Pteridines in the control of pigmentation

Tetrahydrobiopterin as a Trigger for Vitiligo: Phototransformation during UV Irradiation

Vitiligo is a type of hypomelanosis. Tetrahydrobiopterin (H4Bip), the coenzyme of the initial stage of melanogenesis, appears to be a trigger for vitiligo. H4Bip is present in vitiligo in 3-5-fold excess and causes oxidative stress by triggering an autocatalytic cycle of excess hydrogen peroxide synthesis. Using quantum-chemical calculations, we have evaluated the possibility of H4Bip reactions occurring in the dark and under ultraviolet (UV) irradiation, including the formation of dihydropterin dimers. In order to simulate the oxidative stress, oxidative modification of human serum albumin (HSA) has been carried out in the presence of excessive H4Bip using the fluorescence method. The fraction of oxidized protein (FOP) has been calculated. It has been established that there is a strong oxidative modification of amino acids chromophores (tryptophan and tyrosine) in the protein (FOP 0.64). Under UV irradiation of the system (HSA + H4Bip), FOP is reduced to 0.39. Apparently, a part of H4Bip transforms into dihydropterin dimers and does not participate in the oxidative modification of the protein. The data on oxidative modification of HSA are consistent with dynamic light scattering: H4Bip promotes HSA aggregation with the formation of particles with a hydrodynamic radius Rh ≥ 2000 nm, which can become immunogenic.

A Comprehensive Review of Mammalian Pigmentation: Paving the Way for Innovative Hair Colour-Changing Cosmetics

The natural colour of hair shafts is formed at the bulb of hair follicles, and it is coupled to the hair growth cycle. Three critical processes must happen for efficient pigmentation: (1) melanosome biogenesis in neural crest-derived melanocytes, (2) the biochemical synthesis of melanins (melanogenesis) inside melanosomes, and (3) the transfer of melanin granules to surrounding pre-cortical keratinocytes for their incorporation into nascent hair fibres. All these steps are under complex genetic control. The array of natural hair colour shades are ascribed to polymorphisms in several pigmentary genes. A myriad of factors acting via autocrine, paracrine, and endocrine mechanisms also contributes for hair colour diversity. Given the enormous social and cosmetic importance attributed to hair colour, hair dyeing is today a common practice. Nonetheless, the adverse effects of the long-term usage of such cosmetic procedures demand the development of new methods for colour change. In this context, case reports of hair lightening, darkening and repigmentation as a side-effect of the therapeutic usage of many drugs substantiate the possibility to tune hair colour by interfering with the biology of follicular pigmentary units. By scrutinizing mammalian pigmentation, this review pinpoints key targetable processes for the development of innovative cosmetics that can safely change the hair colour from the inside out.

Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 1758)

The shell color of the Mollusca has attracted naturalists and collectors for hundreds of years, while the molecular pathways regulating pigment production and the pigments themselves remain poorly described. In this study, our aim was to identify the main pigments and their molecular pathways in the pearl oyster Pinctada margaritifera—the species displaying the broadest range of colors. Three inner shell colors were investigated—red, yellow, and green. To maximize phenotypic homogeneity, a controlled population approach combined with common garden conditioning was used. Comparative analysis of transcriptomes (RNA-seq) of P. margaritifera with different shell colors revealed the central role of the heme pathway, which is involved in the production of red (uroporphyrin and derivates), yellow (bilirubin), and green (biliverdin and cobalamin forms) pigments. In addition, the Raper–Mason, and purine metabolism pathways were shown to produce yellow pigments (pheomelanin and xanthine) and the black pigment eumelanin. The presence of these pigments in pigmented shell was validated by Raman spectroscopy. This method also highlighted that all the identified pathways and pigments are expressed ubiquitously and that the dominant color of the shell is due to the preferential expression of one pathway compared with another. These pathways could likely be extrapolated to many other organisms presenting broad chromatic variation.

Protective Effects of Melatonin on the Skin: Future Perspectives

When exposed to hostile environments such as radiation, physical injuries, chemicals, pollution, and microorganisms, the skin requires protective chemical molecules and pathways. Melatonin, a highly conserved ancient molecule, plays a crucial role in the maintenance of skin. As human skin has functional melatonin receptors and also acts as a complete system that is capable of producing and regulating melatonin synthesis, melatonin is a promising candidate for its maintenance and protection. Below, we review the studies of new metabolic pathways involved in the protective functions of melatonin in dermal cells. We also discuss the advantages of the topical use of melatonin for therapeutic purposes and skin protection. In our view, endogenous intracutaneous melatonin production, together with topically-applied exogenous melatonin and its metabolites, represent two of the most potent defense systems against external damage to the skin.

Melatonin: A Cutaneous Perspective on its Production, Metabolism, and Functions

Melatonin, an evolutionarily ancient derivative of serotonin with hormonal properties, is the main neuroendocrine secretory product of the pineal gland. Although melatonin is best known to regulate circadian rhythmicity and lower vertebrate skin pigmentation, the full spectrum of functional activities of this free radical-scavenging molecule, which also induces/promotes complex antioxidative and DNA repair systems, includes immunomodulatory, thermoregulatory, and antitumor properties. Because this plethora of functional melatonin properties still awaits to be fully appreciated by dermatologists, the current review synthesizes the main features that render melatonin a promising candidate for the management of several dermatoses associated with substantial oxidative damage. We also review why melatonin promises to be useful in skin cancer prevention, skin photo- and radioprotection, and as an inducer of repair mechanisms that facilitate the recovery of human skin from environmental damage. The fact that human skin and hair follicles not only express functional melatonin receptors but also engage in substantial, extrapineal melatonin synthesis further encourages one to systematically explore how the skin's melatonin system can be therapeutically targeted in future clinical dermatology and enrolled for preventive medicine strategies.

The metabolic analysis of psoriasis identifies the associated metabolites while providing computational models for the monitoring of the disease

The majority of studies on psoriasis have focused on explaining the genetic background and its associations with the immune system’s response. The aim of this study was to identify the low-molecular weight compounds contributing to the metabolomic profile of psoriasis and to provide computational models that help with the classification and monitoring of the severity of the disease. We compared the results from targeted and untargeted analyses of patients’ serums with plaque psoriasis to controls. The main differences were found in the concentrations of acylcarnitines, phosphatidylcholines, amino acids, urea, phytol, and 1,11-undecanedicarboxylic acid. The data from the targeted analysis were used to build classification models for psoriasis. The results from this study provide an overview of the metabolomic serum profile of psoriasis along with promising statistical models for the monitoring of the disease.

