Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.10.3.1 (tyrosinase)
 9,065 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Since the characterization of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as a major melanogenic intermediate, the fate of this compound and the mechanisms of its incorporation into the melanin polymer have become major issues in the study of melanogenesis. DHICA is a stable dihydroxyindole with a low rate of spontaneous oxidation, suggesting that enzymatic mechanism(s) might contribute to its evolution. The most obvious candidates are the melanosomal tyrosinases. We have recently shown that mouse melanosomes contain two electrophoretically distinct tyrosinase isoenzymes, termed low electrophoretic mobility tyrosinase (LEMT) and high electrophoretic mobility tyrosinase (HEMT), that can be resolved and purified. In this study, we report immunological evidence indicating that LEMT corresponds to the protein encoded by the brown locus (termed tyrosinase-related protein-1, TRP1), while HEMT corresponds to the tyrosinase encoded by the albino locus. We have compared the ability of both isoenzymes to catalyze DHICA evolution as determined by high performance liquid chromatography; although LEMT is a relatively poor tyrosine hydroxylase and DOPA oxidase as compared to HEMT, it was readily able to accelerate DHICA consumption concomitant with the production of a brownish product. However, the DHICA conversion activity of HEMT was barely detectable. The ability of purified LEMT to catalyze DHICA conversion could be almost completely abolished by treatment with heat or trypsin, and was inhibited in a concentration dependent way by the tyrosinase inhibitor 2-phenylthiourea and by L-tyrosine. Moreover, in the presence of low concentration of ascorbate, the DHICA conversion activity of LEMT displayed a lag period which was progressively longer at higher ascorbate concentrations. Based on the relationship between ascorbate added, enzyme activity, and lag period, it is very likely that the DHICA converting activity is indeed a DHICA oxidase activity. This was further proven by the demonstration that the product reacts rapidly and efficiently with the quinone trapping reagent 3-methyl-2-benzothiazolinone hydrazone, yielding a colored adduct similar to the one obtained with DOPAquinone. The DHICA oxidase activity of LEMT displayed a Km for DHICA of about 0.8 mM, as compared to 1.9 mM for L-DOPA and 0.23 nM for L-tyrosine. These results suggest that TRP1, the product of the brown locus, is indeed a tyrosinase with DHICA oxidase activity. However, as opposed to the tyrosinase encoded by the albino locus, TRP1's role in melanogenesis could be more directly related to DHICA metabolism than to the first steps of the pathway.
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PMID:A new enzymatic function in the melanogenic pathway. The 5,6-dihydroxyindole-2-carboxylic acid oxidase activity of tyrosinase-related protein-1 (TRP1). 802 58

Patients with the depigmentation disorder vitiligo lack the capacity to synthesize the melanins from L-tyrosine via the essential activity of tyrosinase. The aim of this study has been to examine both the supply of the substrate (L-tyrosine) and the regulation of tyrosinase in the epidermis of subjects with vitiligo. Patients with this depigmentation disorder have a 3- to 5-fold increase in GTP-cyclohydrolase I activity leading to an excessive de novo synthesis of (6R)5,6,7,8 tetrahydrobiopterin (6-BH4). Continuous production of 6-BH-4 leads to: (1) an accumulation of the non-enzymatic byproduct 7-tetrahydropterin (7-BH4) in the epidermis, and (2) increased synthesis of the catecholamines in keratinocytes, leading to an excess of norepinephrine in both the plasma and urine of these patients. In vitiligo, the time-dependent production of 7-BH4 is caused by decreased 4a-hydroxytetrahydrobiopterin dehydratase activity; the essential enzyme for recycling and maintaining normal levels of 6-BH-4. In the epidermis and in cultured melanocytes, 7-BH4 is a potent competitive inhibitor of phenylalanine hydroxylase (Ki = 10(-6) M) and its accumulation in the epidermis of patients with vitiligo blocks the supply of L-tyrosine from L-phenylalanine. 4a-hydroxytetrahydrobiopterin dehydratase has a dual function as the activator/dimerization catalyst for the transcription factor hepatocyte nuclear factor I (HNF-I). HNF-I binds to a 16-base inverted palindrome which seems to be present on the promoters of both the tyrosinase and phenylethanolamine-N-methyl transferase (PNMT) genes. Therefore, defective 4a-hydroxytetrahydrobiopterin dehydratase in vitiligo influences not only the supply of L-tyrosine but also the transcription of the tyrosinase gene in melanocytes. Furthermore, a similar transcriptional regulation of the PNMT gene in keratinocytes offers a possible explanation for the accumulation of norepinephrine in these patients.
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PMID:Defective tetrahydrobiopterin and catecholamine biosynthesis in the depigmentation disorder vitiligo. 820 66

