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Liu Y, Zhao Y, Chen H, Wang W, Zou M. Isolation, purification, and biological activities of polysaccharides from Amorpha fruticosa flowers. Nat Prod Res 2024:1-8. [PMID: 38684011 DOI: 10.1080/14786419.2024.2346272] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2024] [Accepted: 04/17/2024] [Indexed: 05/02/2024]
Abstract
The extraction, isolation, structural characterisation and biological activities of polysaccharides from Amorpha fruticosa flowers were investigated. First, the crude polysaccharide AFP was extracted, and two major purified polysaccharide fractions AFP-2 and AFP-3 were isolated. The molecular weight and monosaccharide compositions of AFP-2 and AFP-3 were determined. Then the antioxidant activities of AFP, AFP-2 and AFP-3 were assessed by DPPH radical, β-Carotene bleaching and hydroxyl radical assays. All three tested polysaccharides showed good antioxidant activity while AFP was the strongest one. The study also showed that AFP, AFP-2 and AFP-3 have good tyrosinase inhibition, moisture absorption and retention activities. The results will provide a helpful reference for the application of polysaccharide from Amorpha fruticosa flowers as a natural cosmetic ingredient.
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Affiliation(s)
- Yixian Liu
- Henan Institute of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China
| | - Ying Zhao
- Henan Institute of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China
| | - Huiping Chen
- Henan Institute of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China
| | - Weidong Wang
- Henan Institute of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China
| | - Min Zou
- Henan Institute of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China
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KAPLAN A. Investigation of in vitro Enzyme Inhibitory Properties and Antioxidant Activity of Moltkia coerulea (Willd.) Lehm. (Boraginaceae) Growing in Raman Mountain - Batman. INTERNATIONAL JOURNAL OF SECONDARY METABOLITE 2021. [DOI: 10.21448/ijsm.950087] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022] Open
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Maria-Solano MA, Ortiz-Ruiz CV, Muñoz-Muñoz JL, Teruel-Puche JA, Berna J, Garcia-Ruiz PA, Garcia-Canovas F. Further insight into the pH effect on the catalysis of mushroom tyrosinase. ACTA ACUST UNITED AC 2016. [DOI: 10.1016/j.molcatb.2015.12.008] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Action of ellagic acid on the melanin biosynthesis pathway. J Dermatol Sci 2016; 82:115-22. [PMID: 26899308 DOI: 10.1016/j.jdermsci.2016.02.004] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2015] [Revised: 01/07/2016] [Accepted: 02/10/2016] [Indexed: 11/23/2022]
Abstract
BACKGROUND Tyrosinase is an enzyme involved in the first steps of the melanogenesis process. It catalyzes the hydroxylation of monophenols to o-diphenols and the oxidation of the latter to o-quinones. Ellagic acid (EA) is a phenolic compound which has been described as a tyrosinase inhibitor and is used in the cosmetic industry as a whitening agent. However, it has hydroxyl groups in ortho position and could act as a substrate rather than inhibitor. This aspect should be taken into consideration when using this compound as a cosmetic ingredient due to the reactive character of o-quinones. OBJECTIVE To determine whether ellagic acid is a substrate or an inhibitor of tyrosinase, to characterize it kinetically and interpret its role in the melanogenesis process. METHODS UV-vis spectrophotometry was used to follow the action of tyrosinase on typical substrates and ellagic acid. A chronometric method was chosen for the kinetic characterization of ellagic acid. RESULTS Ellagic acid is not an inhibitor per se but an alternative substrate of tyrosinase. It is oxidized by the enzyme to an unstable o-quinone. Its kinetic characterization provided low Michaelis and catalytic constants (KM(EA)=138±13μM and kcat(EA)=0.47±0.02s(-1)). Furthermore, ellagic acid, which is a powerful antioxidant, may chemically reduce the o-quinones (o-dopaquinone) and semiquinones, in this way inhibiting the melanogenesis. CONCLUSION Ellagic acid is oxidized by tyrosinase, producing reactive o-quinones. As an antioxidant it can inhibit the melanogenesis process. This first aspect should be taken into consideration in its application as a cosmetic ingredient due to the toxicity of o-quinones and its ability to modify the redox status of the cell.
