epidermal melanocytes through receptor-mediated signaling pathways.

Pheomelanin production in the epidermis from newborn agouti mice is induced by the expression of the agouti gene in the dermis

The present study was designed to clarify the role of the agouti gene in the regulation of the proliferation and differentiation of mouse epidermal melanocytes using serum-free primary culture of epidermal melanocytes from 0.5-d-old black (a/a; C57BL/10JHir) mice and congenic, agouti (A/A; C57BL/10JHir-A/A) mice. There was no significant difference in the proliferation or differentiation of melanocytes between a/a and A/A mice. However, the content of pheomelanin in culture media from A/A melanocytes was increased by L-tyrosine compared with a/a melanocytes. In addition, the content of the pheomelanin precursor, 5-S-cysteinyldopa, in culture media from A/A melanocytes was dramatically increased by L-tyrosine. Moreover, pheomelanin content in the epidermis from 3.5- and 5.5-d-old A/A mice was much higher than in a/a mice. Analysis of the A gene using reverse transcription-polymerase chain reaction revealed that cultured keratinocytes and melanocytes do not express the A gene. Moreover, the A gene was expressed in the A/A dermis of 0.5-, 3.5- and 5.5-d-old mice, but not in the a/a dermis nor in the A/A or a/a epidermis. These results suggest that A/A epidermal melanoblasts are influenced by the A gene from the dermis of neonatal mice, and are capable of synthesizing pheomelanin in the culture. Pheomelanin production in the epidermis from 3.5- and 5.5-d-old A/A mice may be induced by the expression of the agouti gene in the dermis.

Preservation of eumelanin hair pigmentation in proopiomelanocortin-deficient mice on a nonagouti (a/a) genetic background

The original strain of proopiomelanocortin (POMC)-deficient mice (Pomc-/-) was generated by homologous recombination in 129X1/SvJ (A(w)/A(w))-derived embryonic stem cells using a targeting construct that deleted exon 3, encoding all the known functional POMC-derived peptides including alpha MSH, from the Pomc gene. Although these Pomc-/- mice exhibited adrenal hypoplasia and obesity similar to the syndrome of POMC deficiency in children, their agouti coat color was only subtly altered. To further investigate the mechanism of hair pigmentation in the absence of POMC peptides, we studied wild-type (Pomc+/+), heterozygous (Pomc+/-), and homozygous (Pomc-/-) mice on a nonagouti (a/a) 129;B6 hybrid genetic background. All three genotypes had similar black fur pigmentation with yellow hairs behind the ears, around the nipples, and in the perianal area characteristic of inbred C57BL/6 mice. Histologic and electron paramagnetic resonance spectrometry examination demonstrated that hair follicles in back skin of Pomc-/- mice developed with normal structure and eumelanin pigmentation; corresponding molecular analyses, however, excluded local production of alpha MSH and ACTH because neither Pomc nor putative Pomc pseudogene mRNAs were detected in the skin. Thus, 129;B6 Pomc null mutant mice produce abundant eumelanin hair pigmentation despite their congenital absence of melanocortin ligands. These results suggest that either the mouse melanocortin receptor 1 has sufficient basal activity to trigger and sustain eumelanogenesis in vivo or that redundant nonmelanocortin pathway(s) compensate for the melanocortin deficiency. Whereas the latter implies feedback control of melanogenesis, it is also possible that the two mechanisms operate jointly in hair follicles.

Identification of heat-induced degradation products from purified betanin, phyllocactin and hylocerenin by high-performance liquid chromatography/electrospray ionization mass spectrometry

Betanin, phyllocactin (malonylbetanin) and hylocerenin (3-hydroxy-3-methylglutarylbetanin) were isolated from purple pitaya (Hylocereus polyrhizus [Weber] Britton and Rose) juice, and their degradation products generated by heating at 85 degrees C were subsequently monitored by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry. Thermal degradation of phyllocactin and hylocerenin in purified solution excluding the alleged protective effects by the juice matrix is reported for the first time. Betanin was predominantly degraded by hydrolytic cleavage, while decarboxylation and dehydrogenation were of minor relevance. In contrast, hylocerenin showed a strong tendency to decarboxylation and dehydrogenation, hydrolytic cleavage of the aldimine bond occurring secondarily. Phyllocactin degradation was most complex because of additional decarboxylation of the malonic acid moiety as well as generation and subsequent degradation of betanin due to phyllocactin demalonylation. Upon prolonged heating, all betacyanins under observation formed degradation products characterized by an additional double bond at C2-C3. Hydrolytic cleavage of the aldimine bond of phyllocactin and hylocerenin yielded previously unknown acylated cyclo-dopa derivatives traceable by positive ionization, while application of ESI(-) facilitated the detection of a glycosylated aminopropanal derivative and dopamine, which have never been described before as betanin degradation products.

