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1.
DOPAchrome tautomerase (DCT) is known to control the ratio of DHICA/DHI formed within the melanocyte, but physiologic significance of this activity is not yet fully elucidated. In this study the two melanin monomers are shown to inhibit with different efficacy the initial, tyrosinase-controlled, melanogenic reaction, namely conversion of L-tyrosine to DOPAchrome (2-carboxy-2,3-dihydroindole-5,6-quinone). This is demonstrated in the test tube assay system whereby formation of DOPAchrome is catalyzed by i) isolated premelanosomes (PMS), ii) tyrosinase-rich PMS glycoproteins, or iii) tyrosinase purified from fibroblasts transfected with human tyrosinase gene. Both DHI and DHICA suppress the conversion of L-tyrosine to DOPAchrome when added to reaction mixture but the inhibitory effect is far more strongly pronounced by DHI. DHI inhibits both activities of tyrosinase—tyrosine-hydroxylation and DOPA-oxidation—more strongly than DHICA. The different extent of inhibition is shown to reflect i) the ability of the two monomers to compete with tyrosinase substrates for the enzyme's active center and ii) the rate of interaction between melanin monomers and DOPAquinone. Consequently, we demonstrate that the tyrosinase-catalyzed DOPAchrome formation can be modulated by the ratio of DHICA/DHI among melanin monomers with the increased proportion of DHICA resulting in more efficient DOPAchrome formation. These results raise the possibility that DOPAchrome tautomerase plays a role in positive control of the tyrosinase-catalyzed early phase of melanogenesis.  相似文献   

2.
Although melanins can be formed in vitro by the unique action of tyrosinase on L-tyrosine, it is now well accepted that other enzymes termed tyrosinase-related proteins are involved in mammalian melanogenesis. However, some aspects of their roles in the regulation of the pathway are still unknown. The action of dopachrome tautomerase on L-dopachrome yields DHICA, a stable dihydroxyindole with a low rate of spontaneous oxidation. However, DHICA is efficiently incorporated to the pigment, as judged by the high content of carboxylated indole units in natural melanins. Therefore, the fate of this melanogenic intermediate and the mechanisms of its incorporation to the melanin polymer are major issues in the study of melanogenesis. We have recently shown that mouse melanosomes contain two electrophoretically distinguishable tyrosinase isoenzymes, LEMT and HEMT, that can be purified and completely resolved (Jiménez-Cervantes et al., 1993a). Herein, we have compared the ability of these tyrosinases to catalyze DHICA oxidation. Although highly purified LEMT shows a very low specific activity for dopa oxidation in comparison to HEMT, it is able to catalyze DHICA oxidation. However, the DHICA oxidase activity of HEMT was very low, if significant. The ability of purified LEMT to catalyze DHICA oxidation was abolished by heat, trypsin, or phenylthiourea treatments. LEMT acting on DHICA caused the formation of a brownish soluble color similar to DHICA-melanin. Immunoprecipitation of the DHICA oxidase activity of LEMT by specific antibodies suggests that this activity corresponds to TRP1. These results indicate that LEMT, most probably identical to the product of the b locus, is a tyrosinase having a specific DHICA oxidase activity. Opposite to HEMT, the true tyrosinase encoded by the albino locus, its role in melanogenesis would be related to the incorporation of DHICA into eumelanin rather than to the first steps of the pathway.  相似文献   

3.
Several studies have confirmed that a melanocyte-specific enzyme, dopachrome tautomerase (EC 5.3.2.3), catalyzes the isomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) (Pawelek, 1991). Here we report that DHICA, produced either enzymatically with dopachrome tautomerase or through chemical synthesis, spontaneously polymerized to form brown melanin that was soluble in aqueous solutions above pH 5. Under the same reaction conditions, solutions of either DOPA, DOPAchrome, or 5,6-dihydroxyindole (DHI) formed black, insoluble melanin precipitates. When DHICA and DHI were mixed together, with DHICA in molar excess, little or no precipitation of DHI-melanin occurred and the rate and extent of soluble melanin formation was markedly enhanced over that achieved with DHICA alone, suggesting co-polymerization of DHICA and DHI. With or without DHI, DHICA-melanins absorbed throughout the ultraviolet and visible spectra (200-600 nm). The DHICA-melanins precipitated below pH 5, at least in part because of protonation of the carboxyl groups. DHICA-melanins could be passed through 0.22 micron filters but could not be dialyzed through semi-permeable membranes with exclusion limits of 12,000-14,000 daltons. HPLC/molecular sieve analyses revealed apparent molecular weights ranging from 20,000 to 200,000 daltons, corresponding to 100-1,000 DHICA monomers per molecule of melanin. DHICA-melanins were stable to boiling, lyophilization, freezing and thawing, and incubation at room temperature for more than 1 year. The natural occurrence of oligomers of DHICA was first reported by Ito and Nichol (1974) in their studies of the brown tapetal pigment in the eye of the sea catfish (Arius felis L.).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