Neuroendocrinology of the skin: An overview and selective analysis

The concept on the skin neuro-endocrine has been formulated ten years ago, and recent advances in the field further strengthened this role. Thus, skin forms a bidirectional platform for a signal exchange with other peripheral organs, endocrine and immune systems or brain to enable rapid and selective responses to the environment in order to maintain local and systemic homeostasis. In this context, it is not surprising that the function of the skin is tightly regulated by systemic neuro-endocrine system. Skin cells and skin appendages not only respond to neuropeptides, steroids and other regulatory signals, but also actively synthesis variety of hormones. The stress responses within the skin are tightly regulated by locally synthesized factors and their receptor expression. There is growing evidence for alternative splicing playing an important role in stress signaling. Deregulation of the skin neuro-endocrine signaling can lead or/and be a marker of variety of skin diseases. The major problem in this area relates to their detailed mechanisms of crosstalk between skin and brain and between the local and global endocrine as well as immune systems.

Ultraviolet-A irradiation to the eye modulates intestinal mucosal functions and properties of mast cells in the mouse

We previously reported that topical irradiation of the eye by ultraviolet-B (UVB) activated hypothalamo-pituitary-adrenal axis (HPA-A) of the mouse to increase 3, 4-dihydroxyphenylalanine (DOPA)-positive melanocytes in the skin by an inducible nitric oxide synthase (iNOS)-dependent mechanism. This work demonstrates that irradiation of the eye by ultraviolet-A (UVA) specifically increased DOPA-positive cells in the mucosa of the jejunum and colon of C57BL/6J mice by some HPA- and iNOS-independent mechanism. UVA-induced increase in DOPA-positive cells in the intestine was inhibited by the administration of hexamethonium or prazosin plus propranolol, blockers for the sympathetic nervous system. UVA irradiation of the eye increased DOPA- and histidine decarboxylase (HDC)-positive cells in the intestinal mucosa of both C57BL/6J and WBB6F1/J mice but not in the mutant strain W/Wv of the latter that lack mast cells. UVA irradiation of the eye suppressed the intestinal peristalsis of control, hypophysectomized or iNOS(-/-) C57BL/6J mice by the mechanism that was inhibited by hexamethonium or prazosin plus propranolol. These observations suggest that UVA irradiation of the eye stimulated the sympathetic nervous system to increase the mucosal DOPA- and HDC-positive mast cells and suppressed the peristalsis of the small intestine of the mouse.

Regulation of melanogenesis--controversies and new concepts

Despite many efforts, regulation of skin and hair pigmentation is still not fully understood. This article focuses mainly on controversial aspects in pigment cell biology which have emerged over the last decade. The central role of tyrosinase as the key enzyme in initiation of melanogenesis has been closely associated with the 6BH4 dependent phenylalanine hydroxylase (PAH) and tyrosine hydroxylase isoform I (THI) providing evidence for an old concept of the three enzyme theory in the initiation of the pigmentation process. In this context, it is noteworthy that intracellular L-phenylalanine uptake and turnover to L-tyrosine via PAH is vital for substrate supply of THI and tyrosinase. While PAH acts in the cytosol of melanocytes, THI and tyrosinase are sitting side by side in the melanosomal membrane. THI at low pH provides L-3,4-hydroxyphenylalanine L-DOPA which in turn is required for activation of met-tyrosinase. After an intramelanosomal pH change, possibly by the p-protein, has taken place, tyrosinase is subject to control by 6/7BH4 and the proopiomelanocortin (POMC) peptides alpha-MSH melanocyte stimulating hormone and beta-MSH in a receptor independent manner. cAMP is required for the activation of microphthalmia-associated transcription factor to induce expression of tyrosinase, for transcription of THI and for activation of PAH. The redundancy of the cAMP signal is discussed. Finally, we propose a novel mechanism involving H2O2 in the regulation of tyrosinase via p53 through transcription of hepatocyte nuclear factor 1alpha which in turn can also affect the POMC response.

Melatonin in the skin: synthesis, metabolism and functions

Melatonin, a ubiquitous methoxyindole, is produced by and metabolized in the skin. Melatonin affects skin functions and structures through actions mediated by cell-surface and putative-nuclear receptors expressed in skin cells. Melatonin has both receptor-dependent and receptor-independent effects that protect against oxidative stress and can attenuate ultraviolet radiation-induced damage. The widespread expression and pleiotropic activity of the cutaneous melatoninergic system provides for a high level of cell-specific selectivity. Moreover, intra-, auto- and para-crine mechanisms equip this system with exquisite functional selectivity. The properties of endogenous melatonin suggest that this molecule is an important effector of stress responses in the skin. In this way, melatonin actions may counteract or buffer both environmental and endogenous stressors to maintain skin integrity.

The skin as a mirror of the soul: exploring the possible roles of serotonin

Serotonin (5-hydroxytryptamine; 5-HT) is an important mediator of bidirectional interactions between the neuroendocrine system and the skin. The rate of synthesis of 5-HT from l-tryptophan can be enhanced by brain-derived neuronal growth factor, cytokines, exposure to ultraviolet light and steroids. The major source of 5-HT in the skin are platelets, which, upon aggregation, release this biogenic amine. Moreover, the epidermal and dermal skin express the enzymes required for the transformation of tryptophan to 5-HT, and certain skin cells, such as melanocytes, have been demonstrated to produce 5-HT. In addition, rodent mast cells produce 5-HT, but human mast cells have not yet been fully examined in this respect. Skin cells express functionally active, membrane-bound receptors for 5-HT, as well as proteins that transport 5-HT. The interactions of 5-HT with these various proteins determines the nature, magnitude and duration of serotonergic responses. The immune and vasculature systems in the skin are traditional targets for bioregulation by 5-HT. Moreover, recent findings indicate that keratinocytes, melanocytes and dermal fibroblasts also respond to this amine in various ways. Thus, mammalian skin is both a site for the production of and a target for bioregulation by 5-HT. This indicates that agonists and antagonists directed towards specific 5-HT receptors could be useful in connection with treatment of skin diseases. Based on our increasing knowledge concerning these receptors and their plasticity, future research will focus on the development of serotonergic drugs that exert metabotrophic effects on the cells of the skin without affecting the central nervous system.