The genetics of sun sensitivity in humans

Humans vary >100-fold in their sensitivity to the harmful effects of ultraviolet radiation. The main determinants of sensitivity are melanin pigmentation and less-well-characterized differences in skin inflammation and repair processes. Pigmentation has a high heritability, but susceptibility to cancers of the skin, a key marker of sun sensitivity, is less heritable. Despite a large number of murine coat-color mutations, only one gene in humans, the melanocortin 1 receptor (MC1R), is known to account for substantial variation in skin and hair color and in skin cancer incidence. MC1R encodes a 317-amino acid G-coupled receptor that controls the relative amounts of the two major melanin classes, eumelanin and pheomelanin. Most persons with red hair are homozygous for alleles of the MC1R gene that show varying degrees of diminished function. More than 65 human MC1R alleles with nonsynonymous changes have been identified, and current evidence suggests that many of them vary in their physiological activity, such that a graded series of responses can be achieved on the basis of (i) dosage effects (of one or two alleles) and (ii) individual differences in the pharmacological profile in response to ligand. Thus, a single locus, identified within a Mendelian framework, can contribute significantly to human pigmentary variation.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Isolation and biophysical studies of natural eumelanins: applications of imaging technologies and ultrafast spectroscopy

The major pigments found in the skin, hair, and eyes of humans and other animals are melanins. Despite significant research efforts, the current understanding of the molecular structure of melanins, the assembly of the pigment within its organelle, and the structural consequences of the association of melanins with protein and metal cations is limited. Likewise, a detailed understanding of the photochemical and photophysical properties of melanins has remained elusive. Many types of melanins have been studied to date, including natural and synthetic model pigments. Such studies are often contradictory and to some extent the diversity of systems studied may have detracted from the development of a basic understanding of the structure and function of the natural pigment. Advances in the understanding of the structure and function of melanins require careful characterization of the pigments examined so as to assure the data obtained may be relevant to the properties of the pigment in vivo. To address this issue, herein the influence of isolation procedures on the resulting structure of the pigment is examined. Sections describing the applications of new technologies to the study of melanins follow this. Advanced imaging technologies such as scanning probe microscopies are providing new insights into the morphology of the pigment assembly. Recent photochemical studies on photoreduction of cytochrome c by different mass fraction of sonicated natural melanins reveal that the photogeneration of reactive oxygen species (ROS) depends upon aggregation of melanin. Specifically, aggregation mitigates ROS photoproduction by UV-excitation, suggesting the integrity of melanosomes in tissue may play an important role in the balance between the photoprotective and photodamaging behaviors attributed to melanins. Ultrafast laser spectroscopy studies of melanins are providing insights into the time scales and mechanisms by which melanin dissipates absorbed light energy.

Indirect oxidation of 6-tetrahydrobiopterin by tyrosinase

6-Tetrahydrobiopterin is known to bind to an allosteric site of tyrosinase to directly inhibit the enzyme. However, simultaneous measurements of ultraviolet-visible absorption spectra and oxygen consumption led us to conclude that the inhibition was due to oxidation of 6-tetrahydrobiopterin by dopaquinone. Immediately after addition of 6-tetrahydrobiopterin, tyrosinase stopped producing dopachrome from either tyrosine or dopa. Duration of inhibition was proportional to the concentration of added 6-tetrahydrobiopterin and the enzyme activity was fully restored after the inhibition. Surprisingly, there was a rapid consumption of oxygen during the inhibition period. In addition, absorption spectra indicated that the only reaction that occurred during the inhibition was oxidation of 6-tetrahydrobiopterin to 7,8-dihydrobiopterin. In the absence of tyrosine or dopa, tyrosinase did not oxidize 6-tetrahydrobiopterin, suggesting that a reaction intermediate between dopa and dopachrome was a target for the inhibition. We propose a new mechanism in which dopa is oxidized to dopaquinone and the latter, instead of producing dopachrome, is reduced back to dopa by 6-tetrahydrobiopterin.

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.

Rate constants for the first two chemical steps of eumelanogenesis

The kinetics of the initial cyclization and redox exchange reactions involved in the eumelanogenic pathway have been studied previously but because of the difficulty of detecting the intermediate cyclodopa by optical means (because its absorbance is in the same range as dopa which is present in excess in the experimental system) no accurate value for the redox exchange reaction has so far been obtained and there is no available analytical methodology that can be applied to the successive first- and second-order reactions involved. We have synthesized cyclodopa and examined the kinetics of the formation of dopachrome following the pulse radiolytic generation of dopaquinone in its presence. From this direct measurement we determined that the rate constant of the reaction between cyclodopa and dopaquinone is 5.3 x 10(6)/M/s. Employing this value in a computational model of the combined cyclization and redox exchange reactions we calculate that the observed kinetics of dopaquinone decay and dopachrome formation are compatible with a cyclization rate constant of 3.8/s.