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Ortiz-Ruiz CV, Garcia-Molina MDM, Serrano JT, Tomas-Martinez V, Garcia-Canovas F. Discrimination between alternative substrates and inhibitors of tyrosinase. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY 2015; 63:2162-2171. [PMID: 25665009 DOI: 10.1021/jf5051816] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Many phenolic compounds have been described in the scientific literature as inhibitors of tyrosinase. In this work a test is proposed that allows us to distinguish whether a molecule is an enzyme inhibitor or substrate. The test has several stages. First, the degree of inhibition of the studied molecule is determined on the monophenolase activity (i(M)) and on the diphenolase activity (i(D)). If i(M) = i(D), it is an inhibitor. If i(M) ≠ i(D), the molecule could be substrate or inhibitor. Several additional stages are proposed to solve this ambiguity. The study described herein was carried out using the following molecules: benzoic acid, cinnamic acid, guaiacol, isoeugenol, carvacrol, 4-tert-butylphenol, eugenol, and arbutin.
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Affiliation(s)
- Carmen Vanessa Ortiz-Ruiz
- GENZ-Group of Research on Enzymology, Department of Biochemistry and Molecular Biology-A, Regional Campus of International Excellence "Campus Mare Nostrum", University of Murcia , 0100 Espinardo, Murcia, Spain
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Manivasagan P, Venkatesan J, Sivakumar K, Kim SK. Actinobacterial melanins: current status and perspective for the future. World J Microbiol Biotechnol 2013; 29:1737-50. [PMID: 23595596 DOI: 10.1007/s11274-013-1352-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2013] [Accepted: 04/11/2013] [Indexed: 01/18/2023]
Abstract
Melanins are enigmatic pigments that are produced by a wide variety of microorganisms including several species of bacteria and fungi. Melanins are biological macromolecules with multiple important functions, yet their structures are not well understood. Melanins are frequently used in medicine, pharmacology, and cosmetics preparations. Melanins also have great application potential in agriculture industry. They have several biological functions including photoprotection, thermoregulation, action as free radical sinks, cation chelators, and antibiotics. Plants and insects incorporate melanins as cell wall and cuticle strengtheners, respectively. Actinobacteria are the most economically as well as biotechnologically valuable prokaryotes. However, the melanin properties are, in general, poorly understood. In this review an evaluation is made on the present state of research on actinobacterial melanins and its perspectives. The highlights include the production and biotechnological applications of melanins in agriculture, food, cosmetic and medicinal fields. With increasing advancement in science and technology, there would be greater demands in the future for melanins produced by actinobacteria from various sources.
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Affiliation(s)
- Panchanathan Manivasagan
- Marine Biotechnology Laboratory, Department of Chemistry and Marine Bioprocess Research Center, Pukyong National University, Busan, 608-737, Republic of Korea,
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Tyrosinase Inhibitory Activity, 3D QSAR, and Molecular Docking Study of 2,5-Disubstituted-1,3,4-Oxadiazoles. J CHEM-NY 2013. [DOI: 10.1155/2013/849782] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
In continuation with our research program, in search of potent enzyme tyrosinase inhibitor, a series of synthesized 2,5-disubstituted 1,3,4-oxadiazoles have been evaluated for enzyme tyrosinase inhibitory activity. Subsequently, 3D QSAR and docking studies were performed to find optimum structural requirements for potent enzyme tyrosinase inhibitor from this series. The synthesized 20 compounds of 2,5-disubstituted-1,3,4-oxadiazole series were screened for mushroom tyrosinase inhibitory activity at various concentrations by enzyme inhibition assay. The percentage enzyme inhibition was calculated by recording absorbance at 492 nm with microplate reader. 3D QSAR and docking studies were performed using VLife MDS 3.5 software. In the series 2,5-disubstituted-1,3,4-oxadiazoles enzyme tyrosinase inhibitory activity was found to be dose dependent with maximum activity for compounds4c,4h,4m, and4r. 3D QSAR and docking studies revealed that more electropositive and less bulky substituents if placed on 1,3,4-oxadiazole nucleus may result in better tyrosinase inhibitory activity in the series.
