Sunlight, melanogenesis and radicals in the skin

A Multifunctional and Possible Skin UV Protectant, (3R)-5-Hydroxymellein, Produced by an Endolichenic Fungus Isolated from Parmotrema austrosinense

Lichens are considered a great bio-resource because they produce large numbers of secondary metabolites with many biological activities; however, they have not been cultivated under artificial conditions to date. As a result, lichen substances from natural sources are limited and have not been widely utilized in commercial applications. Accordingly, interest in lichen-associated fungi, especially endogenic fungi, has increased. Ultraviolet (UV) radiation in sunlight is harmful to human health, resulting in demand for effective UV filtering agents for use in sunscreen. In this study, we purified (3R)-5-hydroxymellein, which has UVA absorption activity, from the secondary metabolites of an endolichenic fungus (ELF000039). The antioxidant properties were then assessed by in vitro tests. The antioxidant activity of (3R)-5-hydroxymellein was high when compared to the recognized antioxidants ascorbic acid (ASA) and butyl hydroxyl anisole (BHA). Moreover, the compound exhibited no cytotoxicity toward mouse melanoma cell lines, B16F1 and B16F10, or the normal cell line, HaCaT. Furthermore, (3R)-5-hydroxymellein recovered the damage caused by UVB irradiation and inhibited melanin synthesis. Taken together, these results suggest that (3R)-5-hydroxymellein could have an interesting and vital profile to go further development as a multifunctional skin UV protectant.

One-step deposition of a melanin-like polymer on individual Escherichia coli cells exhibiting a special UV resistance effect

Melanin-like polydopamine encapsulated E. coli cells could retained cells viability, inhibited cell division and protected cells from UV radiation. These provide both fundamental research and applications of cell encapsulation for UV resistance.

Aqueous Extract of Chrysanthemum morifolium ( Jú Huā) Enhances the Antimelanogenic and Antioxidative Activities of the Mixture of Soy Peptide and Collagen Peptide

The possible synergistic effect between the aqueous extract of Chrysanthemum morifolium ( Jú Huā) (AECM) and the peptide mixture (PM) containing soy peptide and collagen peptide was investigated in an ultraviolet (UV) irradiation-induced skin damage mouse model. The irradiated mice were treated with the PM or PM + AECM (containing PM and AECM), respectively. Both PM and PM + AECM groups displayed an apparent photoprotective effect on the UV-irradiated skin damage of mice. Histological evaluation demonstrated that the epidermal hyperplasia and melanocytes in the basal epidermal layer of the UV-irradiated skin in mice decreased when treated with either PM or PM + AECM. Further study showed that soy peptide, collagen peptide, and AECM also inhibited the activities of mushroom tyrosinase with IC50 values of 82.3, 28.2, and 1.6 μg/ml, respectively. Additionally, PM + AECM reduced melanogenesis by 46.2% at the concentration of 10 mg/ml in B16 mouse melanoma cells. Meanwhile, the UV-induced increase of antioxidative indicators, including glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), and malondialdehyde (MDA), was reduced significantly after treatment with 1.83 g/kg/dbw of PM + AECM. This evidence supported the synergistic antioxidative effect of AECM with PM. These results demonstrated that oral intake of PM and AECM had synergistic antimelanogenic and antioxidative effects in UV-irradiated mice.

UV radiation and the skin

UV radiation (UV) is classified as a "complete carcinogen" because it is both a mutagen and a non-specific damaging agent and has properties of both a tumor initiator and a tumor promoter. In environmental abundance, UV is the most important modifiable risk factor for skin cancer and many other environmentally-influenced skin disorders. However, UV also benefits human health by mediating natural synthesis of vitamin D and endorphins in the skin, therefore UV has complex and mixed effects on human health. Nonetheless, excessive exposure to UV carries profound health risks, including atrophy, pigmentary changes, wrinkling and malignancy. UV is epidemiologically and molecularly linked to the three most common types of skin cancer, basal cell carcinoma, squamous cell carcinoma and malignant melanoma, which together affect more than a million Americans annually. Genetic factors also influence risk of UV-mediated skin disease. Polymorphisms of the melanocortin 1 receptor (MC1R) gene, in particular, correlate with fairness of skin, UV sensitivity, and enhanced cancer risk. We are interested in developing UV-protective approaches based on a detailed understanding of molecular events that occur after UV exposure, focusing particularly on epidermal melanization and the role of the MC1R in genome maintenance.