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.
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PMID:Tyrosinase isoenzymes in mammalian melanocytes. 1. Biochemical characterization of two melanosomal tyrosinases from B16 mouse melanoma. 822 98

1. Intermediates in the process of melanin synthesis formed through oxidation of catechols by tyrosinase produced the inactivation of ornithine decarboxylase (ODC), a key enzyme in the polyamine biosynthesis pathway. 2. The inactivation was dependent on the substrate used (dihydroxybenzylamine > L-3,4-dihydroxyphenylalanine > L-tyrosine) and on the concentration of intermediate produced rather than on the rate of formation. 3. Sulfhydryl compounds (dithiothreitol and glutathione) or quinone-reducing agents (ascorbic acid) prevented the inactivation of ODC; L-ornithine, but not other amino acids, also protected partially ODC. The results suggest that different cysteine residues in ODC molecule are implicated in the inactivatory event. 4. When 14C-labeled catechols were used, numerous polypeptides resulted labeled, showing that the reactive quinones formed as intermediates in the process of melanin biosynthesis bind covalently to many cellular proteins.
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PMID:Inactivation of ornithine decarboxylase by intermediates of tyrosinase-catalyzed reaction. 846 26

It is proposed that the phenol oxidase enzyme of schistosomes and other trematodes has a crucial role in the cross-linking of precursor proteins in the formation of the eggshell. However, to date there is no direct evidence to show that the enzyme catalyzes the reactions necessary for the posttranslational modification of eggshell precursor proteins. In this report we demonstrate that an eggshell precursor protein acts as a substrate for a schistosome fraction that catalyzed the 2 steps in the oxidation of tyrosine. This action of the phenol oxidase-containing worm fraction resulted in the tyrosine-dependent insolubilization and aggregation of the protein, suggesting a role for the enzyme in the posttranslational modification and cross-linking of schistosome eggshell proteins. The enzyme-rich fraction from Schistosoma mansoni catalyzed both steps of the reactions necessary for the conversion of tyrosine residues on putative eggshell precursor protein (p48) to quinones. The parasite fraction also catalyzed the hydroxylation of free L-tyrosine to DOPA (monophenol oxidase activity) and the oxidation of L-DOPA to dopaquinone (diphenol oxidase activity). Both activities of the enzyme are avidly bound to membranous structures, are susceptible to agents known to inhibit a functionally related enzyme, tyrosinase, and may reside on the same protein.
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PMID:Characteristics of phenol oxidase of Schistosoma mansoni and its functional implications in eggshell synthesis. 850 88

Serum samples for the analysis of tyrosinase activity were obtained from 10 healthy subjects in autumn, winter and summer. Tyrosinase was purified from 100 microl serum by adsorption to concanavalin A sepharose, the tyrosinase adsorbed to the gel being separated from other components by centrifugation. The gel was suspended in a buffer containing 5-hydroxy-indole-3-acetic acid as an antioxidant and incubated for 2 min with L-cysteine and D-L-dopa at 37 degrees C. The 5-S-L-cysteinyl-L-dopa formed was measured by HPLC and electrochemical detection. Tyrosinase has high stereo-specificity for the L-enantiomer of dopa, and correction for non-specific oxidation was made by simultaneous measurement of 5-S-L-cysteinyl-D-dopa formed from D-dopa. Whereas the oxidation of L-dopa catalysed by tyrosinase ws inhibited by L-tyrosine, the non-specific oxidation of D-dopa was not. Mean serum tyrosinase activity was 0.9 nkatal/l in summer, 0.8 nkatal/l in autumn and 0.4 nkatal/l in winter. The range of tyrosinase activity was much higher in summer and autumn than in winter.
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PMID:Seasonal variation of tyrosinase activity in serum. 857 49

Arbutin, a naturally occurring beta-D-glucopyranoside of hydroquinone, is effective in the topical treatment of various cutaneous hyperpigmentations characterized by hyperactive melanocyte function. We examined the mechanism of its depigmenting action in human melanocyte cultures. Arbutin inhibited the tyrosinase activity of cultured human melanocytes at noncytotoxic concentrations. It did not affect the expression of tyrosinase mRNA. Melanin production was inhibited significantly by arbutin, as determined by measuring eumelanin radicals with an electron spin resonance spectrometer. The study of the kinetics and mechanism for inhibition of tyrosinase confirms the reversibility of arbutin as a competitive inhibitor of this enzyme. The utilization of L-tyrosine or L-dopa as the substrate suggests a mechanism involving competition with arbutin for the L-tyrosine binding site at the active site of tyrosinase. These results suggest that the depigmenting mechanism of arbutin in humans involves inhibition of melanosomal tyrosinase activity, rather than suppression of the expression and synthesis of tyrosinase.
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PMID:Arbutin: mechanism of its depigmenting action in human melanocyte culture. 863 48