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Extracellular tyrosinase from the fungus Trichoderma reesei shows product inhibition and different inhibition mechanism from the intracellular tyrosinase from Agaricus bisporus. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2012; 1824:598-607. [DOI: 10.1016/j.bbapap.2011.12.012] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/28/2011] [Revised: 12/20/2011] [Accepted: 12/23/2011] [Indexed: 11/18/2022]
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Lebedev AV, Ivanova MV, Ruuge EK. Calcium-dioxolene complexes: Rate constants of pyrocatechol oxidation in the presence of Ca2+. Biophysics (Nagoya-shi) 2011. [DOI: 10.1134/s0006350911020187] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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Muñoz-Muñoz J, García-Molina F, García-Ruiz P, Arribas E, Tudela J, García-Cánovas F, Rodríguez-López J. Enzymatic and chemical oxidation of trihydroxylated phenols. Food Chem 2009. [DOI: 10.1016/j.foodchem.2008.07.076] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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11
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Stereospecific inactivation of tyrosinase by l- and d-ascorbic acid. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2009; 1794:244-53. [DOI: 10.1016/j.bbapap.2008.10.002] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/27/2008] [Revised: 09/30/2008] [Accepted: 10/16/2008] [Indexed: 11/18/2022]
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12
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Munoz-Munoz JL, García-Molina F, Molina-Alarcón M, Tudela J, García-Cánovas F, Rodríguez-López JN. Kinetic characterization of the enzymatic and chemical oxidation of the catechins in green tea. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY 2008; 56:9215-9224. [PMID: 18788750 DOI: 10.1021/jf8012162] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
The oxidation of green tea catechins by polyphenol oxidase/O2 and peroxidase/H2O2 gives rise to o-quinones and semiquinones, respectively, which inestability, until now, have hindered the kinetic characterization of enzymatic oxidation of the catechins. To overcome this problem, ascorbic acid (AH2) was used as a coupled reagent, either measuring the disappearance of AH2 or using a chronometric method in which the time necessary for a fixed quantity of AH2 to be consumed was measured. In this way, it was possible to determine the kinetic constants characterizing the action of polyphenol oxidase and peroxidase toward these substrates. From the results obtained, (-) epicatechin was seen to be the best substrate for both enzymes with the OH group of the C ring in the cis position with respect to the B ring. The next best was (+) catechin with the OH group of the C ring in the trans position with respect to the B ring. Epigallocatechin, which should be in first place because of the presence of three vecinal hydroxyls in its structure (B ring), is not because of the steric hindrance resulting from the hydroxyl in the cis position in the C ring. The epicatechin gallate and epigallocatechin gallate are very poor substrates due to the presence of sterified gallic acid in the OH group of the C ring. In addition, the production of H2O2 in the auto-oxidation of the catechins by O2 was seen to be very low for (-) epicatechin and (+) catechin. However, its production from the o-quinones generated by oxidation with periodate was greater, underlining the importance of the evolution of the o-quinones in this process. When the [substrate] 0/[IO4 (-)] 0 ratio = 1 or >>1, H2O2 formation increases in cases of (-) epicatechin and (+) catechin and practically is not affected in cases involving epicatechin gallate, epigallocatechin, or epigallocatechin gallate. Moreover, the antioxidant power is greater for the gallates of green tea, probably because of the greater number of hydroxyl groups in its structure capable of sequestering and neutralizing free radicals. Therefore, we kinetically characterized the action of polyphenol oxidase and peroxidase on green tea catechins. Furthermore, the formation of H2O2 during the auto-oxidation of these compounds and during the evolution of their o-quinones is studied.
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Affiliation(s)
- J L Munoz-Munoz
- GENZ, Grupo de Investigacion de Enzimologia, Departamento de Bioquimica y Biologia Molecular-A, Facultad de Biologia, Universidad de Murcia, E-30100 Espinardo, Murcia, Spain
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Roy S. Melanin, melanogenesis, and vitiligo. FORTSCHRITTE DER CHEMIE ORGANISCHER NATURSTOFFE = PROGRESS IN THE CHEMISTRY OF ORGANIC NATURAL PRODUCTS. PROGRES DANS LA CHIMIE DES SUBSTANCES ORGANIQUES NATURELLES 2007; 88:131-85. [PMID: 17302180 DOI: 10.1007/978-3-211-49389-2_3] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Affiliation(s)
- Shyamali Roy
- Institute of Natural Products, 8, J. N. Roy Lane, Kolkata 700006, India.
