Tyrosinase isozyme heterogeneity in differentiating B16/C3 melanoma

The Glutathione Derivative, GSH Monoethyl Ester, May Effectively Whiten Skin but GSH Does Not

Glutathione in its reduced form (GSH) is an antioxidant and also is involved in pheomelanin formation. Thus, it has been long believed that GSH has a skin whitening effect. However, its actual or direct effect is unproven. We evaluated the anti-melanogenic effects of GSH and its derivatives in vitro. We examined change of melanogenesis and its related proteins by GSH itself and its derivatives, including GSH monoethyl ester (GSH-MEE), GSH diethyl ester (GSH-DEE) and GSH monoisopropyl ester (GSH-MIPE) in Melan-A cells, Mel-Ab cells, and B16F10 cells. GSH and GSH-MEE did not display cytotoxic activity, but GSH-MIPE and GSH-DEE did. Intriguingly, GSH itself had no inhibitory effect on melanin production or intracellular tyrosinase activity. Rather, it was GSH-MEE and GSH-MIPE that profoundly reduced the amount of melanin and intracellular tyrosinase activity. Thus, GSH-MEE was selected as a suitable candidate skin-whitening agent and it did not alter melanogenesis-associated proteins such as microphthalmia-associated transcription factor (MITF), tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2, but it did increase the amount of suggested pheomelanin and suggested pheomelanin/eumelanin ratio. GSH-MEE was effective for anti-melanogenesis, whereas GSH itself was not. GSH-MEE could be developed as a safe and efficient agent for the treatment of hyperpigmentation skin disorders.

Microbial tyrosinases: promising enzymes for pharmaceutical, food bioprocessing, and environmental industry

Tyrosinase is a natural enzyme and is often purified to only a low degree and it is involved in a variety of functions which mainly catalyse the o-hydroxylation of monophenols into their corresponding o-diphenols and the oxidation of o-diphenols to o-quinones using molecular oxygen, which then polymerizes to form brown or black pigments. The synthesis of o-diphenols is a potentially valuable catalytic ability and thus tyrosinase has attracted a lot of attention with respect to industrial applications. In environmental technology it is used for the detoxification of phenol-containing wastewaters and contaminated soils, as biosensors for phenol monitoring, and for the production of L-DOPA in pharmaceutical industries, and is also used in cosmetic and food industries as important catalytic enzyme. Melanin pigment synthesized by tyrosinase has found applications for protection against radiation cation exchangers, drug carriers, antioxidants, antiviral agents, or immunogen. The recombinant V. spinosum tryosinase protein can be used to produce tailor-made melanin and other polyphenolic materials using various phenols and catechols as starting materials. This review compiles the recent data on biochemical and molecular properties of microbial tyrosinases, underlining their importance in the industrial use of these enzymes. After that, their most promising applications in pharmaceutical, food processing, and environmental fields are presented.

Laccase-like activity in the hemolymph of Venerupis philippinarum: characterization and kinetic properties

The phenoloxidases (POs) include tyrosinases (EC 1.14.18.1), catecholases (EC 1.10.3.1) and laccases (EC 1.10.3.2) and are known to play a role in the immune defences of many invertebrates. For the Manila clam, Venerupis philippinarum, the exact role is not known, especially with regard to defences against Brown Ring Disease (BRD), which leads to high mortalities along European coasts. In order to understand the role and functioning of PO in V. philippinarum, the first step, and aim of this study, was to biochemically characterize the PO activity in the circulating hemolymph. Various substrates were tested and the common PO substrates L-DOPA, Catechol and dopamine exhibited good affinity with the enzyme and consequent low K(m) values (3.75, 1.97, 4.91 mM, respectively). A single tyrosinase-specific substrate, PHPPA, was oxidized, but the affinity for it was low (K(m) = 47.33 mM). Three tested laccase-specific substrates were oxidized by V. philippinarum PO (PPD, OPD and hydroquinone) and affinity was higher than for PHPPA. The results obtained with the substrate were confirmed by the use of different inhibitors: CTAB, a laccase-specific inhibitor inhibited PO activity greatly but not completely, whereas 4-Hr, specific to catecholases and tyrosinases, inhibited PO activity to a lesser extent. The results lead us to conclude that V. philippinarum PO activity in the circulating hemolymph, is mainly a laccase-like activity but there is also a smaller-scale tyrosinase-like activity. The inhibition mechanisms were also determined using dose-response substrate concentration for an inhibitor concentration equal or close to the IC50. Optimal conditions for the enzyme activity were also determined using L-DOPA as substrate, showing that its optimal temperature and pH are around 40 °C and 8.4 respectively. The enzyme is denatured for temperatures above 50 °C.

Fermented Viola mandshurica inhibits melanogenesis in B16 melanoma cells

We assessed the effects of chloroform extract of fermented Viola mandshurica (CEFV) on melanogenesis B16 melanoma cells. CEFV treatment significantly decreased melanin content and tyrosinase activity in dose-dependent manners. To elucidate the mechanism of the inhibitory effects of CEFV on melanogenesis, we performed RT-PCR and Western blotting for melanogenesis-related genes such as tyrosinase, tyrosinase-related protein-1 (TRP-1), TRP-2, and microphthalmia-associated transcription factor (MITF). CEFV strongly inhibited mRNA as well as the protein expression of tyrosinase and MITF, but had no significant effect on TRP-1 or TRP-2 expressions. It markedly decreased the phosphorylation of cAMP responsive element binding protein (CREB), and induced the duration of extracellular signal-regulated kinase (ERK) activation, leading to reduction of MITF expression and subsequently that of tyrosinase. Therefore, we suggest that CEFV induces downregulation of melanogenesis through decreased CREB phosphorylation and ERK activation.

Down-regulation of tyrosinase expression by acetylsalicylic acid in murine B16 melanoma

Acetylsalicylic acid (aspirin; ASA) is widely used as an analgesic/antipyretic drug. ASA exhibits a wide range of biological effects, including preventative effects against heart attack, stroke, and the development of some types of cancer. However, the effects of ASA on melanogenesis are not well known. Therefore, we investigated the effect of ASA on melanin production using B16 murine melanoma cells and demonstrated a new biological effect of ASA. In the presence of alpha-melanocyte stimulating hormone (alpha-MSH), B16 melanoma cells are stimulated to enhance melanin synthesis. ASA (2 mM) inhibited alpha-MSH-enhanced melanin synthesis in melanoma more strongly than other well-known anti-melanogenic agents such as arbutin (2 mM) and kojic acid (200 microM). Interestingly, ASA did not inhibit the catalytic activity of mushroom tyrosinase (concentration range 0.5-4.0 mM). To clarify the target of ASA action in melanogenesis, we performed Western blotting for tyrosinase, which is a key melanogenic enzyme. ASA inhibited tyrosinase expression in a dose-dependent manner. Therefore, the depigmenting effect of ASA might be due to inhibition of tyrosinase expression or enhancement of tyrosinase degradation. This study suggests that ASA is a candidate anti-melanogenic agent and it might be effective in hyperpigmentation disorders.