Advances in melanocyte basic science research

This article focuses on recent advances in melanocyte biology and physiology. The major function of this neural crest-derived cell is the production of melanins. A "three enzyme theory" in the initiation of pigmentation is put forward and backed up by recent findings. A receptor-independent role for alpha-MSH and the cofactor (6R)-l-erythro-5,6,7,8-terahydrobiopterin (6BH(4)) in the control of tyrosinase is described. The importance of intramelanosomal pH for melanogenesis is covered. Finally, the redundancy of the cAMP and IP3/DAG/calcium signal in melanocytes together with the downstream events are highlighted. The main message of this article is that the intracellular H(2)O(2)- redox-equilibrium controls melanocyte function in a concentration-dependent manner.

On the role of melatonin in skin physiology and pathology

Melatonin has been experimentally implicated in skin functions such as hair growth cycling, fur pigmentation, and melanoma control, and melatonin receptors are expressed in several skin cells including normal and malignant keratinocytes, melanocytes, and fibroblasts. Melatonin is also able to suppress ultraviolet (UV)-induced damage to skin cells and shows strong antioxidant activity in UV exposed cells. Moreover, we recently uncovered expression in the skin of the biochemical machinery involved in the sequential transformation of l-tryptophan to serotonin and melatonin. Existence of the biosynthetic pathway was confirmed by detection of the corresponding genes and proteins with actual demonstration of enzymatic activities for tryptophan hydroxylase, serotonin N-acetyl-transferase, and hydroxyindole-O-methyltransferase in extracts from skin and skin cells. Initial evidence for in vivo synthesis of melatonin and its metabolism was obtained in hamster skin organ culture and in one melanoma line. Therefore, we propose that melatonin (synthesized locally or delivered topically) could counteract or buffer external (environmental) or internal stresses to preserve the biological integrity of the organ and to maintain its home-ostasis. Furthermore, melatonin could have a role in protection against solar radiation or even in the management of skin diseases.

Decreased phenylalanine uptake and turnover in patients with vitiligo

The human epidermis has the full machinery for autocrine L-phenylalanine turnover to L-tyrosine in keratinocytes and melanocytes. Phenylalanine hydroxylase (PAH) activities increase linearly with inherited skin colour (skin phototype I-VI, Fitzpatrick classification) yielding eightfold more activities in black skin compared to white skin. Moreover, UVB irradiation (1 MED) significantly increases epidermal PAH activities 24 h after exposure. Importantly, L-phenylalanine uptake and turnover in the pigment forming melanocytes is vital for initiation of melanogenesis. In this context it was shown that the uptake of this amino acid is regulated by calcium. The depigmentation disorder vitiligo provides a unique model to follow impaired L-phenylalanine turnover in the skin as well as in serum because affected individuals hold an impaired epidermal 6BH4 de novo synthesis/recycling and regulation including low epidermal PAH activities. After overnight fasting and oral loading with L-phenylalanine (100 mg/kg body weight), 29.6% of 970 patients tested (n=287/970) yielded serum phenylalanine/tyrosine ratios >or=4 and 35.3% (n=342/970) had mild to moderate hyperphenylalaninaemia (HPA), while 9.3% (n=90/970) had both serum L-phenylalanine levels >or=2.0 mg/dl and phe/tyr ratios >or=4.0. Isolated HPA was found in 26% (n=252/970), whereas 20.3% had only increased ratios (n=197/970). None of the patients had phenylketonuria and the family history for this metabolic disease was negative. The IQ followed normal Gaussian distribution. In vitro L-phenylalanine uptake/turnover studies on primary epidermal melanocytes originating from these patients demonstrated a significantly decreased calcium dependent L-phenylalanine uptake and turnover compared to healthy control cells. Based on our observation, we would like to propose that phenylalanine uptake/turnover is under tight control by calcium which in turn could offer an additional novel mechanism in the aetiology of HPA.

The cutaneous serotoninergic/melatoninergic system: securing a place under the sun

It was recently discovered that mammalian skin can produce serotonin and transform it into melatonin. Pathways for the biosynthesis and biodegradation of serotonin and melatonin have been characterized in human and rodent skin and in their major cellular populations. Moreover, receptors for serotonin and melatonin receptors are expressed in keratinocytes, melanocytes, and fibroblasts and these mediate phenotypic actions on cellular proliferation and differentiation. Melatonin exerts receptor-independent effects, including activation of pathways protective of oxidative stress and the modification of cellular metabolism. While serotonin is known to have several roles in skin-e.g., pro-edema, vasodilatory, proinflammatory, and pruritogenic-melatonin has been experimentally implicated in hair growth cycling, pigmentation physiology, and melanoma control. Thus, the widespread expression of a cutaneous seorotoninergic/melatoninergic syste,m(s) indicates considerable selectivity of action to facilitate intra-, auto-, or paracrine mechanisms that define and influence skin function in a highly compartmentalized manner. Notably, the cutaneous melatoninergic system is organized to respond to continuous stimulation in contrast to the pineal gland, which (being insulated from the external environment) responds to discontinuous activation by the circadian clock. Overall, the cutaneous serotoninergic/melatoninergic system could counteract or buffer external (environmental) or internal stresses to preserve the biological integrity of the organ and to maintain its homeostasis.-Slominski, A. J., Wortsman, J., Tobin, D. J. The cutaneous serotoninergic/melatoninergic system: securing a place under the sun.

