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

p-Coumaroyl-D-glucose hydroxylase in sweet potato (Ipomoea batatas Lam.) has been purified to apparent electrophoretic homogeneity using a combination of anion-and cation-exchange, hydrophobic and gel filtration chromatography. The purified enzyme was a monomer with a molecular weight of 33,000 and pI of 8.3. The purified enzyme showed not only hydroxylase activity but also polyphenol oxidase activity. L-Ascorbic acid was the best electron donor for the hydroxylation reaction, which had an optimum pH of 7.0. The enzyme hydroxylated p-coumaroyl-D-glucose, p-coumaric acid, and p-cresol but did not act on o-coumaric acid, m-coumaric acid, 4-hydroxy-3-methoxycinnamic acid, p-hydroxybenzoic acid or L-tyrosine. While the enzyme utilized p-coumaroyl-D-glucose and p-coumaric acid equally at pH 7.0, it hydroxylated only p-coumaroyl-D-glucose at pH 5.5. The enzyme oxidized diphenols such as D,L-(3,4-dihydroxyphenyl) alanine and caffeic acid, but exhibited no clear pH optimum in this reaction characteristic of polyphenol oxidase. Both the hydroxylase and the polyphenol oxidase activities were strongly inhibited by beta-mercaptoethanol, diethyldithiocarbamate, KCN, and p-coumaric acid (in concentrations higher than 5 mM). Ammonium sulfate and sodium chloride activated the hydroxylase activity but not the polyphenol oxidase activity of the enzyme. The enzyme activity and L-ascorbic acid contents changed in a manner suggesting their involvements in chlorogenic acid biosynthesis during incubation of sliced sweet potato root tissues.
 ...
PMID:Purification and characterization of p-coumaroyl-D-glucose hydroxylase of sweet potato (Ipomoea batatas) roots. 198 92

Certain mono- and dihydroxybenzene derivatives cause depigmentation of skin and hair, and appear to be selectively cytotoxic for melanized pigment cells. As direct physical and/or chemical interaction between depigmenter (DP) and pigment melanin may play a role in depigmentation, we have carried out preliminary studies in model systems where such interactions may easily be separated from effects due to tyrosinase, melanosomal proteins, and other components. We have used synthetic L-3,4-hydroxyphenylalanine (L-DOPA)-melanin as a protein-free model pigment and potassium ferricyanide as a model electron acceptor. Compounds studied were catechol, 4-t-butylcatechol, 4-methylcatechol, 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylacetic acid, hydroquinone, 4-methoxyphenol, 4-t-butylphenol, and 2,6, di-t-butyl-4-methylphenol (BHT) in 0.1 M phosphate buffer, pH 7.4. These compounds vary widely in their ability to depigment hair and skin. Ferricyanide reduction by DP in the presence and absence of melanin was monitored spectrophotometrically. The sparingly soluble BHT and 4-t-butylphenol did not reduce ferricyanide in the absence or presence of melanin. For the other compounds, kinetic analysis demonstrated direct interaction between each DP and melanin. Except for dihydroxyphenylacetic acid, reduction kinetics were consistent with a mechanism involving noninteractive binding of both DP and ferricyanide to melanin prior to coupled electron transfer through the melanin backbone. Kinetic analysis afforded KB, a thermodynamic constant (M-1) for DP-melanin binding, and k', a rate parameter (M s-1) for electron transfer. A dimensionless enhancement factor (EF) was defined as k'KB/ks, with ks a pseudo-first-order constant (s-1) for ferricyanide reduction in the absence of melanin. Depending on the reductant, melanin either retards (EF less than 1) or accelerates (EF greater than 1) the rate of ferricyanide reduction. There appears to be a direct relationship between EF and depigmenting potency. There is no relationship between depigmenting power and the ability per se of the DP to bind to melanin or to reduce ferricyanide.
 ...
PMID:Interaction of several mono- and dihydroxybenzene derivatives of various depigmenting potencies with L-3,4-dihydroxyphenylalanine-melanin. 308 Sep 56

A study has been made of the kinetics of cresolase and catecholase activities of tyrosinase on the p-cresol/4-methyl-catechol pair in the presence of L-serine. For this, a spectrophotometer assay for both activities based on the spectrophotometric and stoichiometric characteristics of the chemical reactions in the evolution of 4-methyl-o-benzoquinone is proposed. The presence of L-serine in the reaction medium caused a modification in the lag period and the steady-state rate expressed during the cresolase activity of tyrosinase, but no modification was observed during the expression of catecholase activity. These results can be explained taking into account the complex mechanism proposed for tyrosinase which included the chemical steps involved in the process. Furthermore, a singular form of regulation of enzymatic activity by L-serine has been clarified, not by any direct interaction between the protein molecule and the nucleophile, but by modification of the chemical reactions involved in the mechanism of tyrosinase.
 ...
PMID:Enzymatic oxidation by frog epidermis tyrosinase of 4-methylcatechol and p-cresol. Influence of L-serine. 311 38