Depigmentation of Melanocytes by the Treatment of Extracts from Traditional Chinese Herbs: a Cell Culture Assay

OBJECTIVE

To obtain potential skin whitening agents from traditional Chinese herbs, we tested changes of melanin content in melanocyte cell lines after treatment with extracts of 90 traditional Chinese herbs.

METHODS

Mouse melanocyte cell lines were used. Depigmentation activity of the herb extracts were first screened in Mel-Ab cells, and then re-evaluated in melan-a cells and co-culture of melan-a and SP-1 cells. Melanin content and cell viability were the two indications for evaluation. Tyrosinase activity and the expression of melanin synthesis related enzymes in cells treated with the herb extracts were also tested.

RESULTS

Nine herb extracts were proved to have depigmentation activity similar to or better than that of arbutin and low cytotoxicity to melanocytes. Two of them were more effective in co-cultured melan-a cells. Most of the effective herb extracts inhibited tyrosinase activity and the expression of tyrosinase. Some of them also inhibited tyrosinase related protein-1 and/or tyrosinase related protein-2 in cultured cells.

CONCLUSIONS

We have found 9 herb extracts to be promising skin whitening agents. Among them, water extract of Galla Chinensis and ethanol extract of Radix Clematidis exhibited higher depigmentation activity and caused lower tyrosinase activity in cell culture assays and are worthy to be further studied.

Human placental lipid induces melanogenesis by increasing the expression of tyrosinase and its related proteins in vitro

Lipids, particularly sphingolipids, are emerging as novel regulators of cellular activity. A placental total lipid fraction (PTLF), the total lipid prepared from an hydroalcoholic extract of fresh term human placenta, was previously shown to have a pigment-inducing activity in an animal model. The PTLF contains sphingolipids which stimulate DNA synthesis and melanin formation with marked morphological changes in B16F10 melanoma cells. In order to identify the mechanism underlying the increased melanin synthesis, B16F10 cells were treated with PTLF to assess the catalytic activities of tyrosinase (i.e. tyrosine hydroxylase and DOPA oxidase), the key regulatory enzyme of melanin synthesis. Tyrosine hydroxylase (estimated by the release of (3)H(2)O) as well as DOPA oxidase (measured spectrophotometrically and also in non-denaturing gels), was stimulated significantly by PTLF. Western blot analysis demonstrated an increase in the expression of tyrosinase, tyrosinase related proteins 1 and 2 (TRP1 and TRP2) at the protein level and RT-PCR analysis revealed stimulated transcription of tyrosinase, TRP1 and TRP2 mRNAs in PTLF-treated B16F10 cells. Actinomycin D and cycloheximide, inhibitors of transcription and translation, respectively, inhibited PTLF-induction of tyrosinase activity with a corresponding decrease in melanogenesis. In all cases, the response to PTLF was similar to that induced by alpha-melanocyte stimulating hormone, a well-known stimulator of melanogenesis. Thus, these results provide the basis of action of PTLF stimulated melanogenesis in B16F10 cells showing that this placental extract is a strong inducer of pigmentation at the transcriptional and translational levels.

Quercetin enhances melanogenesis by increasing the activity and synthesis of tyrosinase in human melanoma cells and in normal human melanocytes

Quercetin (3,3',4',5,7-pentahydroxyflavone) is a diphenyl propanoid widely distributed in edible plants. In this study, we examined the effect of quercetin on melanogenesis in human HMVII melanoma cells and in normal human epidermal melanocytes (NHEM) in the absence of ultraviolet radiation. Upon the addition of quercetin to the culture medium, the melanin content in melanoma cells (HMVII) increased remarkably in time- and dose-dependent manners. In addition, quercetin induced melanogenesis in cultured NHEM. As compared with controls, melanin content was increased about sevenfold by treatment with 20 microM (HMVII) or 1 microM (NHEM) quercetin for 7 d. Tyrosinase activity was also increased, to 61.8-fold higher than the control. The expression of tyrosinase protein was slightly increased by the addition of quercetin. However, quercetin did not affect the expression of tyrosinase mRNA. Tyrosinase activation by quercetin was blocked by actinomycin-D or by cycloheximide demonstrating that its actions in stimulating melanogenesis may involve both transcriptional and translational events. Tyrosinase activity was increased dramatically whereas the level of melanogenic inhibitor was remarkably decreased following quercetin treatment. Taken together, these results demonstrate that in human melanoma cells and in NHEM, quercetin stimulates melanogenesis by increasing tyrosinase activity and decreasing other factors such as melanogenic inhibitors.

New Tyrosinase Inhibitors, (+)-Catechin−Aldehyde Polycondensates

In this study, new tyrosinase inhibitors, (+)-catechin-aldehyde polycondensates, have been developed. Tyrosinase is a copper-containing enzyme that catalyzes the hydroxylation of a monophenol (monophenolase activity) and the oxidation of an o-diphenol (diphenolase activity). In the measurement of tyrosinase inhibition activity, (+)-catechin acted as substrate and cofactor of tyrosinase. On the other hand, the polycondensates inhibited the tyrosine hydroxylation and L-DOPA oxidation by chelation to the active site of tyrosinase. The UV-visible spectrum of a mixture of tyrosinase and the polycondensate exhibited a characteristic shoulder peak ascribed to the chelation of the polycondensate to the active site of tyrosinase. Furthermore, circular dichroism measurement showed a small red shift of the band due to the interaction between tyrosinase and the polycondensate. These data support that the polycondensate acts as an inhibitor of tyrosinase.