On the role of melatonin in skin physiology and pathology

Melatonin has been experimentally implicated in skin functions such as hair growth cycling, fur pigmentation, and melanoma control, and melatonin receptors are expressed in several skin cells including normal and malignant keratinocytes, melanocytes, and fibroblasts. Melatonin is also able to suppress ultraviolet (UV)-induced damage to skin cells and shows strong antioxidant activity in UV exposed cells. Moreover, we recently uncovered expression in the skin of the biochemical machinery involved in the sequential transformation of l-tryptophan to serotonin and melatonin. Existence of the biosynthetic pathway was confirmed by detection of the corresponding genes and proteins with actual demonstration of enzymatic activities for tryptophan hydroxylase, serotonin N-acetyl-transferase, and hydroxyindole-O-methyltransferase in extracts from skin and skin cells. Initial evidence for in vivo synthesis of melatonin and its metabolism was obtained in hamster skin organ culture and in one melanoma line. Therefore, we propose that melatonin (synthesized locally or delivered topically) could counteract or buffer external (environmental) or internal stresses to preserve the biological integrity of the organ and to maintain its home-ostasis. Furthermore, melatonin could have a role in protection against solar radiation or even in the management of skin diseases.

Hair follicle pigmentation

Hair shaft melanin components (eu- or/and pheomelanin) are a long-lived record of precise interactions in the hair follicle pigmentary unit, e.g., between follicular melanocytes, keratinocytes, and dermal papilla fibroblasts. Follicular melanogenesis (FM) involves sequentially the melanogenic activity of follicular melanocytes, the transfer of melanin granules into cortical and medulla keratinocytes, and the formation of pigmented hair shafts. This activity is in turn regulated by an array of enzymes, structural and regulatory proteins, transporters, and receptors and their ligands, acting on the developmental stages, cellular, and hair follicle levels. FM is stringently coupled to the anagen stage of the hair cycle, being switched-off in catagen to remain absent through telogen. At the organ level FM is precisely coupled to the life cycle of melanocytes with changes in their compartmental distribution and accelerated melanoblast/melanocyte differentiation with enhanced secretory activity. The melanocyte compartments in the upper hair follicle also provides a reservoir for the repigmentation of epidermis and, for the cyclic formation of new anagen hair bulbs. Melanin synthesis and pigment transfer to bulb keratinocytes are dependent on the availability of melanin precursors, and regulation by signal transduction pathways intrinsic to skin and hair follicle, which are both receptor dependent and independent, act through auto-, para- or intracrine mechanisms and can be modified by hormonal signals. The important regulators are MC1 receptor its and adrenocorticotropic hormone, melanocyte stimulating hormone, agouti protein ligands (in rodents), c-Kit, and the endothelin receptors with their ligands. Melanin itself has a wide range of bioactivities that extend far beyond its determination of hair color.

Regulation of tyrosinase by tetrahydropteridines and H2O2

Recently two alternative mechanisms have been put forward for the inhibition of tyrosinase by 6R-l-erythro 5,6,7,8-tetrahydrobiopterin (6BH(4)). Initially allosteric uncompetitive inhibition was demonstrated due to 1:1 binding of 10(-6)M 6BH(4) to a specific domain 28 amino acids away from the Cu(A) active site of the enzyme. Alternatively it was then shown that 10(-3)M 6BH(4) inhibit the reaction by the reduction of the product dopaquinone back to l-dopa. In the study presented herein we have used two structural analogues of 6BH(4) (i.e., 6,7-(R,S)-dimethyl tetrahydrobiopterin and 6-(R,S)-tetrahydromonapterin) confirming classical uncompetitive inhibition due to specific binding of the pyrimidine ring of the pterin moiety to the regulatory domain on tyrosinase. Under these conditions there was no reduction of l-dopaquinone back to l-dopa by both cofactor analogues. Inhibition of tyrosinase by 6BH(4) occurs in the concentration range of 10(-6)M after preactivation with l-tyrosine and this mechanism uncouples the enzyme reaction producing H(2)O(2) from O(2). Moreover, a direct oxidation of 6BH(4) to 7,8-dihydrobiopterin by tyrosinase in the absence of the substrate l-tyrosine was demonstrated. The enzyme was activated by low concentrations of H(2)O(2) (<0.3 x 10(-3)M), but deactivated at concentrations in the range 0.5-5.0 x 10(-3)M. In summary, our results confirm a major role for 6BH(4) in the regulation of human pigmentation.

Ocular Melanogenesis: The Role of Antioxidants

Given the propensity of a large number of melanogenic pathways that can be modulated by cellular redox status, a causal role of the deficiency of ocular pigments such as melanin in the pathogenesis of age-related macular degeneration and evidence that melanin production does occur in the adult eye, it seems not improbable that antioxidants (or agents that modify cellular redox status) may have melanin stimulatory (or inhibitory) effects that are superimposible on their effects as mere free radical scavengers. More empirical studies are needed to investigate this phenomenon so that antioxidant therapy may prove more beneficial to patients with ocular degenerative diseases.

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.

Human phenylalanine hydroxylase is activated by H2O2: a novel mechanism for increasing the l-tyrosine supply for melanogenesis in melanocytes

Epidermal phenylalanine hydroxylase (PAH) produces L-tyrosine from the essential amino acid L-phenylalanine supporting melanogenesis in human melanocytes. Those PAH activities increase linearly in the different skin phototypes I-VI (Fitzpatrick classification) and also increase up to 24h after UVB light with only one minimal erythemal dose. Since UVB generates also H(2)O(2), we here asked the question whether this reactive oxygen species could influence the activity of pure recombinant human PAH. Under saturating conditions with the substrate L-phenylalanine (1x10(-3)M), the V(max) for enzyme activity increased 4-fold by H(2)O(2) (>2.0x10(-3)M). Lineweaver-Burk analysis identified a mixed activation mechanism involving both the regulatory and catalytic domains of PAH. Hyperchem molecular modelling and Deep View analysis support oxidation of the single Trp(120) residue to 5-OH-Trp(120) by H(2)O(2) causing a conformational change in the regulatory domain. PAH was still activated by H(2)O(2) in the presence of the electron donor/cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin despite slow oxidation of this cofactor. In vivo FT-Raman spectroscopy confirmed decreased epidermal phenylalanine in association with increased tyrosine after UVB exposure. Hence, generation of H(2)O(2) by UVB can activate epidermal PAH leading to an increased L-tyrosine pool for melanogenesis.