Effects of thymol on mushroom tyrosinase-catalyzed melanin formation

The novel inhibitory mechanism of thymol (2-isopropyl-5-methylphenol) on dopachrome formation by mushroom tyrosinase (EC 1.14.18.1) was identified. The UV-vis spectrum and oxygen consumption assays showed dopachrome formation using L-tyrosine as a substrate was suppressed by thymol. This inhibitory activity was reversed by the addition of a well-known radical scavenger, butylated hydroxyanisole (BHA). Further investigations using N-acetyl-L-tyrosine as a substrate with HPLC analysis suggested that thymol inhibits chemical redox reactions between dopaquinone and leukodopachrome instead of enzymatic reaction. This redox inhibitory activity of thymol was examined by using a model redox reaction with L-dihydroxyphenylalanine (L-DOPA) and p-benzoquinone. Thymol successfully inhibited oxidation of L-DOPA to dopaquinone, coupled with reduction of p-benzoquinone. Hence, the suppression of dopachrome formation by thymol is due to the inhibition of conversion of leukodopachrome to dopachrome. The antioxidant property of thymol is a key characteristic for the inhibitory mechanism of melanin synthesis.

The pheomelanin precursor 5-S-cysteinyldopa protects melanocytes from membrane damage induced by ultraviolet A radiation

Pheomelanin and pheomelanin precursors have been implicated as risk factors for induction of melanoma by ultraviolet radiation. The pheomelanin precursor, 5-S-cysteinyldopa, has been shown to sensitise DNA to oxidative damage by ultraviolet radiation. We here show that 5-S-cysteinyldopa significantly protects melanocytes from membrane damage (permeability) induced by ultraviolet A radiation. Thus, 5-S-cysteinyldopa, may at the same time sensitise DNA and protect membranes from damage induced by ultraviolet radiation.

Lightening Effect on Ultraviolet-Induced Pigmentation of Guinea Pig Skin by Oral Administration of a Proanthocyanidin-Rich Extract from Grape Seeds

Antioxidants such as vitamins C and E have been reported to inhibit the progression of ultraviolet (UV) radiation-induced pigmentation in the skin of hairless mice. However, little is known of the lightening effect of proanthocyanidin, a powerful polyphenolic antioxidant, on UV-induced pigmentation of the skin. We investigated the lightening effect of oral administration of a proanthocyanidin-rich grape seed extract (GSE) using guinea pigs with UV-induced pigmentation. These pigmented guinea pigs were fed diets containing 1% GSE or 1% vitamin C (w/w) for 8 weeks. GSE-feeding had an apparent lightening effect on the guinea pigs' pigmented skin. Histologic evaluation demonstrated a decrease in the number of 3,4-dihydroxyphenylalanine (DOPA)-positive melanocytes as well as 8-hydroxy-2'-deoxyguanosine (8-OHdG)-positive, Ki-67-positive, proliferating cell nuclear antigen (PCNA)-positive melanin-containing cells in the basal epidermal layer of the UV-irradiated skin in GSE-fed guinea pigs. In contrast, these parameters did not change in the skin of vitamin C-fed or control guinea pigs. GSE inhibited the activity of mushroom tyrosinase and also inhibited melanogenesis without inhibiting the growth of cultured B16 mouse melanoma cells. In conclusion, we demonstrated that oral administration of GSE is effective in lightening the UV-induced pigmentation of guinea pig skin. This effect may be related to the inhibition of melanin synthesis by tyrosinase in melanocytes and the reactive oxygen species (ROS)-related proliferation of melanocytes.

Photoprotective properties of skin melanin

Melanins are a ubiquitous class of biological pigments, which play an important role in the photoprotection of skin. Recent advances in the chemistry of melanins have demonstrated their diversity. The various types of melanin show different physico-chemical properties, suggesting that their photobiological properties are not unique. In this review, the implications of melanin diversity for the natural photoprotection of skin are discussed.