Tyrosine hydroxylase isoenzyme I is present in human melanosomes: a possible novel function in pigmentation

Both human epidermal melanocytes and keratinocytes have the full capacity for de novo synthesis of 6(R) L-erythro 5,6,7,8, tetrahydrobiopterin, the essential cofactor for the rate limiting step in catecholamine synthesis, via tyrosine hydroxylase. Catecholamine synthesis has been demonstrated in proliferating keratinocytes of the epidermis in human skin. This study presented herein identified for the first time the expression of tyrosine hydroxylase isozyme I mRNA within the melanocyte. The location of the enzyme was demonstrated in both the cytosol and melanosomes of human epidermal melanocytes, using immunohistochemistry and immunofluorescence double staining as well as immunogold electron microscopy. High-performance liquid chromatography (HPLC) analysis of pure melanosomal extracts from the human melanoma cell line, FM94, confirmed the production of L-dopa within these organelles. In addition, enzyme activities for both tyrosine hydroxylase and tyrosinase were measured in the same preparations, by following the catalytic release of tritiated water from L-[3,5-3H]tyrosine. The melanosomal membrane location of tyrosine hydroxylase together with tyrosinase implies a coupled interaction, where L-dopa production facilitates the activation of tyrosinase. Our results support a direct function for tyrosine hydroxylase in the melanosome via a concerted action with tyrosinase to promote pigmentation.

Progressive appearance of pigmentation in amelanotic melanoma lesions

We previously described a transgenic mouse line (TG-3) that spontaneously develops pigmented cutaneous melanoma. The generation of several albino mice that developed amelanotic melanoma has also been reported. In this report, we describe an unanticipated result with crosses between C57BL/6-c2j and TG-3 mice. C57BL/6-c2j has the same genetic background as TG-3 (C57BL/6), except for a single base mutation (nucleotide 291) in the tyrosinase locus, resulting in albino coat colour. Only albino F2 mice generated from (TG-3 x C57BL/6-c2j) F1s were selected for further studies. Mice that contained the transgene showed a very high incidence of tumor development as early as 4-6 weeks of age. Raised amelanotic tumors developed on the ear pinnae and perianal region in young F2 albino mice, similar phenotypes as those described earlier for the other albino inbred strains. However, with time, these amelanotic tumors not only increased in size, but unexpectedly developed foci of dark pigmentation. DNA sequence analysis on reverse transcriptase-polymerase chain reactions (RT-PCRs) of tyrosinase mRNA showed that the original tyrosinase mutation was still present in the tumors, indicating that no reversion at this nucleotide had occurred in the tumors. Two different tyrosinase activity assays were used and tyrosinase activity was detected in most tumor samples. Furthermore, Western blot analysis demonstrated various levels of tyrosinase protein in ear tumor samples. These results suggest that tyrosinase and/or melanin are not directly involved in the establishment of melanoma, but that late events occurring within the tumors may generate some tyrosinase activity and production of melanin.

Cats require more dietary phenylalanine or tyrosine for melanin deposition in hair than for maximal growth

In 1986, the NRC recommended a dietary concentration of 4.0 g/kg of phenylalanine and 8.5 g/kg of total aromatic amino acids for growing kittens on the basis of maximal growth rate and nitrogen balance. Black hair-coated cats were given purified diets containing the following phenylalanine + tyrosine (Phe + Tyr) concentrations (g/kg diet): 4 + 2, 4 + 4, 4 + 6, 4 + 8, 10 + 0, 10 + 2, 10 + 4, 10 + 6, 10 + 8, 10 + 10, 24 + 0 for at least 6 mo. All other amino acids were present at about twice the requirements. Total melanin and the oxidation product of eumelanin, pyrrole-2,3,5-tricarboxylic acid (PTCA) were measured in hair. There was a significant linear relationship between the concentrations of tyrosine in plasma and PTCA in hair. The relationship between PTCA concentration in hair and Phe + Tyr in the diet had a point of inflection at approximately 16 g/kg Phe + Tyr. Cats fed diets with <16 g Phe + Tyr developed "red hair." We confirmed the anecdotal reports that the black hair of cats can change from black to reddish brown. An aromatic amino acid concentration > or =18 g/kg is recommended for the prevention of visually discernible red hair in black-coated cats. Dietary concentrations >18 g total aromatic amino acids/kg diet promote a greater ratio of PTCA:total melanin in hair. We are unaware of a secondary nutrient requirement being so much greater than the requirement for growth.

Aberrant pH of melanosomes in pink-eyed dilution (p) mutant melanocytes

In past studies, we cloned the mouse p gene and its human homolog P, which is associated with oculocutaneous albinism type 2. Both mouse and human genes are expressed in melanocytes and encode proteins predicted to have 12 membrane-spanning domains with structural homology to known ion transporters. We have also demonstrated that the p protein is localized to the melanosomal membrane and does not function as a tyrosine transporter. In this study, immunohistochemistry and confocal microscopy were used to show that the p protein plays an important role in the generation or maintenance of melanosomal pH. Melanosomes (and their precursor compartments) were defined by antiserum directed against the melanosomal marker tyrosinase related protein 1. Acidic vesicles were identified by 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine incorporation, visualized with anti-dinitrophenol. In C57BL/6+/+ (wild-type) melanocytes, 94.2% of vesicles demonstrated colocalization of tyrosinase related protein 1 and 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine, indicating that almost all melanosomes or their precursors were acidic. By contrast, only 7%-8% of the staining vesicles in p mutant cell lines (pJ/pJ and pcp/p6H) showed colocalization of tyrosinase related protein 1 and 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine. Thus, without a functional p protein, most melanosomes and their precursors are not acidic. As mammalian tyrosinase activity in situ is apparently dependent on low pH, we postulate that in the absence of a low pH environment brought about by ionic transport mediated by the p protein, tyrosinase activity is severely impaired, leading to the minimal production of melanin that is characteristic of p mutants. Additionally (or alternatively), an abnormal pH may also impair the assembly of the normal melanogenic complex.