Characterization of the serotoninergic system in the C57BL/6 mouse skin

We showed expression of the tryptophan hydroxylase gene and of tryptophan hydroxylase protein immunoreactivity in mouse skin and skin cells. Extracts from skin and melanocyte samples acetylated serotonin to N-acetylserotonin and tryptamine to N-acetyltryptamine. A different enzyme from arylalkylamine N-acetyltransferase mediated this reaction, as this gene was defective in the C57BL6 mouse, coding predominantly for a protein without enzymatic activity. Serotonin (but not tryptamine) acetylation varied according to hair cycle phase and anatomic location. Serotonin was also metabolized to 5-hydroxytryptophol and 5-hydroxyindole acetic acid, probably through stepwise transformation catalyzed by monoamine oxidase, aldehyde dehydrogenase and aldehyde reductase. Activity of the melatonin-forming enzyme hydroxyindole-O-methyltransferase was notably below detectable levels in all samples of mouse corporal skin, although it was detectable at low levels in the ears and in Cloudman melanoma (derived from the DBA/2 J mouse strain). In conclusion, mouse skin has the molecular and biochemical apparatus necessary to produce and metabolize serotonin and N-acetylserotonin, and its activity is determined by topography, physiological status of the skin, cell type and mouse strain.

In situ and in vitro evidence for DCoH/HNF-1 alpha transcription of tyrosinase in human skin melanocytes

Human epidermal melanocytes hold the full capacity for autocrine de novo synthesis/regulation/recycling of the essential cofactor 6-tetrahydrobiopterin (6BH(4)) for conversion of L-phenylalanine via phenylalanine hydroxylase to L-tyrosine and for production of L-Dopa via tyrosine hydroxylase to initiate both pigmentation and catecholamine synthesis in these neural crest-derived cells. Earlier we have demonstrated pterin-4a-carbinolamine dehydratase (PCD) mRNA and enzyme activities in epidermal melanocytes and keratinocytes. This protein dimerises also the transcription factor hepatocyte nuclear factor 1 (HNF-1), leading to activation of multiple genes. This study demonstrates for the first time DCoH/HNF-1 alpha expression and transcriptional activity in human epidermal melanocytes in vitro and in situ and identified tyrosinase, the key enzyme for pigmentation, as a new transcriptional target. Specific binding of DCoH/HNF-1 complex to the human tyrosinase promoter was confirmed by gel shift analysis. These results provide a novel mechanism in the regulation of skin pigmentation.

Serotoninergic system in hamster skin

We have cloned the tryptophan hydroxylase cDNA from hamster pituitary and demonstrated its expression in the skin, melanotic and amelanotic melanomas, spleen, heart, and the eye. We further demonstrated that skin, melanomas, spleen, pituitary, and eye but not heart expressed arylalkylamine N-acetyltransferase mRNA. The cutaneous expression of the arylalkylamine N-acetyltransferase gene was accompanied by enzymatic activity for the conversion of serotonin and tryptamine to N-acetylserotonin and N-acetyltryptamine, respectively. There was marked regional variation in the serotonin N-acetyltransferase activity, which was higher in ear skin than in corpus skin, and was lower in melanomas than in normal skin. Serotonin N-acetyltransferase activity was significantly inhibited by Cole bisubstrate at low concentration (</= 1 micro m); this evidence in conjunction with arylalkylamine N-acetyltransferase mRNA expression implies an involvement of arylalkylamine N-acetyltransferase in serotonin metabolism in the skin. We also documented both the in vitro transformation of serotonin to N-acetylserotonin using liquid chromatography/mass spectrometry and the generation/storage of N-acetylserotonin in cultured melanoma cells. Thus, we have uncovered a cutaneous pathway displaying capabilities for serotonin biosynthesis and/or its metabolism to N-acetylserotonin in rodent skin. As serotonin has powerful vasodilator, immunomodulator, and growth factor actions, this pathway could be involved in skin physiology and/or pathology.

Putative conventional protein kinase C inhibitor Gödecke 6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] stimulates transglutaminase activity in primary mouse epidermal keratinocytes

Much data in the literature suggest a role for protein kinase C (PKC) in regulating keratinocyte proliferation and differentiation. Nevertheless, the exact role of this family of isoenzymes is unclear, since PKC agonists (e.g., phorbol esters) are known to stimulate expression of both proliferative and differentiative markers in keratinocytes. Similarly, PKC inhibitors have been demonstrated both to inhibit [2-[1-3(aminopropyl)indol-3-yl]-3(1-methyl-1H-indol-3-yl)maleimide, acetate (Ro 31-7549) and 3-[1-[3-(amidinothio)propyl-1H-indol-3-(1-methyl-1H-indol-3yl) maleimide (Ro 31-8220)] and to induce (staurosporine) keratinocyte differentiation. In this study, we examined the role of the PKC inhibitor, Gödecke 6976 (Gö6976) [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo (3,4-c)-carbazole], on keratinocyte proliferation, as measured by DNA synthesis, and differentiation, as monitored by transglutaminase activity. This compound is reported to be selective for the conventional PKC isoforms, of which keratinocytes express only PKCalpha, and for protein kinase D (PKD; also known as PKCmu). We report that Gö6976 stimulated transglutaminase activity. Consistent with this effect, Gö6976 also potently inhibited [(3)H]thymidine incorporation (a half-maximal inhibitory concentration of approximately 0.1 microM). In addition, Gö6976 (1 microM) was able to enhance the stimulation of transglutaminase activity by 1,25-dihydroxyvitamin D(3) but had no effect on D(3)-induced expression of keratin-1. Conversely, Gö6983 [2-[1-(3-dimethylaminopropy)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide], a similar compound that also selectively inhibits conventional PKCalpha, but not PKD, had little or no effect on DNA synthesis or transglutaminase activity (up to 1 microM). The effect of Gö6976 was not due to cytotoxicity as its effect on thymidine incorporation was largely reversible, and its stimulation of transglutaminase activity could be inhibited by another general PKC inhibitor, bisindolylmaleimide I. Therefore, our results suggest a proproliferative, antidifferentiative role for PKD in epidermal maturation.