4.
After dopachrome?   总被引:4,自引:0,他引:4  
Dopachrome, an intermediate in melanin biosynthesis, exhibits some unusual properties. At physiologic pH (e.g., pH 6-8) it is unstable and spontaneously loses its carboxyl group to form 5,6-dihydroxyindole (DHI) and CO2. However, over this same pH range, if various metals or a melanocyte-specific enzyme are present, it rapidly rearranges to its isomer form--5,6-dihydroxyindole-2-carboxylic acid (DHICA)--which is far more stable than dopachrome in its ability to retain the carboxyl group. Whether or not the carboxyl group is retained could have important implications for the regulation of melanogenesis, since in the presence of oxygen DHI spontaneously forms a black precipitate, whereas DHICA forms a golden-brown solution. The solubility of "DHICA-melanin" is due to the presence of carboxyl groups, which provide negative charges and hydrophilicity. Thus, in vivo, the extent to which dopachrome is converted to DHI or DHICA may well influence the solubility and color of the melanin formed. The purpose of this article is to review recent findings in these areas and to discuss the possible significance of dopachrome conversion in the regulation of melanogenesis and color formation.  相似文献   

5.
The expression of various melanogenic proteins, including tyrosinase, the tyrosinase-related proteins 1 (TRP1) and 2 (TRP2/DOPAchrome tautomerase), and the silver protein in human melanocytes was studied in six different human melanoma cell lines and compared to a mouse derived melanoma cell line. Analysis of the expression of tyrosinase, TRP1, TRP2, and the silver protein using flow cytometry revealed that in general there was a positive correlation between melanin formation and the expression of those melanogenic enzymes. Although several of the melanoma cell lines possessed significant activities of TRP2, the levels of DOPAchrome tautomerase in extracts of human cells were relatively low compared to those in murine melanocytes. Melanins derived from melanotic murine JB/MS cells, from melanotic human Ihara cells and HM-IY cells, from sepia melanin, and from C57BL/6 mouse hair were chemically analyzed. JB/MS cells, as well as Ihara cells and HM-TY cells, possessed significant amounts of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) derived melanins, this being dependent on the activity of TRP2. Kinetic HPLC assays showed that 5,6-dihydroxyindole (DHI) produced during melanogenesis was metabolized quickly to melanin in pigmented KHm-1/4 cells, whereas DHI was stable in amelanotic human SK-MEL-24 cells. A melanogenic inhibitor that has been purified from SK-MEL-24 cells that suppressed oxidation of DHI in the presence or absence of tyrosinase, but had no effect on DHICA oxidation. The sum of these results suggest that the expression of melanogenic enzymes as well as the activity of a melanogenic inhibitor are critical to the production of melanin synthesis in humans.  相似文献   

6.
Several studies have confirmed that a melanocyte-specific enzyme, dopachrome tautomerase (EC 5.3.2.3), catalyzes the isomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) (Pawelek, 1991). Here we report that DHICA, produced either enzymatically with dopachrome tautomerase or through chemical synthesis, spontaneously polymerized to form brown melanin that was soluble in aqueous solutions above pH 5. Under the same reaction conditions, solutions of either DOPA, DOPAchrome, or 5,6-dihydroxyindole (DHI) formed black, insoluble melanin precipitates. When DHICA and DHI were mixed together, with DHICA in molar excess, little or no precipitation of DHI-melanin occurred and the rate and extent of soluble melanin formation was markedly enhanced over that achieved with DHICA alone, suggesting co-polymerization of DHICA and DHI. With or without DHI, DHICA-melanins absorbed throughout the ultraviolet and visible spectra (200-600 nm). The DHICA-melanins precipitated below pH 5, at least in part because of protonation of the carboxyl groups. DHICA-melanins could be passed through 0.22 μm filters but could not be dialyzed through semi-permeable membranes with exclusion limits of 12,000-14,000 daltons. HPLC/molecular sieve analyses revealed apparent molecular weights ranging from 20,000 to 200,000 daltons, corresponding to 100-1,000 DHICA monomers per molecule of melanin. DHICA-melanins were stable to boiling, lyophilization, freezing and thawing, and incubation at room temperature for more than 1 year. The natural occurrence of oligomers of DHICA was first reported by Ito and Nichol (1974) in their studies of the brown tapetal pigment in the eye of the sea catfish (Arius felis L.). In experiments reported here, brown, but not black, melanins from mouse hairs, human melanoma cells, and peacock feathers were soluble in aqueous buffers. Since DHICA-melanins are both soluble and brown, the results raise the possibility that they are determinants of brown colors in the animal kingdom.  相似文献   