Inhibition of tyrosinase reduces cell viability in catecholaminergic neuronal cells

The biosynthesis of dopamine (DA) in catecholaminergic neurons is regulated by tyrosine hydroxylase, which converts tyrosine into 3, 4-dihydroxyphenylalanine (L-DOPA). In melanocytes, tyrosinase catalyzes both the hydroxylation of tyrosine and the consequent oxidation of L-DOPA to form melanin. Although it has been demonstrated that tyrosinase is also expressed in the brain, the physiological role of tyrosinase in the brain is still obscure. In this study, to investigate the role of tyrosinase in catecholaminergic neuronal cells, we examined the effects of tyrosinase inhibition on the viability of CATH.a and SH-SY5Y cells using tyrosinase inhibitors-specifically, phenylthiourea (PTU) and 5-hydroxyindole (5-HI)-and the transfection of antisense tyrosinase cDNA. Both inhibitors significantly reduced the cell viability of CATH.a cells in a dose-dependent manner. PTU also specifically enhanced DA-induced cell death, but 5-HI did not. This discrepancy in cell death is probably due to the inhibitors' different mechanism of action: 5-HI inhibits the hydroxylation of tyrosine as a competitor for the substrate to induce cell death that may be due to depletion of DA, whereas PTU mainly inhibits the enzymatic oxidation of L-DOPA and DA rather than tyrosine hydroxylation to increase consequently autooxidation of DA. Indeed, the intracellular DA content in CATH.a cells was enhanced by PTU exposure. In contrast, PTU showed no enhancing effects on DA-induced cell death of SH-SY5Y cells, which express little tyrosinase. Furthermore, transfection with antisense tyrosinase cDNA into CATH.a cells dramatically reduced cell viability and significantly enhanced DA-induced cell death. These results suggest that tyrosinase controls the intracellular DA content by biosynthesis or enzymatic oxidation of DA, and the dysfunction of this activity induces cell death by elevation of intracellular DA level and consequent gradual autooxidation of DA to generate reactive oxygen species.

Expression and characterization of human tyrosinase from a bacterial expression system

To carry out biochemical characterizations of human tyrosinase and to provide an unlimited source of the enzyme for further study, an expression plasmid, pHis-Tyrosinase, which contains the entire coding sequence except the signal sequence of a human tyrosinase was constructed and expressed in Escherichia coli. The expressed enzyme was simply purified by an immobilized metal affinity chromatography. The recombinant enzyme had the same electrophoretic mobility as the native enzyme from human melanoma cell and cross-reacted with the polyclonal antibody raised against the native enzyme. The recombinant enzyme retained its catalytic function with both hydroxylating and oxidative activities. Km values for L-tyrosine and L-3,4-dihydroxy-phenylalanine of the recombinant enzyme were 0.17 and 0.36 mM, respectively. The activity of the recombinant enzyme was optimal at pH 7.5. Glutathione notably inhibited the enzymatic activity. This work is a further enzymatic characterization of human tyrosinase.

The depigmenting effect of alpha-tocopheryl ferulate on human melanoma cells

Oral vitamin E (alpha-tocopherol, alpha-T) supplementation has been reported to improve facial hyperpigmentation. alpha-Tocopheryl ferulate (alpha-TF) is a compound of alpha-T and ferulic acid connected by an ester bond; ferulic acid is also an antioxidant, and could scavenge free radicals induced by ultraviolet (UV) radiation, and thus maintain the long-lasting antioxidative effect of alpha-T. Our aim was to see whether alpha-TF might be useful as a whitening agent and an antioxidant to improve and prevent facial hyperpigmentation following UV exposure. In this study, the inhibitory effect of alpha-TF on melanogenesis was examined biochemically using human melanoma cells in culture. The results show that alpha-TF, solubilized in ethanol or in 0.5% lecithin, inhibited melanization significantly, as did alpha-T at a concentration of 100 microg/mL, without inhibiting cell growth. This phenotypic change was associated with inhibition of tyrosinase and 5, 6-dihydroxyindole-2-carboxylic acid polymerase activities, and the degree of inhibition was dose dependent. No significant effect on DOPAchrome tautomerase activity was observed. alpha-TF did not directly inhibit tyrosinase activity of the large granule fraction extracted from human melanoma cells, and Western blotting revealed that there were no changes in protein content or in molecular size of tyrosinase, tyrosinase-related protein (TRP)-1 or TRP-2. Therefore, the inhibition of tyrosinase activity by alpha-TF might be due to effects at the post-translational level, and possibly by a secondary molecule activated by alpha-TF. These results suggest that alpha-TF is a candidate for an efficient whitening agent which suppresses melanogenesis and inhibits biological reactions induced by reactive oxygen species.

Inhibition of N-glycan processing in B16 melanoma cells results in inactivation of tyrosinase but does not prevent its transport to the melanosome

Tyrosinase is the key enzyme in melanin biosynthesis, catalyzing multiple steps in this pathway. The mature glycoprotein is transported from the Golgi to the melanosome where melanin biosynthesis occurs. In this study, we have investigated the effects of inhibitors of N-glycan processing on the synthesis, transport, and catalytic activity of tyrosinase. When B16 mouse melanoma cells were cultured in the presence of N-butyldeoxynojirimycin, an inhibitor of the endoplasmic reticulum-processing enzymes alpha-glucosidases I and II, the enzyme was synthesized and transported to the melanosome but almost completely lacked catalytic activity. The cells contained only 2% of the melanin found in untreated cells. Structural analysis of the N-glycans from N-butyldeoxynojirimycin-treated B16 cells demonstrated that three oligosaccharide structures (Glc3Man7-9) predominated. Removal of the glucose residues with alpha-glucosidases I and II failed to restore enzymatic activity, suggesting that the glucosylated N-glycans do not sterically interfere with the enzyme's active sites. The mannosidase inhibitor deoxymannojirimycin had no effect on catalytic activity suggesting that the retention of glucosylated N-glycans results in the inactivation of this enzyme. The retention of glucosylated N-glycans does not therefore result in misfolding and degradation of the glycoprotein, as the enzyme is transported to the melanosome, but may cause conformational changes in its catalytic domains.

Glycosylation: heterogeneity and the 3D structure of proteins

Glycoproteins generally exist as populations of glycosylated variants (glycoforms) of a single polypeptide. Although the same glycosylation machinery is available to all proteins that enter the secretory pathway in a given cell, most glycoproteins emerge with characteristic glycosylation patterns and heterogeneous populations of glycans at each glycosylation site. The factors that control the composition of the glycoform populations and the role that heterogeneity plays in the function of glycoproteins are important questions for glycobiology. A full understanding of the implications of glycosylation for the structure and function of a protein can only be reached when a glycoprotein is viewed as a single entity. Individual glycoproteins, by virtue of their unique structures, can selectively control their own glycosylation by modulating interactions with the glycosylating enzymes in the cell. Examples include protein-specific glycosylation within the immunoglobulins and immunoglobulin superfamily and site-specific processing in ribonuclease, Thy-1, IgG, tissue plasminogen activator, and influenza A hemagglutinin. General roles for the range of sugars on glycoproteins such as the leukocyte antigens include orientating the molecules on the cell surface. A major role for specific sugars is in recognition by lectins, including chaperones involved in protein folding. In addition, the recognition of identical motifs in different glycans allows a heterogeneous population of glycoforms to participate in specific biological interactions.