Melanocyte function and its control by melanocortin peptides

Melanocytes are cells of neural crest origin. In the human epidermis, they form a close association with keratinocytes via their dendrites. Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively. One of the factors that regulates melanocytes and skin pigmentation is the locally produced melanocortin peptide alpha-MSH. The effects of alpha-MSH on melanogenesis are mediated via the MC-1R and tyrosinase, the rate-limiting enzyme in the melanogenesis pathway. Binding of alpha-MSH to its receptor increases tyrosinase activity and eumelanin production, which accounts for the skin-darkening effect of alpha-MSH. Other alpha-MSH-related melanocortin peptides, such as ACTH1-17 and desacetylated alpha-MSH, are also agonists at the MC-1R and could regulate melanocyte function. Recent evidence shows that melanocytes have other functions in the skin in addition to their ability to produce melanin. They are able to secrete a wide range of signal molecules, including cytokines, POMC peptides, catecholamines, and NO in response to UV irradiation and other stimuli. Potential targets of these secretory products are keratinocytes, lymphocytes, fibroblasts, mast cells, and endothelial cells, all of which express receptors for these signal molecules. Melanocytes may therefore act as important local regulators of a range of skin cells. It has been shown that alpha-MSH regulates NO production from melanocytes, and it is possible that the melanocortins regulate the release of other signalling molecules from melanocytes. Therefore, the melanocortin signaling system is one of the important regulators of skin homeostasis.

Thioredoxin reductase - its role in epidermal redox status

The human epidermis with an area of 1.8 m(2) is the outer most layer of the human body. Hence, this organ plays a pivotal role in the defence against reactive oxygen species (ROS) generated by UV or X-ray exposure, heat and other sources. Consequently, a plethora of defence mechanisms exist controlling the redox status in this compartment. The role of thioredoxin reductase (TR), thioredoxin (T) in antioxidant defence has gained widespread recognition. In the past it has been shown that thioredoxin protects against UVB-induced skin injury, as well as against peroxidative damage. Under normal conditions, TR reduces oxidised thioredoxin in the presence of NADPH. Reduced thioredoxin serves as an electron donor for thioredoxin peroxidase (TPx) which consequently reduces H(2)O(2) to H(2)O. In this context, it has been demonstrated that membrane associated TR correlates with different skin photo types I-VI (Fitzpatrick classification), where darker skin has significantly higher enzyme activity compared to very fair skin, underlining the importance of this system in ROS defence. Moreover, it was only recently demonstrated in vivo with non-invasive Fourier-Transform Raman spectroscopy that UVB generates H(2)O(2) in the epidermis in a dose-dependent manner. H(2)O(2) can oxidise the selenocysteine residue in the penultimate position of the carboxyl terminus of TR with a K(m) of 2.5 mM. This oxidation is followed by an upregulation of mRNA expression of the enzyme. Hence, it can be concluded that UVB generated H(2)O(2) induces TR. However, permanent H(2)O(2) levels induce the tumour suppressor p53 which in turn downregulates cytosolic TR. Therefore TR activities are under fine control by H(2)O(2). This conclusion is also supported by the observation that thioredoxin, the substrate for TR, migrates from the cytosol to the nucleus after UVB exposure. A new function for the TR/T/TPx system in epidermal cells has been discovered in the control of the important cofactor (6R)-L-erythro 5,6,7,8 tetrahydrobiopterin (6BH(4)) homeostasis. Full oxidation of 6BH(4) to 6 biopterin via H(2)O(2) can lead to a cytotoxic environment for epidermal melanocytes. This cascade of events is observed in the depigmentation disorder vitiligo, where millimolar levels of H(2)O(2) can accumulate in the epidermis of affected individuals, consequently leading to cellular vacuolation in this compartment.

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

Pro-opiomelanocortin-related peptides, prohormone convertases 1 and 2 and the regulatory peptide 7B2 are present in melanosomes of human melanocytes

Recently, it has been shown that alpha-melanocyte stimulating hormone can directly activate tyrosinase by removing the allosteric regulator 6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin resulting in a stable alpha-melanocyte stimulating hormone/6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin complex. As melanin production occurs in the melanosome, a specific organelle of the melanocyte, it seemed important to investigate whether these organelles themselves actually produce pro-opiomelanocortin-related peptides in their acidic environment. The presence of alpha-melanocyte stimulating hormone and adrenocorticotropin in the epidermis and melanocytes has been shown by several investigators. In order to follow possible pro-opiomelanocortin processing in the melanosome, human melanocytes were established in MCDB 153 medium and utilized for immunohistochemistry, immunogold electron microscopy, and western blotting. For this purpose antibodies against alpha-melanocyte stimulating hormone, adrenocorticotropin, prohormone convertases 1 and 2 (PC1 and PC2) and the PC2 regulatory protein 7B2 were used. Our results demonstrated the presence of the entire system for pro-opiomelanocortin processing in the melanosome. Considering the pH optima of these convertases, the results are in agreement with an autocrine intramelanosomal production of pro- opiomelanocortin-related peptides and an autocrine production and recycling of the cofactor 6(R)-L- erythro 5,6,7,8 tetrahydrobiopterin in melanocytes. Based on these novel observations, we would like to propose that the pigmentation process may not necessarily involve a melanocortin-1 receptor-mediated mechanism.

alpha-melanocyte-stimulating hormone modulates nitric oxide production in melanocytes