Kojic acid reduces the cytotoxic effects of sulfur mustard on cultures containing human melanoma cells in vitro

In vivo experiments have shown that melanocytes are more sensitive than keratinocytes to the cytotoxic effects of sulfur mustard when it is applied topically to pig skin.1 It has been hypothesized that this is caused by the uncoupling of the melanogenic pathway by depletion of cellular glutathione, resulting in the uncontrolled production of cytotoxic quinone free-radical species by tyrosinase.2. In the present study, the feasibility of blocking the melanogenic pathway as a means of reducing the cytotoxicity of sulfur mustard was evaluated using kojic acid. Kojic acid is a topically applied depigmenting agent that exerts its effect by acting as a slow-binding, competitive inhibitor of tyrosinase.3 Preincubation of G361 pigmented melanoma cells and mixed cultures of G361 cells and SVK keratinocytes with 2.5 mM kojic acid resulted in significant increases in the viability of these cultures as determined by neutral red (NR) and gentian violet (GV) dye binding assays for up to 48 h following exposure to 50 microM sulfur mustard. The highest levels of protection were seen in the G361 cultures, with a 26.8% increase in culture viability (NR assay) compared with the sulfur-mustard-only controls at 24 h. Preincubation of SVK cells alone with kojic acid resulted in lower increases in viability (2.5% at 24 h by the NR assay). Inhibition of the melanogenic pathway reduces the sensitivity of cultures containing pigment cells to sulfur mustard.

The role of melanin in the induction of oxidative DNA base damage by ultraviolet A irradiation of DNA or melanoma cells

Highly pigmented, dark skin is more resistant to the harmful effects of solar ultraviolet radiation than light-colored human skin. The extent to which tanning protects skin from harmful effects including induction of skin cancer is not known, however. We have investigated whether the skin pigment, melanin, sensitizes or protects isolated DNA or nuclear DNA in melanoma cells from the induction of the premutagenic oxidative DNA base damage, 8-hydroxy-deoxyguanosine, by ultraviolet A irradiation. Synthetic eumelanin sensitized isolated DNA to induction of the oxidative DNA base damage by ultraviolet A, but it also induced the oxidative DNA base damage in the dark. To study the role of natural melanin in mammalian melanoma cells in the induction of oxidative DNA base damage, melanin synthesis was modulated 5-7-fold in the human melanoma cells GLL19 and IGR1 (which contain both pheomelanin and eumelanin) as well as in the mouse melanoma cells B16 (which contain mainly eumelanin). Increased melanin synthesis clearly did not protect against ultraviolet A-induced oxidative DNA base damage in cells. On the contrary, the human melanoma cells with high melanin content accumulated two times more 8-hydroxy-deoxyguanosine after ultraviolet A irradiation than cells with low melanin content. Furthermore, preirradiation of the human melanoma cells, IGR1, with ultraviolet A 4 h before a second ultraviolet A exposure produced an altered amount of induced 8-hydroxy-deoxyguanosine dependent on the melanin content of the cells. We conclude that stimulation of melanin synthesis, but probably not melanin itself, increases the susceptibility of human melanoma cells to induction of premutagenic oxidative DNA base damage by ultraviolet A irradiation.

Potentials and limitations of the natural antioxidants RRR-alpha-tocopherol, L-ascorbic acid and beta-carotene in cutaneous photoprotection

Sun exposure has been linked to several types of skin damage including sun burn, photoimmunosuppression, photoaging and photocarcinogenesis. In view of the increasing awareness of the potentially detrimental long term side effects of chronic solar irradiation there is a general need for safe and effective photoprotectants. One likely hypothesis for the genesis of skin pathologies due to solar radiation is the increased formation of reactive oxidants and impairment of the cutaneous antioxidant system. Consequently, oral antioxidants that scavenge reactive oxidants and modulate the cellular redox status may be useful; systemic photoprotection overcomes some of the problems associated with the topical use of sunscreens. Preclinical studies amply illustrate the photoprotective properties of supplemented antioxidants, particularly RRR-alpha-tocopherol, L-ascorbate and beta-carotene. However, clinical evidence that these antioxidants prevent, retard or slow down solar skin damage is not yet convincing. The purpose of this review is to provide the reader with current information on cutaneous pathophysiology of photoxidative stress, to review the literature on antioxidant photoprotection and to discuss the caveats of the photo-oxidative stress hypothesis.

Nitric oxide-induced oxidation of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid under aerobic conditions: non-enzymatic route to melanin pigments of potential relevance to skin (photo)protection

Diffusible melanin-related metabolites have recently been suggested to subserve a variety of functions that are critical for protection of skin against inflammatory stimuli and oxidative tissue injury. We report here the results of in vitro studies showing that 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) exhibit a marked reactivity toward potentially cytotoxic nitrogen oxides produced by autoxidation of nitric oxide (NO) under physiologically relevant conditions. Exposure of DHI or DHICA to NO in air-equilibrated 0.1 M phosphate buffer, pH 7.4, resulted in a fast, concentration-dependent consumption of the substrates and the concomitant deposition of dark melanin-like pigments. All NO-induced oxidations were completely inhibited in the absence of oxygen. Addition of 10 microM DHI and DHICA completely prevented the oxidation of 10 microM alpha-tocopherol in 0.1 M phosphate buffer, pH 7.4 in the presence of 300 microM NO. Overall, these results shed light on novel oxidative pathways of melanin-related metabolites of possible relevance to the mechanisms of skin hyperpigmentation under oxidative stress conditions.