7.
The mouse b locus controls black/brown coat coloration. Its product, the b-protein or TRP-1, has significant homology to tyrosinase, and this has led to suggestions that the b-protein is itself a melanogenic enzyme. In order to investigate its function, we have used lines of mouse fibroblasts stably expressing the b-protein. We were unable to con-firm previous reports that the b-protein has tyrosinase or catalase activity, but detected stereospecific dopachrome tautomerase activity in b-protein-expressing fibroblasts. This dopachrome tautomerase binds to Concanavalin A-Sepharose, and the major product of its action on L-dopachrome is 5,6-dihydroxyindole-2-carboxylic acid, as expected for the mammalian enzyme. Since this activity is not present in untransfected fibroblasts we conclude that the b-protein has dopachrome tautomerase activity. Further supporting evidence comes from the analysis of melanin metabolites produced by fibroblasts expressing tyrosinase alone, or in combination with the b-protein. Culture medium from the line expressing both proteins contains significant amounts of methylated carboxylated indoles, such as 6-hydroxy-5-methoxyindole-2-carboxylic acid, which would be expected in cells with an active dopachrome tautomerase. The levels of these compounds in medium from cells expressing tyrosinase alone are approximately 20-fold lower, and not significantly above background. Hence, it appears that the b-protein acts as a dopachrome tautomerase in vivo as well as in vitro.  相似文献   

8.
The production of melanin pigment in mammals requires tyrosinase, an enzyme which hydroxylates the amino acid tyrosine to DOPA (3,4-dihydroxyphenylalanine), thus allowing the cascade of reactions necessary to synthesize that biopolymer. However, there are other regulatory steps that follow the action of tyrosinase and modulate the quantity and quality of the melanin produced. DOPAchrome tautomerase is one such melanogenic enzyme that isomerizes the pigmented intermediate DOPAchrome to DHICA (5,6-dihydroxyindole-2-carboxylic acid) rather than to DHI (5,6-dihydroxyindole), which would be generated spontaneously. This enzyme thus regulates a switch that controls the proportion of carboxylated subunits in the melanin biopolymer. Efforts to clone the gene for tyrosinase have resulted in the isolation of a family of tyrosinase related genes which have significant homology and encode proteins with similar predicted structural characteristics. Using specific antibodies generated against synthetic peptides encoded by unique areas of several of those proteins, we have immuno-affinity purified them and studied their melanogenic catalytic functions. We now report that TRP-2 (tyrosinase related protein-2), which maps to and is mutated at the slaty locus in mice, encodes a protein with DOPAchrome tautomerase activity.  相似文献   

9.
In this study, a new skin-depigmenting agent, 2,6-dimethoxy-N-(4-methoxyphenyl)benzamide (DMPB), was synthesized using a combination of benzoic acid and aniline. DMPB exhibited significant depigmentation ability on the UV B-induced hyperpigmentation of the brown guinea pig skin. In addition, the 100ppm treatment with this compound had a 30% inhibitory effect on melanin pigment generation in the melan-a cell line without significant cell toxicity. To search for relationship with the depigmentation, the effects of DMPB on the tyrosinase and dopachrome tautomerase were evaluated. DMPB had no effect on tyrosinase. However, it accelerated dopachrome transformation into 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in the presence of dopachrome tautormerase. In addition, intracellular level of dopachrome tautomerase in melan-a cells was increased by treatment of DMPB. These results suggest that the pigment-lightening effects of DMPB might be due to biased production of DHICA-eumelanin induced by dopachrome tautormerase activation.  相似文献   

10.

Background

Tautomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) is a biologically crucial reaction relevant to melanin synthesis, cellular antioxidation, and cross-talk among epidermal cells. Since dopachrome spontaneously converts into 5,6-dihydroxyindole (DHI) via decarboxylation without any enzymes at physiologically usual pH, the mechanism of how tautomerization to DHICA occurs in physiological system is a subject of intense debate. A previous work has found that Cu(II) is an important factor to catalyze the tautomerization of dopachrome to DHICA. However, the effect of Cu(II) on the tautomerization has not been clarified at the atomic level.