We have previously observed that melanocytes produce nitric oxide in response to ultraviolet radiation and lipopolysaccharide and in this study have examined how these responses are affected by alpha-melanocyte-stimulating hormone. Nitric oxide production by cultured cells was measured electrochemically in real time using an ISO-nitric oxide sensor probe. B16 mouse melanoma cells released nitric oxide in response to lipopolysaccharide and the effects were enhanced in cells that had been grown in the presence of 10-11-10-9 M alpha-melanocyte-stimulating hormone prior to stimulation. At concentrations in excess of 10-9 M alpha-melanocyte-stimulating hormone decreased nitric oxide production. Preincubation with lipopolysaccharide, a well-known inducer of inducible nitric oxide synthase, also increased nitric oxide production but this response was reduced by alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone also increased the levels of nitric oxide produced in response to ultraviolet radiation (20-100 mJ per cm2) in B16 cells. The same effect was seen in human melanocytes and as this was inhibited by aminoguanidine would appear to involve an induction of inducible nitric oxide synthase. Reverse transcription-polymerase chain reaction showed that melanocytic cells express inducible nitric oxide synthase mRNA. Western blotting analysis and immunocytochemistry confirmed the presence of inducible nitric oxide synthase protein in B16 cells and FM55 human melanoma cells and that the levels were increased in response to alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone, however, decreased inducible nitric oxide synthase protein expression, which occurred in response to lipopolysaccharide. These results suggest that alpha-melanocyte-stimulating hormone regulates nitric oxide production in melanocytic cells by modulating the induction of inducible nitric oxide synthase. Additional experiments showed that nitric oxide increased melanin production by B16 cells and human melanocytes. This is in keeping with a melanogenic role for nitric oxide but whether its production by melanocytes in response to alpha-melanocyte-stimulating hormone is associated with such a role or whether it has some other significance relating to melanocyte differentiation or in mediating immunomodulatory actions of alpha-melanocyte-stimulating hormone remains to be seen.

alpha-MSH can control the essential cofactor 6-tetrahydrobiopterin in melanogenesis

In the human epidermis both keratinocytes and melanocytes express POMC m-RNA. Immunohistochemical studies of both cell types demonstrate significantly higher levels of alpha-MSH in melanocytes than in keratinocytes. Both cell types also hold the full capacity for de novo synthesis/recycling of the essential cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH4). 6BH4 is critical for the hydroxylation of the aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophan, for nitric oxide production and in various immune modulatory processes. Recently it was shown that tyrosinase activity is regulated by 6BH4 through a specific allosteric inhibition. The tyrosinase/6BH4 inhibition can be activated by 1:1 complex formation between 6BH4 and alpha-MSH, but an excess of alpha-MSH over 6BH4 can inhibit tyrosinase due to complex formation by tyr2 in the alpha-MSH sequence. In both melanocytes and keratinocytes 6BH4 controls the L-tyrosine supply via phenylalanine hydroxylase (PAH). Recently we were able to show that the cellular uptake of L-phenylalanine and its intracellular turnover to L-tyrosine is crucial for melanogenesis. alpha-MSH can promote the production of L-tyrosine via PAH due to activation of the PAH tetramer to the more active dimer by removing 6BH4 from the regulatory binding domain on the enzyme. In conclusion, alpha-MSH can control (1) intracellular L-tyrosine formation from L-phenylalanine in both melanocytes and keratinocytes, and (2) tyrosinase activity, directly, in melanocytes.

The importance of L-phenylalanine transport and its autocrine turnover to L-tyrosine for melanogenesis in human epidermal melanocytes

A comparative study of (14)C-labeled l-phenylalanine and (3)H-labeled L-tyrosine uptake in cultures of human melanocytes (n = 8) and keratinocytes (n = 2) identified a significantly more rapid active transport for L-phenylalanine in melanocytes, whereas the slower uptake of L-tyrosine followed a concentration-dependent gradient, confirming facilitated diffusion rather than active transport. In addition, a significantly more efficient autocrine turnover of L-phenylalanine to L-tyrosine via intracellular phenylalanine hydroxylase was demonstrated in melanocytes. The incorporation of the newly synthesized radiolabeled L-tyrosine was directly followed into the end product melanin. The presence of L-phenylalanine in the culture medium produced 40% more melanin compared to an equivalent concentration of L-tyrosine alone. The transport of extracellular L-phenylalanine and its intracellular metabolism to L-tyrosine via intracellular phenylalanine hydroxylase are coupled to calcium uptake/efflux, whereas L-tyrosine uptake is calcium independent. Taken together, our results identified for the first time the importance of autocrine calcium-dependent active l-phenylalanine uptake/turnover in melanocytes as a major pathway for melanogenesis.

Coordinated induction of inducible nitric oxide synthase and GTP-cyclohydrolase I is dependent on inflammatory cytokines and interferon-gamma in HaCaT keratinocytes: implications for the model of cutaneous wound repair

Recently we demonstrated a strong expression of inducible nitric oxide synthase (iNOS) and GTP-cyclohydrolase I (GTP-CH I) in the basal keratinocytes of the epidermis adjacent to the wound and of the hyperproliferative epithelium during wound healing. To identify possible mediators of iNOS and GTP-CH I expression during this process, we analyzed the regulation of iNOS and GTP-CH I expression in cultured human keratinocytes. We found a large and long lasting coinduction of iNOS and GTP-CH I expression upon simultaneous treatment of quiescent cells with inflammatory cytokines interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma, but not with serum growth factors. The stimulatory effect of interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma is strongly synergistic on iNOS and GTP-CH I expression, because these factors alone stimulated GTP-CH I expression, although to a much lesser extent. Furthermore, iNOS mRNA levels are not influenced at all by stimulation with IL-1beta and revealed only a weak induction after treatment with tumor necrosis factor-alpha and interferon-gamma. Induction of iNOS and GTP-CH I gene expression upon cytokine and interferon-gamma exposure is independent of de novo protein synthesis. Because these cytokines are present at the wound site, they might be responsible for iNOS and GTP-CH I induction during cutaneous repair. Serum, which is released upon hemorrhage, is likely to play no stimulatory role in iNOS and GTP-CH I induction during wound healing.

Induction of inducible nitric oxide synthase and its corresponding tetrahydrobiopterin-cofactor-synthesizing enzyme GTP-cyclohydrolase I during cutaneous wound repair

Recent work has suggested a possible role of nitric oxide, a free radical gas, during the wound healing process. In this study we investigated the regulation of inducible nitric oxide synthase (iNOS) and GTP-cyclohydrolase I (GTP-CH I), the rate-limiting enzyme in the biosynthesis of the iNOS cofactor (6R) 5,6,7,8-tetrahydrobiopterin (6-BH4), during the repair process. We found a similar time course of induction of iNOS and GTP-CH I expression, whereas absolute expression levels were different for both genes. Immunohistochemical analysis revealed colocalization of iNOS and GTP-CH I proteins in the wound. Systemic treatment with glucocorticoids significantly altered the expression levels of iNOS and GTP-CH I. Expression of iNOS and GTP-CH I was suppressed by glucocorticoids in normal, and to a much greater extent in wounded skin. Furthermore, a role of nitric oxide as a novel mediator of gene regulation during healing is suggested by the demonstration of nitric oxide-mediated induction of vascular endothelial growth factor expression in keratinocytes. These findings may provide an explanation for the beneficial effects of orally supplemented L-arginine on wound healing, and suggest that a disturbed induction of iNOS and GTP-CH I expression may at least partially underlie the wound healing defect seen in glucocorticoid-treated animals.