Spontaneous melanocytosis in transgenic mice

Clone B is a 2-kb fragment of cloned genomic DNA involved in adipocyte differentiation in vitro. Insertion of this DNA fragment into the genome of a variety of cell lines results in committing the recipient cells to undergo adipocyte differentiation. Construction of transgenic mice with Clone B DNA resulted in an unexpected phenotype--spontaneous melanocytosis. The present study describes the distribution and morphology of melanin-containing lesions in these transgenic mice. Spontaneous melanin-containing dermal lesions appeared on the ears, snout, and perianal regions of transgenic mice by the age of 3-4 months. Multifocal dermal masses rapidly developed into raised lesions, which appeared to spread to adjacent skin. Ultrastructural examination of lymph nodes, spleen, and dermal lesions of these mice revealed membrane-bound melanin with effacement f the organelle structure of severely affected cells. Protein gel electrophoresis revealed elevated activity of tyrosinase in the pinnae, skin, perianal mass, and lymph nodes. This line of transgenic mice may provide a useful model for investigation of the etiology and progression of benign and malignant melanin-containing tumors.

Dialkyldithiocarbamates inhibit tyrosine hydroxylase activity in PC12 cells and in fibroblasts that express tyrosine hydroxylase

Dithiocarbamates and CS2 have been associated with neurobehavioural changes suggestive of central dopaminergic dysfunction. Diethyldithiocarbamate (DEDC), dimethyldithiocarbamate (DMDC), and methyldithiocarbamate (MDC) were examined for their ability to inhibit tyrosine hydroxylase (TH) activity in PC12 cells and transfected CHO fibroblasts that expressed TH (CHO/TH) activity when tetrahydrobiopterin (BH4) was added to medium. DEDC or DMDC did not significantly alter viability of PC12 cells or CHO/TH cells at < or = 100 microM for 18 h; the EC50 for each compound was approximately 5 mM in both cell lines. In contrast, the EC50 for MDC was 41 or 74 microM in PC12 or CHO/TH cultures, respectively. There was no change in immunodetectable levels of TH in PC12 or CHO/TH cells following exposure to subcytotoxic concentrations of dithiocarbamates. DEDC and DMDC (5 to 100 microM) produced concentration-dependent reductions in PC12 cell dopamine and dopac levels as well as in dopa levels in CHO/TH cultures. Reduction of PC12 catechols was not due to altered vesicular storage. In vitro PC12 TH activity was 80.2 +/- 3.4% or 82.4 +/- 2.9% of control following exposure to 100 microM DEDC or DMDC, respectively, and was not fully restored by incubation with Fe2+. These results show that DEDC and DMDC, but not MDC, are low potency cytotoxins that decrease TH activity in cultured cells through mechanisms other than inhibition of BH4 biosynthesis or iron chelation.

New thioureas and related substances intended for melanoma targeting

Thiouracil and a few related drugs are known to be melanoma-seeking agents owing to specific incorporation into nascent melanin. The melanin-affinic properties are apparently due to binding to intermediates, preferably dopaquinone, produced in the melanin synthetic pathway by tyrosinase-catalysed oxidation of tyrosine. In the present paper, in vitro screening methods have been used for the identification of possible melanoma seekers according to the above principle. The binding of test substance to dopaquinone suppressed dopachrome formation by the withdrawal of dopaquinone from the reaction of the mixture, and the decrease in dopachrome concentration was monitored spectrophotometrically at 475 nm. In order to eliminate false results caused by tyrosinase inhibition, which also will decrease the dopachrome concentration, the oxygen consumption was followed potentiometrically. To avoid the effect of tyrosinase inhibition on dopachrome formation, additional experiments with autoxidation of L-dopa in the presence of test substance were performed. Of the 22 substances (mainly thioureylenes and thioamides) studied, 4,5,6-triamino-2(H)- pyrimidinehtionsulfate, trithiocyanuric acid, 2-thiouracil, 6-methyl-2-thiouracil, and 4- amino-2-mercaptopyrimidine most effectively decreased the dopachrome formation with no or little inhibition of tyrosinase activity. They should therefore be regarded as potential melanoma seekers. In a complementary autoradiographic study on the uptake of the potent tyrosinase inhibitor mercaptobenzothiazole (MBT) in B 16 melanoma transplanted to mice, it was found that strong tyrosinase inhibition seems to decrease incorporation into melanin in vitro. MBT was partially accumulated in restricted areas of the tumor which may be explained by the molar dose injected.

Isoenzymic patterns of tyrosinase in the rabbit choroid and retina/retinal pigment epithelium

Tyrosinase in crude extracts from the adult rabbit choroid and retina/retinal pigment epithelium was found to be differently affected by the inhibitors sodium metabisulfite, cyanide, diethyldithiocarbamate and 2,2'-dipyridyl:the latter inhibited the activity in the retina/retinal pigment epithelium extract after 24 hr of incubation, but not that in the choroid. The first three inhibitors, on the other hand, inhibited the activity in the choroid extract, but not in the retina/retinal pigment epithelium extract, after 3 hr of incubation. Both extracts contained one insoluble and two soluble isoenzymes. The mobilities and the apparent molecular weights of the soluble isoenzymes (more markedly so for the most rapidly migrating) differed, as revealed by polyacrylamide gel electrophoresis, resulting in distinct isoenzymic patterns. We suggest that such patterns may be related to the characteristic melanogenic features of the retina/retinal pigment epithelium and choroid.