Diffusible melanin-related metabolites are potent inhibitors of lipid peroxidation

Although it has long been known that epidermal melanocytes produce and excrete a number of melanin-related metabolites, including 5.6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), and 5-S-cysteinyldopa (CD), the possible functional significance of these compounds has been so far largely overlooked. We report now evidence that DHI, DHICA and CD exert potent inhibitory effects in different in vitro models of lipid peroxidation. The compounds, at 100 microM concentration, substantially decreased malondialdehyde (MDA) formation by lipid peroxidation in rat brain cortex homogenates. At 1.2 microM concentration, DHI proved as effective as alpha-tocopherol (alpha-T), one of the most potent endogenous antioxidants, in suppressing azo-induced peroxidation of linoleic acid in phosphate buffer (pH 7.4), containing 0.10 M SDS, whereas CD and DHICA at the same concentration were less active. DHI, CD and DHICA (all in the range 25 microM-0.5 mM) were also found to inhibit Fe (II)/EDTA-induced oxidation of 0.5 mM arachidonic acid at pH 7.4, as well as MDA formation by iron-promoted degradation of 0.5 mM 15-hydroperoxy-5,8,11, 13-eicosatetraenoic acid (15-HPETE). In both cases the inhibitory effects were much greater than those of ascorbic acid and glutathione. These results point to melanin precursors as a novel class of biological antioxidants which may contribute to defense mechanisms against oxidative injury in human skin.

5-S-cysteinyldopa, a diffusible product of melanocyte activity, is an efficient inhibitor of hydroxylation/oxidation reactions induced by the Fenton system

Interest in 5-S-cysteinyldopa (5-S-CD), a major excretion product of normal and malignant melanocytes, has traditionally concentrated on its significance as a biosynthetic precursor of pheomelanins, the characteristic pigments of red hair, and as a specific biochemical marker for monitoring melanoma progression. The present study shows that 5-S-CD is a potent inhibitor of hydroxylation/oxidation reactions mediated by hydrogen peroxide and the Fe2+/EDTA complex under both aerobic and anaerobic conditions. The inhibitory effect of 5-S-CD, as determined by the deoxyribose and salicylic acid assays in phosphate buffer (pH 7.4), is much stronger than that of dopa, acetylsalicylic acid and mannitol, increases with increasing ligand-to-metal ratio, and is inversely proportional to the concentration of EDTA present in the Fenton system. Spectrophotometric evidence and competition experiments indicate that 5-S-CD forms a chelate complex with ferric ions (lambda max = 500 nm at pH 7.4), which may account for both an altered production of hydroxyl radicals by the Fenton reagent and a site-specific localization of oxidative damage on the chelate complex itself.

Dual role of melanins and melanin precursors as photoprotective and phototoxic agents: inhibition of ultraviolet radiation-induced lipid peroxidation

Ultraviolet radiation (UVR) is one of the risk factors for skin cancer and the main inducer of melanin pigmentation, the major protective mechanism of mammalian skin against radiation damage. The melanin pigments, eumelanin and pheomelanin, are likely to be important in protection against UVR, but their precursors are generally considered as phototoxic. The available data suggest DNA damage as the mechanism of phototoxicity. However, the effect of melanin precursors on membrane damage through lipid peroxidation, another important and probably more relevant (from the point-of-view of the melanosomal confinement of these molecules) mechanism of phototoxicity, not known. As a model system for UVR-melanin-membrane interactions, we irradiated liposomes in the presence of eumelanin, pheomelanin and two of their major precursors, 5,6-dihydroxyindole (DHI) and 5-S-cysteinyldopa (SCD). The presence of the two melanin precursors substantially reduced the formation of lipid peroxidation products resulting from UVR exposure. The antioxidant activity of the melanin precursors was diminished under strong prooxidant conditions (presence of Fe3+). These results suggest that melanin precursors may have an important role in the protection of skin against the harmful effects of UVR including photocarcinogenesis.