Does alpha-MSH have a role in regulating skin pigmentation in humans?

Over the years there has been much debate as to whether alpha-MSH has a role as a pigmentary hormone in humans. There are two main reasons for this. First, despite the observations in the 1960s that alpha-MSH increased skin darkening in humans, there are reports that the peptide has no effect on melanogenesis in cultured human melanocytes. Second, the human pituitary, unlike that of most mammals, secretes very little alpha-MSH and circulatory levels of the peptide in humans are extremely low. However, there is now evidence from several groups that alpha-MSH is capable of stimulating melanogenesis in cultured human melanocytes. Rather than producing an overall increase in melanin production, it appears that the peptide acts specifically to increase the synthesis of eumelanin. Such an action could well explain the previously observed skin darkening effects of alpha-MSH. It is also now known that alpha-MSH is not produced exclusively in the pituitary but has been found at numerous sites, including the skin where it is produced by several cell types. Related Proopiomelanocortin (POMC) peptides such as ACTH are also produced in human skin. The ACTH peptides act at the same receptor (MC-1) as alpha-MSH and certain of these would appear to be more potent than alpha-MSH in stimulating melanogenesis. The ACTH peptides are also present in greater amounts than alpha-MSH in human epidermis and it is likely that they play an important role in regulating pigmentary responses. These POMC peptides are released from keratinocytes in response to ultraviolet radiation (UVR) and it has been proposed that they serve as paracrine factors in mediating UV induced pigmentation. Their production by keratinocytes could therefore be critical in determining pigmentary responses and any changes in the availability of these POMC peptides might explain the variations in tanning ability seen in different individuals. However, the possibility that tanning ability is also dependent upon differences at the level of the MC-1 receptor cannot be ruled out and it has been suggested that an inability to tan may depend upon the presence of non-functional changes at the MC-1 receptor. alpha-MSH does, of course, affect human melanocytes in several ways and its stimulation of melanogenesis could be the consequence of some other fundamental action in the melanocyte. The peptide also has many other target sites in the skin and while it may have a role in regulating skin pigmentation in humans, it should not be viewed solely as a pigmentary peptide. alpha-MSH clearly has many different actions and its primary role in the skin may be to maintain homeostasis.

Perturbed epidermal pterin metabolism in Hermansky-Pudlak syndrome

In Hermansky-Pudlak Syndrome (HPS) a mutation in a 79.3 kDa transmembrane protein has been shown. The function of this protein has escaped definition so far. This study unveils a defective (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH4) de novo synthesis/recycling for this cofactor in HPS, where activities of the key enzyme GTP-cyclohydrolase I are in the normal range, but total biopterin levels are significantly decreased in homozygotes (n = 5) compared with unaffected controls (n = 4) (p = 0.00001). Phenylalanine hydroxylase and 4a-hydroxy-6BH4-dehydratase activities are significantly lower. mRNA of all enzymes involved in 6BH4 biosynthesis/recycling and GTP-cyclohydrolase I feedback regulatory protein were expressed in keratinocytes from homozygotes, heterozygotes, and healthy controls. Thioredoxin/thioredoxin reductase can directly control the redox status of 6BH4. These activities are allosterically controlled by calcium. Therefore calcium would directly affect this redox status. In HPS these enzyme activities are low concomitant with a defective calcium uptake, suggesting an extracellular accumulation of this second messenger. In this context phenylalanine hydroxylase is subject to phosphorylation/activation by calcium/calmodulin activated kinases. Therefore it was anticipated that calcium could directly affect the cellular L-phenylalanine turnover to L-tyrosine. A significantly more rapid L-phenylalanine uptake and its turnover to L-tyrosine was identified in normal human melanocytes (n = 5) and keratinocytes (n = 2), and was more enhanced in melanocytes in the presence of 2 x 10(-3) M calcium. The turnover to L-tyrosine was significantly slower. Based on all evidence to date, we speculate that the mutated protein in HPS could be primarily involved in maintaining calcium homeostasis in this patient group.

What controls melanogenesis?

The pigments eumelanin and pheomelanin are the visually most striking products of specialized neural crest-derived cells (melanocytes), and provide color to both epidermis and hair shafts. While the intriguing and controversial biological functions of these multifaceted heteropolymers will be discussed in a later feature, here it is explored how their generation (melanogenesis) is controlled. For decades, this has been the object of much controversy, the salient features of which are delineated in the following contributions.

6-Tetrahydrobiopterin functions as a UVB-light switch for de novo melanogenesis

(6R)-L-erythro 5,6,7,8-tetrahydrobiopterin (6-BH4) and its 7-isomer (7-BH4) function as uncompetitive inhibitors of human and mushroom tyrosinases. Stoichiometry for the binding of [3H]-labeled 6-BH4 to both tyrosinases has been established as 1:1. Stable complexation of 6-BH4 to tyrosinase appears to involve a hydrophilic conserved glutamic acid (Glu131) with a pKa = 4.7. Photo-oxidation by UVB-light and O2 reverses the inhibition of tyrosinase by 6-BH4 and 7-BH4 with the 6-BH4/tyrosinase complex being four-fold more photolabile than 7-BH4/tyrosinase. The photo-oxidation of 6-BH4 by UVB-light can be assessed spectrophotometrically with this reaction yielding 7,8-dihydroxanthopterin as the final product, 7,8-Dihydroxanthopterin neither binds to nor inhibits tyrosinase. By contrast, UVA light does not catalyze the photodegradation of 6-BH4. Taken together, our results indicate that the photo-oxidation of the tetrahydrobiopterins by UVB may represent a photo-switch in the regulation of tyrosinase activity to promote de novo melanogenesis.