Methods

We propose the reaction mechanism of the tautomerization to DHICA by Cu(II) from density functional theory-based calculation.

Results

We clarified that the activation barriers of α-deprotonation, β-deprotonation, and decarboxylation from dopachrome are significantly reduced by coordination of Cu(II) to quinonoid oxygens (5,6-oxygens) of dopachrome, with the lowest activation barrier of β-deprotonation among them. In contrast to our previous work, in which β-deprotonation and quinonoid protonation (O5/O6-protonation) were shown to be important to form DHI, our results show that the Cu(II) coordination to quinonoid oxygens inhibits the quinonoid protonation, leading to the preference of proton rearrangement from β-carbon to carboxylate group but not to the quinonoid oxygens.

Conclusion

Integrating these results, we conclude that dopachrome tautomerization first proceeds via proton rearrangement from β-carbon to carboxylate group and subsequently undergoes α-deprotonation to form DHICA.

General significance

This study would provide the biochemical basis of DHICA metabolism and the generalized view of dopachrome conversion which is important to understand melanogenesis.  相似文献   

11.
The IFPCS presidential lecture: a chemist's view of melanogenesis   总被引:2,自引:0,他引:2  
The significance of our understanding of the chemistry of melanin and melanogenesis is reviewed. Melanogenesis begins with the production of dopaquinone, a highly reactive o-quinone. Pulse radiolysis is a powerful tool to study the fates of such highly reactive melanin precursors. Based on pulse radiolysis data reported by Land et al. (J Photochem Photobiol B: Biol 2001;64:123) and our biochemical studies, a pathway for mixed melanogenesis is proposed. Melanogenesis proceeds in three distinctive steps. The initial step is the production of cysteinyldopas by the rapid addition of cysteine to dopaquinone, which continues as long as cysteine is present (1 microM). The second step is the oxidation of cysteinyldopas to give pheomelanin, which continues as long as cysteinyldopas are present (10 microM). The last step is the production of eumelanin, which begins only after most cysteinyldopas are depleted. It thus appears that eumelanin is deposited on the preformed pheomelanin and that the ratio of eu- to pheomelanin is determined by the tyrosinase activity and cysteine concentration. In eumelanogenesis, dopachrome is a rather stable molecule and spontaneously decomposes to give mostly 5,6-dihydroxyindole. Dopachrome tautomerase (Dct) catalyses the tautomerization of dopachrome to give mostly 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Our study confirmed that the role of Dct is to increase the ratio of DHICA in eumelanin and to increase the production of eumelanin. In addition, the cytotoxicity of o-quinone melanin precursors was found to correlate with binding to proteins through the cysteine residues. Finally, it is still unknown how the availability of cysteine is controlled within the melanosome.  相似文献   

12.
The significance of our understanding of the chemistry of melanin and melanogenesis is reviewed. Melanogenesis begins with the production of dopaquinone, a highly reactive o‐quinone. Pulse radiolysis is a powerful tool to study the fates of such highly reactive melanin precursors. Based on pulse radiolysis data reported by Land et al. (J Photochem Photobiol B: Biol 2001;64:123) and our biochemical studies, a pathway for mixed melanogenesis is proposed. Melanogenesis proceeds in three distinctive steps. The initial step is the production of cysteinyldopas by the rapid addition of cysteine to dopaquinone, which continues as long as cysteine is present (1 μM). The second step is the oxidation of cysteinyldopas to give pheomelanin, which continues as long as cysteinyldopas are present (10 μM). The last step is the production of eumelanin, which begins only after most cysteinyldopas are depleted. It thus appears that eumelanin is deposited on the preformed pheomelanin and that the ratio of eu‐ to pheomelanin is determined by the tyrosinase activity and cysteine concentration. In eumelanogenesis, dopachrome is a rather stable molecule and spontaneously decomposes to give mostly 5,6‐dihydroxyindole. Dopachrome tautomerase (Dct) catalyses the tautomerization of dopachrome to give mostly 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA). Our study confirmed that the role of Dct is to increase the ratio of DHICA in eumelanin and to increase the production of eumelanin. In addition, the cytotoxicity of o‐quinone melanin precursors was found to correlate with binding to proteins through the cysteine residues. Finally, it is still unknown how the availability of cysteine is controlled within the melanosome.  相似文献   