Tyrosinase isoenzymes: two melanosomal tyrosinases with different kinetic properties and susceptibility to inhibition by calcium

Two forms of tyrosinase from B16 mouse melanoma were identified by nonreducing SDS-PAGE after solubilization of crude melanosomal preparations with the nonionic detergent Brij 35. These forms, named LEMT and HEMT (low and high electrophoretic mobility tyrosinase, respectively), were purified by a combination of differential detergent extraction and chromatographic techniques. They displayed tyrosine hydroxylase and dopa oxidase activity and were stereospecific and sensitive to phenylthiourea, providing that they are true tyrosinases. However, based on its kinetic parameters, HEMT is a much more efficient enzyme. Immunoprecipitation and Western blots performed with the specific antibody alpha PEP1, directed against the b protein carboxyl terminus, suggested that LEMT is identical to the b protein. Both forms of tyrosinase were noncompetitively inhibited by Ca2+ at physiologically relevant concentrations. However, the b protein was apparently more susceptible, since maximal inhibition was reached at lower Ca2+ concentrations for LEMT. Moreover, binding of Ca2+ to the tyrosinases resulted in a noticeable thermal destabilization of the enzymes, which was also more pronounced for LEMT.

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase has been developed to enable large numbers of samples to be screened very rapidly. The assay measures the pink pigment formed by the reaction of Besthorn's hydrazone (3-methyl-2-benzothiazoninone hydrazone, or MBTH) with dopaquinone, the product of oxidation of L-dopa by tyrosinase. Addition of perchloric acid stops the reaction and precipitates protein, enabling turbid as well as non-turbid samples to be assayed. Stability of the pink product is enhanced in acid solution and the pigment has a sharp absorbance maximum at 505 nm such that it is easily measured spectrophotometrically. Using the stopped assay, tyrosinase is detectable only in mammalian cell lines expected to express the enzyme, and the specificity of the assay has also been confirmed using tyrosinase inhibitors. The stopped MBTH assay is approx. 15-times more sensitive than the widely used dopachrome assay and can reliably detect the formation of as little as 350 pmol of product.

Cellular and molecular mechanisms in photochemical sensitization: studies on the mechanism of action of psoralens

The interaction of chemicals and light to induce sensitization reactions in the skin is a complex multistep process resulting in physiological changes in both the dermal and epidermal cell layers as well as characteristic inflammatory reactions. It is becoming increasingly apparent that an array of growth factors and cytokines acting on different components of the skin are involved in the regulation of these processes. One of the best characterized classes of chemical photosensitizers are the psoralens, a group of compounds that must be activated by UV light in wavelengths ranging from 320 to 400 nm (UVA) to initiate their biological actions. Recent evidence suggests that the ability of the psoralens to induce sensitization reactions, which include alterations in epidermal cell growth and differentiation, is highly specific and due to interactions with the epidermal growth factor (EGF) receptor. Specific receptor proteins for the psoralens have been identified in cytoplasmic and membrane fractions of responsive cells. Binding of psoralens to these proteins is of high affinity and reversible. UVA light causes psoralens to photoalkylate their receptors, a process thought to activate the receptor. One early biochemical event at the cell surface membrane linked to psoralen-receptor activation is the inhibition of EGF binding and alterations in the structure and function of the EGF receptor. These findings suggest that the cell surface membrane is an important target for chemical photosensitizers such as the psoralens. In addition, since photoactivated psoralens modulate epidermal cell growth and differentiation, the ability of these compounds to modify the function of the EGF receptor may underlie their biological activity as chemical photosensitizers.

Improved tyrosinase activity stains in polyacrylamide electrophoresis gels

Mammalian tyrosinase exists in a variety of subcellular locations and maturation states that result from a complex post-translational processing with possible regulatory implications. So far, SDS-PAGE has proven to be the method of choice for the resolution of tyrosinase isoforms. However, the relatively poor sensitivity of the currently available specific activity stain based on incubation of the gels with L-dopa until the formation of melanin has severely limited the use of electrophoresis in regulation studies. Two alternative staining procedures are presented and discussed. The first one involves the fluorographic detection of radioactive melanin after incubation of the gels in the presence of L-[3-14C]-dopa. A similar method has already been used by others (Tsukamoto et al., 1992, Pigment Cell Res. [Suppl.] 2:84-89), but its performance has not yet been compared to the one of the dopa procedure. The sensitivity of this method can be varied by adjusting the isotopic dilution of the tracer and/or the time of exposure of the gel, but it is at least ten times higher than the one of the colorimetric stain. Moreover, the intensity of the bands is proportional to the initial tyrosinase activity over a wide range. Using this procedure, the activity present in the different subcellular fractions of melanocytes in culture can be easily detected. The second procedure involves the formation of a colored adduct between dopaquinone and MBTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosinase isoenzymes in mammalian melanocytes. 1. Biochemical characterization of two melanosomal tyrosinases from B16 mouse melanoma

B-16 mouse melanoma melanosomes contain two forms of tyrosinase that can be resolved by SDS/PAGE. These forms interact to different extents with the ion-exchanger DEAE-Sephadex and with hydroxyapatite, and have different affinity for the melanosomal membrane and/or the intraorganular matrix. After partial purification and complete separation of the two tyrosinases, several kinetic parameters were analyzed. The form of lower electrophoretic mobility displayed a higher Km for 3,4-dihydroxy-L-phenylalanine (L-dopa) and L-tyrosine, an absolute requirement for the cofactor L-dopa in its tyrosine hydroxylase activity, and a lower ratio of tyrosine hydroxylation to Dopa oxidation. The form of higher electrophoretic mobility displayed lower values of Km for both substrates and was able to exhibit tyrosine hydroxylase activity after a lag period even in the absence of L-dopa. Both forms were stereospecific for the L isomers and sensitive to the specific tyrosinase inhibitor 2-phenylthiourea. These forms do not appear to result from different degrees of glycosylation, nor from limited proteolysis and are also present in the microsomal fraction of B16 mouse melanoma. They might correspond to different gene products, most likely derived from the b and c loci.

Effect of ferrous ions on the monophenolase activity of tyrosinase

The effect of ferrous ions on the monophenolase activity of tyrosinase has been studied. Although a shortening of the lag period which characterizes this hydroxylation reaction was observed, no direct effect on the enzyme was found. The reaction between ferrous ions and molecular oxygen in the presence of chelating agents, such as phosphate or EDTA, produces hydroxyl radicals. These radicals can hydroxylate tyrosine to generate L-3,4-dihydroxyphenylalanine (dopa). Catalase and scavengers of hydroxyl radicals inhibited both the shortening of the lag period and dopa formation. On the basis of these results, it is proposed that the influence of ferrous ions on tyrosinase is due to the formation of dopa in the chemical hydroxylation of tyrosine. Dopa transforms the Emet form of the enzyme (Cu2+Cu2+) into the Edeoxy form (Cu1+Cu1+) and, thus, shortens the lag period.