In vitro experiments are reported showing that NAD(P)H:(quinone acceptor) oxidoreductase (QR), purified from Glycine max seedlings, reduces Leu- and Met-enkephalin-tyrosinase oxidation products, in the presence of NADH or NADPH. QR was not capable to catalyze the reduction of N-acetyl-dopaquinone formed by the cation of mushroom tyrosinase on N-acetyl-L-tyrosine, while it was able to reduce dopachrome. The results support the hypothesis that QR can inhibit the formation of melanin-like compounds, as catalyzed by the action of tyrosinase on Leu-enkephalin and Met-enkephalin. It is proposed that, in the presence of NAD(P)H as the electron donor, the inhibition occurs by the specific conversion of the dopachrome-derivative into the reduced precursor, leucodopachrome-derivative.
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PMID:Effect of NAD(P)H:quinone oxidoreductase on tyrosinase-mediated oxidation of opioid neuropeptides Leu-enkephalin and Met-enkephalin. 867 15

Planarians (Dugesia dorotocephala) were evaluated as bioassay organisms to detect inhibition of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of catecholamines. Thirty planaria per dose were exposed to 0 (control), 0.001, 0.01, 0.1, or 1 mM 3-iodo-L-tyrosine (monoiodotyrosine or MIT) in standard test media beginning 24 hr before decapitation and continuing for 13 days. Complete regeneration of normal heads occurred over the first 6 days in all dose groups, a response reported to be partially dependent on catecholamines. Beginning on Day 7, the black eye pigments began fading in the 0.1 and 1 mM dose groups and were completely absent macroscopically and histologically by Day 11. The 0, 0.001, and 0.01 mM dose groups did not lose visible eye pigments. On Day 13, 3 planaria/dose were harvested for histopathology; 15 planaria/dose were decapitated a second time and remained in MIT solutions; and 12 planaria/dose were left intact, placed in fresh control media, and evaluated for eye repigmentation. Normal head regeneration (including eyes) was detected grossly in all groups, even in those animals devoid of eye pigments at the time of decapitation. As before, eye pigments began fading 7 days after decapitation (Day 20 of experiment) and were completely absent in 73 and 33% of the animals in the 0.1 and 1 mM groups, respectively, on Day 25. All animals moved to control media reformed eye pigments, beginning within 48 hr. Analysis of the decapitated heads by HPLC-ECD on Day 13 revealed a significant decrease in dopamine and 5-hydroxytryptamine (serotonin) concentrations in MIT-exposed animals. Tyrosine hydroxylase activity (and possibly tyrosinase activity) was shown to be inhibited by the highest two concentrations for whole planaria homogenates in vitro.
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PMID:Effects of 3-iodo-L-tyrosine, a tyrosine hydroxylase inhibitor, on eye pigmentation and biogenic amines in the planarian, Dugesia dorotocephala. 881 61

Phenoloxidase (PO) activity in the albumen gland (AG) and egg masses (EM) of Biomphalaria glabrata was assessed using high-performance liquid chromatography combined with electrochemical detection and colorimetric techniques. Both AG and EM extracts catalyzed the hydroxylation of L-tyrosine (monophenol oxidase activity, MPO) and oxidation of L-dopa (diphenol oxidase activity, DPO). However, no PO activity was found in the ovotestis. Both MPO and DPO activities in AG and EM were significantly inhibited by 1-phenyl-2-thiourea and inactivated by boiling. Approximately 35% of MPO and 44% of DPO activities were detected in the soluble fraction of homogenized EM, in contrast to that of homogenized AG, which contained about 5% and 12%, respectively, of MPO and DPO activities. N-acetyl-dopamine, a diphenolic compound, enhanced the hydroxylation of tyrosine by the PO. The presence of both MPO and DPO activities also was confirmed by the accelerated accumulation of dopachrome during incubation of EM extracts with L-tyrosine in the absence of ascorbate. Temperature and pH optima for this enzyme were 30 degrees C and 7.5, respectively. The potential roles of PO in egg formation in B glabrata are discussed.
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PMID:Phenoloxidase activity in the reproductive system and egg masses of the pulmonate gastropod, Biomphalaria glabrata. 884 May 12


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