13.
Eumelanins in animals are biosynthesized by the combined action of tyrosinase, 3,4-dihydroxyphenylalanine (DOPA)chrome isomerase, and other factors. Two kinds of eumelanins were characterized from mammalian systems; these are 5,6-dihydroxyindole (DHI)-melanin and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin. In insects, melanin biosynthesis is initiated by phenoloxidase and supported by DOPAchrome isomerase (decarboxylating). Based on the facts that DOPA is a poor substrate for insect phenoloxidases and DHI is the sole product of insect DOPAchrome isomerase reaction, it is proposed that insects lack DHICA-melanin. Accordingly, the phenoloxidase isolated from the hemolymph of Manduca sexta failed to oxidize DHICA. Control experiments reveal that mushroom tyrosinase, as well as laccase, which is a contaminant in the commercial preparations of mushroom tyrosinase, are capable of oxidizing DHICA. Neither the whole hemolymph nor the cuticular extracts of M. sexta possessed any detectable oxidase activity towards this substrate. Thus, insects do not seem to produce DHICA-eumelanin. A useful staining procedure to localize DHICA oxidase activity on gels is also presented.  相似文献   

14.
5,6-Dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), which are important intermediates in melanogenesis, can be converted into the corresponding melanin pigments by the action of the lipoxygenase/H2O2 system. Kinetic and HPLC analyses indicate that both DHI and DHICA are good substrates for this enzymatic system. Enzyme activity on both substrates was measured in comparison with peroxidase and tyrosinase; the oxidizing behaviour of lipoxygenase is more similar to that of peroxidase rather than that of tyrosinase. The antioxidant properties of DHI- and DHICA-melanins have been investigated in comparison with other kinds of melanins. DHICA-melanin shows a more pronounced antioxidant effect than that of DHI-melanin and this behaviour can be ascribed to the different structure and solubility of the two pigments. The mixed polymer synthesized from DHI and DHICA is the most effective one. Some implications about the possible explanation of the above mentioned behaviour are discussed.  相似文献   

15.
Quinone methide as a new intermediate in eumelanin biosynthesis   总被引:1,自引:0,他引:1  
The conversion of dopachrome to dihydroxyindole(s), a key reaction in eumelanin biosynthetic pathway, has been shown to be under the control of dopachrome conversion factor. Dopachrome conversion factor isolated from the hemolymph of Manduca sexta larvae, which is devoid of any tyrosinase activity, exhibits a narrow substrate specificity and readily bleaches the iminochromes derived from the oxidation of L-dopa, L-dopa methyl ester, and alpha-methyl-L-dopa, but failed to attack the corresponding D-isomers. The product formed in the case of L-dopachrome was identified to be 5,6-dihydroxyindole. Therefore, aromatization of dopachrome seems to accompany its decarboxylation as well. However, the enzyme also converts L-dopachrome methyl ester to an indole derivative indicating that it can deprotonate the alpha-hydrogen when the carboxyl group is blocked. These results are accounted for by the transient formation and further transformation of a reactive quinone methide intermediate during the dopachrome conversion factor-catalyzed reaction. The fact that the enzyme-catalyzed conversion of alpha-methyl dopachrome methyl ester (where both decarboxylation and deprotonation are blocked) resulted in the generation of a stable quinone methide in the reaction mixture confirms this contention and supports our recent proposal that quinone methide and not indolenine is the key transient intermediate in the conversion of dopachrome to dihydroxyindole observed during melanogenesis.  相似文献   

16.
Regulation of mammalian melanogenesis. II: The role of metal cations   总被引:2,自引:0,他引:2  
Melanogenesis can be divided into two phases. The first one involves two tyrosinase-catalyzed oxidations from tyrosine to dopaquinone and a very fast chemical step leading to dopachrome. The second phase, from dopachrome to melanin, can proceed spontaneously through several incompletely known reactions. However, some metal transition ions and protein factors different from tyrosinase might regulate the reaction rate and determine the structure and relative concentrations of the intermediates. The study of the effects of some divalent metal ions (Zn, Cu, Ni and Co) on some steps of the melanogenesis pathway has been approached using different radiolabeled substrates. Zn(II) inhibited tyrosine hydroxylation whereas Ni(II) and Co(II) were activators. Ni(II), Cu(II) and Co(II) accelerated chemical reactions from dopachrome but inhibited its decarboxylation. Dopachrome tautomerase also decreased decarboxylation. When metal ions and this enzyme act together, the inhibition of decarboxylation was greater than that produced by each agent separately, but amount of carboxylated units incorporated to the melanin was not higher than the amount incorporated in the presence of only cations. The amount of total melanin formed from tyrosine was increased by the presence of both agents. The action of Zn(II) was different from other ions also in the second phase of melanogenesis, and its effect on decarboxylation was less pronounced. Since tyrosine hydroxylation is the rate-limiting step in melanogenesis, Zn(II) inhibited the pathway. This ion seems to be the most abundant cation in mammalian melanocytes. Therefore, under physiological conditions, the regulatory role of metal ions and dopachrome tautomerase does not seem to be mutually exclusive, but rather complementary.  相似文献   