Microporous membranes were used as a support material for enzyme immobilization by a supported aqueous-phase. To test the concept, a model reaction was chosen involving the oxidation of p-cresol by tyrosinase. Tyrosinase was first immobilized in a thin film of water formed on the inner surface of the membrane and then allowed to catalyze p-cresol oxidation in chloroform. By choosing optimal operating conditions, tyrosinase functioned catalytically for more than 6 hours with a stable reaction rate. The reaction rate was highly dependent on water content (water wt./enzyme wt. ratio) and permeation flux. Also, enzyme loading was an important factor for maintaining stable activity. This type of high-rate reactor utilized convective flow through an enzyme immobilized microporous membrane and provided high productivity by reducing mass transport limitations.
 ...
PMID:High-rate membrane supported aqueous-phase enzymatic conversion in organic solvent. 776 81

A substrate recycling assay for phenolic compounds was developed using tyrosinase, a copper-containing enzyme, in excess NADH. The reaction of various phenols with the enzyme produced an o-quinone, which was then detected by recycling between reactions with the enzyme and NADH. The recycling of quinones by excess NADH to their original reduced forms prevented the problems of subsequent quinone polymerization and product inactivation which occur in nonrecycling assays. Absorbance measurements of the NADH consumption rate enhanced the assay sensitivity for catechol 100-fold compared to nonrecycling o-quinone detection, giving a detection limit of 240 nM. Fluorescence NADH monitoring permitted a 10-fold improvement over absorbance, with a detection limit of 23 nM. The recycling reaction was selective for o-quinones, and no interference was noted for p-quinones or quinoneimines. The two-step oxidation of phenols was observed as an initial lag phase (ca. 10 min), requiring a higher enzyme concentration to achieve the same sensitivity as that for catechol. The procedure was most useful for assaying catechol, 4-chlorocatechol, phenol, p-cresol, and 4-chlorophenol and may provide selective detection of these components in mixtures. Several other derivatives of catechols, including amine derivatives, were also detected, with relative sensitivity being related to substrate activity of the enzyme.
 ...
PMID:A substrate recycling assay for phenolic compounds using tyrosinase and NADH. 785 38

A 4-coumaroyl-CoA 3-hydroxylase activity was purified 4600-fold from cell cultures of Lithospermum erythrorhizon. The enzyme showed a molecular mass of 42,400 +/- 1700 Da in gel chromatography and required ascorbate, NADH, or NADPH as cofactors. 4-Coumaroyl-CoA, 4-coumarate, p-cresol, and several other phenolic substances, but not tyrosine, were accepted as substrates for the hydroxylation. Besides hydroxylase activity, the enzyme showed diphenol oxidase activity. Both activities were inhibited by diethyldithiocarbamate or beta-mercaptoethanol, although at different concentrations. The enzyme showed striking similarity to a 4-coumaroyl-glucose 3-hydroxylase from sweet potato (Ipomoe batatas) roots, which has reportedly been purified to homogeneity and identified as a specific enzyme of chlorogenic acid biosynthesis. Close examination and comparison to a commercially available polyphenol oxidase, however, suggest that the enzyme activities purified from both Lithospermum and sweet potato are polyphenol oxidases rather than specific enzymes of secondary metabolism.
 ...
PMID:4-Coumaroyl coenzyme A 3-hydroxylase activity from cell cultures of Lithospermum erythrorhizon and its relationship to polyphenol oxidase. 936 32