Differential regulation of tyrosinase activity in skin of white and black individuals in vivo by topical retinoic acid

Tyrosinase activity is a key determinant of melanin production in skin. Because retinoic acid regulates tyrosinase activity in melanoma cells, we analyzed modulation of pigmentation in vivo by retinoic acid. Black and white subjects were either not treated, or treated topically for 4 d under occlusion with vehicle, retinoic acid (0.1%), or the irritant sodium lauryl sulfate (2%). In untreated skin, tyrosinase activity and melanin content were significantly greater (2.3 times, and 3.2 times, respectively) in blacks versus whites. Four days of treatment with topical retinoic acid did not alter tyrosinase activity or melanin content in black skin. In contrast, retinoic acid treatment significantly induced (2.7 times, n = 8) tyrosinase activity, compared to vehicle treatment, in white skin. Melanin content, however, remained unchanged at 4 d. In separate experiments, tyrosinase activity in white subjects (n = 25) was increased 16% (p = 0.01) in sodium lauryl sulfate-treated skin, and 77% (p = 0.0005) in retinoic acid-treated skin, compared to vehicle-treated skin. The effect of retinoic acid on tyrosinase activity could be differentiated from non-specific irritation, because tyrosinase activity in retinoic acid-treated skin was significantly greater (52%, p = 0.004) than sodium lauryl sulfate-treated skin. Similar results were obtained with the dihydroxyphenylalanine reaction done on vehicle, sodium lauryl sulfate-, and retinoic acid-treated white skin. Northern analysis (n = 6) and semi-quantitative polymerase chain reaction (n = 6) demonstrated that retinoic acid treatment did not alter tyrosinase mRNA levels in white skin. Western analysis revealed that induction of tyrosinase activity by retinoic acid also was not associated with increased tyrosinase protein content (n = 9), indicating that regulation of tyrosinase activity by retinoic acid occurs through a post-translational mechanism. These data demonstrate that low tyrosinase activity in white skin in vivo is retinoic acid inducible and high tyrosinase activity in black skin in vivo is neither further induced nor reduced by retinoic acid.

Skin organ culture model for examining epidermal melanization

To clarify the regulatory mechanism of skin pigmentation through epidermal proliferation and differentiation, organ cultures of pigmented guinea pig skins have been studied for their melanogenic responses to exogenous stimuli. Melanogenic activity was measured by both tyrosinase activity assessed by observing release of 3H2O from tissue after incubation with 3H-tyrosine and melanin synthesis, indicated by the incorporation of 14C-2-thiouracil into tissue. Those skin explants that were maintained in serum-free media under air conditions equilibrated with 5% CO2, 50% O2, and 45% N2 formed new, chemically analyzable pigment in the tissue within 4 d of culture. This melanization was accompanied by an increased number of melanocytes as well as by enhanced tyrosinase activity and elevated melanin synthesis. This organ culture system responded well to known tyrosinase inhibitors such as phenylthiourea, which decreased melanogenic activity. Of the arachidonic acid metabolites tested, PGE2, LTC4, LTB4, and 5-HETE were found to significantly stimulate melanogenic activity as indicated by tyrosinase activity, whereas PGF2-alpha, 12-HETE, and 15-HETE did not. Among several known growth factors, only bFGF significantly stimulated melanogenesis in the organ cultured melanocytes. TGF-alpha, which increased DNA synthesis, had a slightly stimulating effect on melanogenesis, whereas TGF beta, which inhibited DNA synthesis, did not stimulate melanogenesis, but rather slightly decreased it. 8-methoxypsoralen+ultraviolet A-treated skin that underwent a marked pigmentation within 14 d in vivo demonstrated enhanced melanogenesis in the organ culture system in proportion to its in vivo pigmentation. Our organ culture system might provide an opportunity to examine the mechanism of action of epidermal melanization.

Proteolysis with trypsin of mammalian tyrosinase isoforms from B16 mouse melanoma

In spite of the central role of tyrosinase in mammalian pigmentation, few data are available on its structure and structure-function relationships based on direct analysis of the protein. A number of reasons have been invoked to account for this situation, including the problems for its purification and its resistance to proteases. However, no study on the effects of proteases on purified tyrosinase has been reported. We have purified the melanosomal and cytosolic tyrosinases from B16 mouse melanoma and analyzed their susceptibility to trypsin digestion. Both isoforms are sensitive to trypsin, and display similar peptide maps and kinetics of proteolysis, suggesting that they are products of the same gene. The peptide maps and the kinetics of appearance of the fragments were consistent with the sequential removal of N-terminal peptides, leading to a core of 55.3 kDa for the melanosomal form and 48.6 kDa for the cytosolic enzyme. This core was apparently resistant to further proteolysis and catalytically inactive. The difference in molecular weight for the core of the cytosolic and melanosomal forms is the same as that calculated for the native isoforms. The kinetics of enzyme inactivation indicate that the tyrosine hydroxylase and Dopa oxidase activities of tyrosinase are lost at the same rate, and should therefore display similar if not identical structural requirements. The results are discussed in terms of the relationship of both isoforms and of the putative protein sequences deduced from the cDNA clones proposed for tyrosinase.

Tyrosinase-like activity and estradiol binding in rat uterine nuclear extracts

Nuclear extracts from the uteri of estradiol-implanted rats contain a tyrosinase-like enzyme that has three activities: monophenolase or cresolase, diphenolase or catecholase, and estrogen binding. When [3H]estradiol was used as a substrate, 3H2O was released from the A ring in the presence of copper and ascorbic acid. The optimal concentrations of these cofactors for the cresolase activity were established. The cresolase activity was lost on attempts at further purification. Estradiol binding was observed in conjunction with the enzymatic activity and was dependent on the presence of ascorbic acid and copper. The most potent inhibitors of 3H2O release from [3H]estradiol were those with a dihydroxyphenol moiety. The reaction was also sensitive to sulfhydryl reagents. These features of the enzyme are distinctive from other oxidases capable of attacking the aromatic ring of estrogens.

Tyrosinase-like polypeptides in the uterus and in the central nervous system of rats

Nuclear and cytosolic fractions of rat uteri and tissues from the central nervous system contain proteins that are recognized by a polyclonal tyrosinase antibody. This antibody eliminates the cresolase activity of uterine nuclear extract when estradiol is used as substrate. Thus, it appears that tyrosinase-like proteins might be present in tissues not generally considered to chain such an enzyme.