17.
Dopachrome conversion, in which dopachrome is converted into 5,6‐dihydroxyindole (DHI) or 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) upstream of eumelanogenesis, is a key step in determining the DHI/DHICA monomer ratio in eumelanin, which affects the antioxidant activity. Although the ratio of DHI/DHICA formed and the conversion rate can be regulated depending on pH, the mechanism is still unclear. To clarify the mechanism, we carried out first‐principles calculations. The results showed the kinetic preference of proton rearrangement to form quinone methide intermediate via β‐deprotonation. We also identified possible pathways to DHI/DHICA from the quinone methide. The DHI formation can be achieved by spontaneous decarboxylation after proton rearrangement from carboxyl group to 6‐oxygen. α‐Deprotonation, which leads to DHICA formation, can also proceed with a significantly reduced activation barrier compared with that of the initial dopachrome. Considering the rate of the proton rearrangements in a given pH, we conclude that the conversion is suppressed at acidic pH.  相似文献   

18.
5,6-Dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) are precursors of eumelanin. The effects of crustacean hemolymph proteins on these eumelanin-related metabolites were investigated. Zymogram analysis indicated that polymers of hemocyanin (Hc) subunits converted DHI into black pigment while no effects were observed using DHICA as a substrate. Spectrum changes for mixtures of purified Hc and DHI showed a profile similar to oxidized DHI by mushroom tyrosinase while Hc had only slight effects on DHICA. Typical inhibitors of tyrosinase and phenoloxidase severely hampered the production of oxidized DHI. Taken together with previous results, these data indicate that Hc plays a crucial role in the conversion of DHI in the hemolymph of crustaceans, which promotes late reactions in the melanin synthetic pathway as well as early reactions (oxidation of tyrosine and DOPA to dopaquinone).  相似文献   

19.
Several genes critical to the regulation of melanin production in mammals have recently been cloned and characterized. They map to the albino, brown, and slaty loci in mice, and encode proteins with similar structures and features, but with distinct catalytic capacities. The albino locus encodes tyrosinase, an enzyme with three distinct catalytic activities—tyrosine hydroxylase, 3,4-dihydroxyphenylalanine (DOPA) oxidase and DHI (5,6-dihydroxyindole) oxidase. The brown locus encodes TRP-l (tyrosinase-related protein-I), which has the same, but greatly reduced, catalytic potential. The slaty locus encodes TRP-2, another tyrosinase related-protein, which has DOPAchrome tautomerase activity. In this study we have examined the enzymatic interactions of these proteins, and their regulation by a novel melanogenic inhibitor. We observed that tyrosinase activity is more stable in the presence of TRP-l and/or TRP-2, but that the catalytic function of TRP-2 is not affected by the presence of TRP-1 or tyrosinase. Other factors also may influence melanogenesis and a unique melanogenic inhibitor suppresses tyrosinase and DOPAchrome tautomerase activities, but does not affect the spontaneous rate of DOPAchrome decarboxylation to DHI. The results demonstrate the catalytic functions of these proteins and how they stably interact within a melanogenic complex in the melanosome to regulate the quantity and quality of melanin synthesized by the melanocyte.  相似文献   

20.
MSG1 is a 27 kDa nuclear protein that is expressed strongly in melanotic B16 melanoma cells but very weakly in amelanotic B16 cells. Transient expression of B16 cells with an expression vector for MSG1 resulted in an increase in levels of the enzyme dopachrome tautomerase but not tyrosinase, as detected by western blotting. Stable transfection of B16 melanoma cells with plasmids containing the full length MSG1 or its deletion mutants, however, generated cell lines that showed an increase in levels of tyrosinase, dopachrome tautomerase and cellular melanin when compared with control transfected cells. Our results suggest that MSG1 plays an important role in melanogenesis, by regulating the levels of the enzymes of the pigmentary system via tyrosinase and dopachrome tautomerase.  相似文献   

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