Photochemistry of 5-S-cysteinyldopa

The photochemical behavior of 5-S-cysteinyldopa (5-S-CD), a colorless product of melanocyte metabolism, was investigated in neutral phosphate buffer with biologically relevant UV radiation. Exposure of 5-S-CD to pyrex-filtered UV light (wavelengths > 320 nm) was found to induce an oxygen-dependent reaction, leading to, besides abundant polymeric materials, the benzothiazine derivatives I and II (two diastereoisomers). Superoxide dismutase exerted a small inhibitory effect on 5-S-CD consumption, whereas other active oxygen scavengers had no effect on the reaction course. Addition of glutathione as a hydrogen donor completely suppressed the reaction. With UVB light (wavelength range 280-320 nm) photolysis of 5-S-CD proceeded mainly with formation of 3,4-dihydroxyphenylalanine, arising presumably by photohomolytic cleavage of the S-CH2 bond followed by desulfuration. These results are of interest in relation to the high susceptibility of fair-complexioned individuals to actinic damage and skin cancer.

Melanized poly(HEMA) hydrogels: basic research and potential use

Synthetic melanogenesis, using epinephrine and other melanin precursors, within the matrix of hydrophilic polymers and copolymers of 2-hydroxyethyl methacrylate resulted in hydrogels able to absorb ultraviolet and visible radiation. This significantly enhances their value as materials for extraocular (contact lenses) or intraocular (artificial crystalline lenses) devices that should protect the retina of aphakic patients from potential damage induced by light. The two-phase morphology of melanized hydrogels, as investigated by TEM, revealed a fine structure that is possibly indicative of a true sequential interpenetrating polymer network. Their biocompatibility was evaluated by a set of different assays involving human choroidal fibroblasts. No cytotoxicity was found in the aqueous extracts of materials. By using an assay with cells and polymers embedded in a collagen gel, a short-range toxic effect was detected, presumably caused by melanin itself. However, in vivo experiments in animal eyes with melanized hydrogel intraocular lenses did not reveal any toxic reaction.

Biochemical and toxicological properties of the oxidation products of catecholamines

The normal catabolism of catecholamines proceeds through enzymatic pathways (monoaminooxidase, catechol-o-methyltranserase, and phenolsulphotransferase). In addition, nonenzymatic oxidative pathways might take place since catechols are readily oxidized. In this review article, the pathways of formation of the oxidation products of catecholamines and their reactions are described. The interactions of these products with different biological systems and their toxicity are examined. Among the reactions known to occur is that with sulfhydryls, which results in either a covalently linked adduct or disulfide production. Another interesting pathway to toxicity involves the oxidation of these catecholamine products by oxygen, with the formation of damaging oxygen-derived species. The action of the oxidation products of catecholamines is outlined, with special attention to the nervous and cardiac systems.

Circadian rhythm of white blood cell aggregation and free radical status in healthy volunteers

Previous studies have demonstrated circadian rhythms in the onset of thrombotic events, which occur most commonly in the morning, and also in the fibrinolytic activity of the blood which has a peak in the evening and a trough in the morning. There has recently been increasing interest in the role of white blood cells (WBCs) and free radicals (FRs) in thrombosis. No one has yet investigated the potential circadian variation of WBC aggregation and FRs in humans. We studied the circadian rhythm of WBC aggregation and FR status in 10 healthy male volunteers. Six blood samples were collected at 4 hourly intervals from 12:00 (mid-day) until 08:00 the following morning. The volunteers carried out normal daily activities until 00:00 at which time they went to bed and they remained in bed until 08:00. The following were measured on each sample: WBC aggregation; thiobarbituric acid reactive substances (TBARs), plasma thiols (PSH), red cell lysate thiols (LSH), glutathione (GSH) and superoxide dismutase (SOD) which are all altered in the presence of FR activity. The following parameters demonstrated significant circadian rhythms, WBC aggregation p less than 0.001, TBARs p less than 0.015, PSH p less than 0.001, LSH p less than 0.002. WBC aggregation was lowest at 09:00 and highest at 00:00-04:00. TBARs and PSH both had a peak at 16:00 and a trough at 04:00. LSH had a peak at 12:00 and a trough at 08:00. As the behaviour of WBCs and FR status influence the flow properties of blood, a circadian rhythm in WBC function and FR status may contribute to the time of onset of thrombotic diseases. Moreover, with many studies being currently undertaken in this area, our work indicates the need to standardize sample times.