Biosynthesis and intracellular movement of the melanosomal membrane glycoprotein gp75, the human b (brown) locus product

A 75-kDa melanosomal glycoprotein (gp75) is the product of a gene that maps to the b (brown) locus, a genetic locus that determines coat color in the mouse. The b locus is conserved (88% identity) between mouse and human. The mouse monoclonal antibody TA99 was used to study the biosynthesis and processing of gp75. gp75 was synthesized as a 55-kDa polypeptide, glycosylated by addition and processing of five or more Asn-linked carbohydrate chains through the cis and trans Golgi, and transported to melanosomes as a mature 75-kDa form. Synthesis and processing of gp75 was rapid (T1/2 less than 30 min), and early steps in processing were required for efficient export of gp75 to melanosomes. Fully processed mature gp75 was quite stable (T1/2 = 22-24 h) in the melanosome. Digestion of high-mannose carbohydrate chains with endo-beta-N-acetylglucosaminidase H revealed two alternative processed forms of gp75 that differed in the number or composition of complex-type carbohydrate chains. The rate of synthesis and movement through intracellular membrane compartments was the same for both glycosylated forms. Studies with inhibitors of steps in oligosaccharide processing showed that alternative forms of gp75 were generated during trimming reactions by mannosidase IA/IB and that further maturation resulted in the two mature forms of gp75. We propose that the kinetics of biosynthesis and processing reflect events in the biogenesis and maturation of melanosomes.

Studies on the reactions between human tyrosinase, superoxide anion, hydrogen peroxide and thiols

Human tyrosinase (5.5 mg) has been purified from a single human melanotic melanoma metastasis (50.5 g). In the presence of dioxygen, L-tyrosine proved to be a very poor substrate for this enzyme with barely detectable activity compared to L-dopa. However, saturating superoxide anion (i.e., greater than 5 x 10(-3) M) enhanced the oxidation rate of L-tyrosine to dopachrome 40-fold. Hydrogen peroxide was shown to be a competitive inhibitor of tyrosinase when L-tyrosine was the substrate. This reversible inhibition is based on a slow pseudocatalase activity for tyrosinase. Monothiols and dithiols inhibit tyrosinase by different mechanisms. Reduced human thioredoxin and 2,3-dithiopropanol are allosteric inhibitors of tyrosinase yielding bis-cysteinate complexes with one of the copper atoms in the enzyme active site. Bis-cysteinate tyrosinase activity is down-regulated to 30% of native enzyme activity in the L-dopa assay; suggesting a true regulatory role for dithiols. Monothiols such as reduced glutathione and beta-mercaptoethanol are much less reactive with tyrosinase although 10(-3) M monothiol totally inhibits enzyme activity. Reduced thioredoxin inhibits tyrosinase 23-fold more than reduced glutathione under the same experimental conditions.

New assays for the tyrosine hydroxylase and dopa oxidase activities of tyrosinase

New assays for the tyrosine hydroxylase and dopa oxidase activities of tyrosinase (EC 1.14.18.1) have been developed. The tyrosine hydroxylase assay uses L-[carboxy-14C]tyrosine as the substrate, 14CO2 is released from the products of the hydroxylation and further metabolism of L-[carboxy-14C]tyrosine by incubation with ferricyanide, and measured radiometrically. D-Dopa is a preferable cofactor to L-dopa for the assay. Dopa oxidase activity is measured spectrophotometrically. Dopaquinone, produced on the oxidation of L-dopa, reacts with Besthorn's hydrazone (3-methyl-2-benzothiazolinone hydrazone) to form a pink pigment with an absorbance maximum at 505 nm. Details of the optimisation of conditions for the assays and their specificities for the two enzyme activities are described.

Differential expression and activity of melanogenesis-related proteins during induced hair growth in mice

In C57 Bl-6 mice, melanogenesis is strictly coupled to the growth phase of the hair cycle (anagen). To further study this phenomenon of concerted developmental and pigmentary activity, we followed the sequence of tyrosinase (key enzyme of melanogenesis) expression and activity and the presence of the melanosomal protein gp 75 during the development of traumatically induced anagen follicles (days 0 = telogen, and days 1-12, after anagen induction studied). In addition to performing Northern and Western blots for tyrosinase, tyrosine hydroxylase activity (THA) and dopa oxidase activity (DOA) were measured. On day 0, DOA was undetectable, and THA was very low. On days 1 and 2, both activities were undetectable; starting from day 3, they increased rapidly, reaching a plateau on days 8 and 12. DO-positive proteins had apparent molecular weights (MW) of 66-68 kD (days 3-12), 72-74 kD (days 5-12), and 130 kD (days 8 and 12). Western blotting emphasized proteins of MW 66-68 kD (tyrosinase), and 73-75 kD (gp 75); tyrosinase was undetectable on day 0, but already present on days 1 and 2; it increased by day 5 and had reached a plateau on days 8 and 12; gp 75 was undetectable on days 0-2; it was present on day 3, increased by day 5, and reached a plateau on days 8 and 12. Northern blot analysis revealed high levels of tyrosinase mRNA on days 5 and 8, low levels on days 1-3, and none on day 0. These data suggest a highly regulated, time frame-restricted, differential pattern of tyrosinase transcription, translation, and enzyme activity during the different stages of the developing murine anagen follicle, possibly as a result of complex interactions between follicular melanocytes and their environment.

Activation of mammalian tyrosinase by ferrous ions

Kinetic experiments are reported showing that mammalian tyrosinase from B16 mouse melanoma is significantly activated by catalytic amounts of ferrous ions. Monitoring of tyrosine oxidation by both dopachrome formation and oxygen consumption showed that ferrous ions at micromolar concentrations induce a marked enzymatic activity with 0.01 U/ml of highly purified tyrosinase, whereas no detectable reaction occurs in the absence of metal over a sufficiently prolonged period of time. The extent of the activating effect, which is specific for the reduced form of iron, is proportional to the concentration of the added metal with a typical saturation profile, no further effect being observed beyond a threshold value. Changing the buffer system from phosphate to hepes or tris results in a marked decrease of the Fe2(+)-induced activation. Scavengers of active oxygen species, such as superoxide dismutase, catalase, formate and mannitol have no detectable effect on the tyrosinase activity. These results are accounted for in terms of an activation mechanism involving reduction of the cupric ions at the active site of the resting enzyme.