The dinucleating ligand 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (H-BPMP) has been used to synthesize the three dinuclear Cu(II) complexes [Cu2(BPMP)(OH)][ClO4](2).0.5C4H8O (1), [Cu2(BPMP)(H2O)2](ClO4)(3).4H2O (2), and [Cu2(H-BPMP)][(ClO4)4].2CH3CN (3). X-ray diffraction studies reveal that 1 is a mu-hydroxo, mu-phenoxo complex, 2 a diaqua, mu-phenoxo complex, and 3 a binuclear complex with Cu-Cu distances of 2.96, 4.32, and 6.92 A, respectively. Magnetization measurements reveal that 1 is moderately antiferromagnetically coupled while 2 and 3 are essentially uncoupled. The electronic spectra in acetonitrile or in water solutions give results in accordance with the solid-state structures. 1 is EPR-silent, in agreement with the antiferromagnetic coupling between the two copper atoms. The X-band spectrum of powdered 2 is consistent with a tetragonally elongated square pyramid geometry around the Cu(II) ions, in accordance with the solid-state structure, while the spectrum in frozen solution suggests a change in the coordination geometry. The EPR spectra of 3 corroborate the solid-state and UV-visible studies. The 1H NMR spectra also lead to observations in accordance with the conclusions from other spectroscopies. The electrochemical behavior of 1 and 2 in acetonitrile or in water solutions shows that the first reduction (Cu(II)Cu(II)-Cu(II)Cu(I) redox couple) is reversible and the second (formation of Cu(I)Cu(I) irreversible. In water, 1 and 2 are reversibly interconverted upon acid/base titration (pK 4.95). In basic medium a new species, 4, is reversibly formed (pK 12.0), identified as the bishydroxo complex. Only 1 exhibits catecholase activity (oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone, vmax = 1.1 x 10(-6) M-1 s-1 and KM = 1.49 mM). The results indicate that the pH dependence of the catalytic abilities of the complexes is related to changes in the coordination sphere of the metal centers.
 ...
PMID:pH-controlled change of the metal coordination in a dicopper(II) complex of the ligand H-BPMP: crystal structures, magnetic properties, and catecholase activity. 1119 11

An amperometric tyrosinase enzyme electrode for the determination of phenols was developed by a simple and effective immobilization method using sol-gel techniques. A grafting copolymer was introduced into sol-gel solution and the composition of the resultant organic-inorganic composite material was optimized, the tyrosinase retained its activity in the sol-gel thin film and its response to several phenol compounds was determined at 0 mV vs. Ag/AgCl (sat. KCl). The dependences of the current response on pH, oxygen level and temperature were studied, and the stability of the biosensor was also evaluated. The sensitivity of the biosensor for catechol, phenol and p-cresol was 59.6, 23.1 and 39.4 microA/mM, respectively. The enzyme electrode maintained 73% of its original activity after intermittent use for three weeks when storing in a dry state at 4 degrees C.
 ...
PMID:Silica sol-gel composite film as an encapsulation matrix for the construction of an amperometric tyrosinase-based biosensor. 1121 53

Phenols are important industrial chemicals, and because they can be volatile, also appear as air pollutants. We examined the potential of tyrosinase to react with the volatile phenol p-cresol. Three lines of evidence support the conclusion that volatile phenols react with tyrosinase and are coupled (i.e., chemisorbed) onto chitosan films. First, phenol-trapping studies indicated that p-cresol can be removed from vapors if the vapors are contacted with tyrosinase-coated chitosan films. Second, the ultraviolet absorbance of tyrosinase-coated chitosan films changes dramatically when they are contacted with cresol-containing vapors, whereas control films are unaffected by contacting with cresol vapors. Third, pressure measurements indicate that tyrosinase-coated chitosan films only react with cresol vapors if the oxygen cosubstrate is present. Additional studies demonstrate the potential of tyrosinase-coated chitosan films/membranes for the detection and removal of phenol vapors.
 ...
PMID:Enzymatic coupling of phenol vapors onto chitosan. 1174 60

The development and optimization of an analytical method using enzymatic biosensors able to operate in organic solvents [organic phase enzyme electrodes (OPEEs)] for the determination of the water content in food fats (butter, margarine) or pharmaceutical or cosmetic ointments is described. The method is based on the increase in enzymatic activity which is related to the increase in the percentage water content in the organic phase into which the biosensor is dipped. The enzymes used to assemble the biosensors were tyrosinase or catalase, the substrates were phenol or p-cresol and tert-butyl hydroperoxide, respectively, and the organic solvents were acetonitrile or dioxane. A gas diffusion amperometric electrode for oxygen measurement was used as electrochemical transducer. The results were compared with those obtained applying the Karl Fischer method to the same food or drug matrices. The correlations among the two methods proved satisfactory, as the difference in the computed values of water content was never higher than 7%. Also, the precision of measurements was acceptable (RSD < 6%) in all the analyses of real matrices.
 ...
PMID:Two OPEEs (organic phase enzyme electrodes) used to check the percentage water content in hydrophobic foods and drugs. 1176 68


1 2 3 4 5 Next >>