In vitro cytotoxicity of melanized poly(2-hydroxyethyl methacrylate) hydrogels, a novel class of ocular biomaterials

Due to their ability to absorb ultraviolet and visible radiation, we have proposed the melanized poly(2-hydroxyethyl methacrylate) hydrogels as biomaterials suitable for the manufacture of soft artificial intraocular lenses. Their biocompatibility has not been evaluated so far. In this study, poly(2-hydroxyethyl methacrylate) containing various amounts of adrenochrome-melanin were synthesized and the cytotoxicity of their aqueous extracts was assessed by using four in vitro testing techniques (trypan blue dye exclusion, inhibition of DNA synthesis, lactate dehydrogenase release, and inhibition of cell growth). Assays were based on incubation with human choroidal fibroblasts. By the first three techniques, no cytotoxicity was found in the extracts. The inhibition of cell growth test showed a slight cellular protein loss, however only in the extracts of polymers with high melanin content. The results suggest that the release of potentially toxic agents from melanized hydrogels into an aqueous medium is not significant. However, when an assay in collagen gel was carried out in the presence of specimens of melanized hydrogels, a toxic reaction was clearly revealed. This can be caused by a delayed release of toxic molecules from melanin, or by some other mechanism. The use of melanin-containing polymers as implant materials becomes questionable and further research is necessary.

Photoprotection by melanin

This paper is an attempt to summarize the current state of information on melanin and epidermal melanin pigmentation (EMP) as photoprotective agents. The chemistry and biochemistry of melanin (the particle) and its interaction, in its various forms, with UV radiation are considered. Methods of attenuation of UV radiation are discussed in terms of structure and chemical constituents. Photoprotection by constitutive and facultative pigmentation is reviewed with minimum erythema dose (MED) as the end point. The issue of acclimatization to UV radiation is discussed in terms of UVB phototherapy for psoriasis. Finally, skin cancer is considered as an end point and the reduction of its incidence with pigment level is discussed. It is concluded that whilst EMP provides protection, its extent depends on the end point chosen for evaluation. MED is a convenient photobiological end point but is rather insensitive, whereas skin cancer is sensitive but impractical for laboratory studies. Our current state of knowledge of melanin lacks information on its absorption and scattering coefficients and its refractive index. Methods for the quantitative measurement of EMP are also urgently required.

Separation of pigmented and albino melanocytes and the concomitant evaluation of endogenous peroxide content using flow cytometry

Flow cytometry (FCM) has been used extensively to analyze various biological properties of the cell. In this report, we describe a method by which FCM was used to determine the light scattering profile of a mixed population of pigmented and non-pigmented melanocytes, plus its subsequent use for the sorting and separation of the two cell types. In addition, the relative peroxide content in pigmented and non-pigmented melanocytes was compared by flow cytometry. Cultured avian melanocytes from a pigmented control and from three genetically distinct albino sources were studied. FCM analysis of forward versus side light scatter within a mixed suspension of pigmented and amelanotic melanocytes distinguished two overlapping populations of cells. Sorting of these two populations demonstrated that the population exhibiting much side and minimal forward light scatter was primarily pigmented melanocytes, while conversely the population exhibiting less side and more forward scatter was principally non-pigmented cells. These two melanocyte types also demonstrated differences in levels of endogenous peroxides. The intracellular content of peroxide in the two subpopulations of cells was measured utilizing the nonfluorescent compound, 2',7'-dichlorofluorescein diacetate (DCFH-DA), which within the cell is oxidized by intracellular peroxides to a fluorescent dichlorofluorescein (DCF). Non-pigmented albino melanocytes had the highest quantity of endogenous peroxides, while heavily pigmented cells had considerably less peroxide-related fluorescence. The amount of this DCF fluorescence could be enhanced by increasing concentrations of DCF used in the assay. These flow cytometric methods are useful for isolating and culturing subpopulations of melanocytes expressing various pigment levels and to investigate the relationship between melanin and its precursors with hydrogen and lipid peroxides in melanocytes.

Toxicity of aminochromes

The first part of the present review deals with the chemical and enzymatic synthesis of adrenochrome and other aminochromes from the corresponding catecholamines. A description of the most significant pathways of formation and the reactivity of the aminochromes is presented. In the second part of the toxicity of aminochromes, mainly at the cardiac and CNS level, is described and some of the molecular mechanisms of the toxic action are outlined. The toxicity of the aminochromes appears to depend mainly on the production of reduced oxygen species through redox cycling. The interaction of aminochromes with sulfhydryl groups and the induced depletion of oxygen, ascorbate and glutathione are additional mechanisms resulting in noxious effects at a cellular level.