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Parvez S, Kang M, Chung HS, Bae H. Naturally occurring tyrosinase inhibitors: mechanism and applications in skin health, cosmetics and agriculture industries. Phytother Res 2007; 21:805-16. [PMID: 17605157 DOI: 10.1002/ptr.2184] [Citation(s) in RCA: 255] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Tyrosinase is a copper-containing enzyme, which is widely distributed in microorganisms, animals and plants and is a key enzyme in melanin biosynthesis, involved in determining the color of mammalian skin and hair. In addition, unfavorable enzymatic browning of plant-derived foods by tyrosinase causes a decrease in nutritional quality and economic loss of food products. The inadequacy of current conventional methods to prevent tyrosinase action encourages researchers to seek new potent tyrosinase inhibitors for food and cosmetics. This article presents a study on the importance of tyrosinase, biochemical characteristics, type of inhibitions, activators from various natural sources with its clinical and industrial importance in recent prospects is discussed in this paper.
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Affiliation(s)
- Shoukat Parvez
- National Institute for Biotechnology and Genetic Engineering (NIBGE), P.O. 577, Faisalabad, Pakistan.
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Molina FG, Muñoz JL, Varón R, López JNR, Cánovas FG, Tudela J. An approximate analytical solution to the lag period of monophenolase activity of tyrosinase. Int J Biochem Cell Biol 2006; 39:238-52. [PMID: 17010655 DOI: 10.1016/j.biocel.2006.08.007] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2006] [Revised: 07/25/2006] [Accepted: 08/12/2006] [Indexed: 11/17/2022]
Abstract
Tyrosinase shows a lag period in its action on monophenols (l-tyrosine). We propose an approximate analytical solution for the lag period, which fulfils the dependences with regard to initial enzyme concentration, and initial monophenol concentration. Furthermore, from a study of the dependences of the lag period on these variables, we can determine experimentally the o-diphenol concentration in the steady state. The Michaelis constant of the o-diphenol in the presence of the monophenol can be determined from the relationship between the o-diphenol concentration in the steady state and the initial monophenol concentration, taking into consideration the experimentally calculated Michaelis constant for the monophenol substrate. Although this Michaelis constant is much lower than the Michaelis constant for diphenol in the absence of monophenol, the binding site is the same. A kinetic analysis of the action mechanism of tyrosinase explains this difference in the values of the Michaelis constants.
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Affiliation(s)
- F García Molina
- GENZ: Grupo de Investigación de Enzimología, Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, E-30100 Espinardo, Murcia, Spain
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Muñoz JL, García-Molina F, Varón R, Rodriguez-Lopez JN, García-Cánovas F, Tudela J. Calculating molar absorptivities for quinones: Application to the measurement of tyrosinase activity. Anal Biochem 2006; 351:128-38. [PMID: 16476401 DOI: 10.1016/j.ab.2006.01.011] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2005] [Revised: 12/22/2005] [Accepted: 01/06/2006] [Indexed: 11/24/2022]
Abstract
The molar absorptivities of the quinones produced from different o-diphenols, triphenols, and flavonoids were calculated by generating the respective quinones through oxidation with an excess of periodate. Oxidation of these substrates by this reagent was analogous to oxidation by tyrosinase with molecular oxygen, although the procedure showed several advantages over the enzymatic method in that oxidation took place almost immediately and quinone stability was favored because no substrate remained. The o-diphenols studied were pyrocatechol, 4-methylcatechol, 4-tert-butylcatechol, 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenylethylamine, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylpropionic acid, and caffeic acid; the triphenols studied were pyrogallol, 1,2,4-benzenetriol, 6-hydroxydopa, and 6-hydroxydopamine; and the flavonoids studied were (+)catechin, (-)epicatechin, and quercetin. In addition, the stability of the quinones generated by oxidation of the compounds by [periodate]0/[substrate]0 << 1 was studied. Taking the findings into account, tyrosinase could be measured by following o-quinone formation in rapid kinetic studies using the stopped-flow method. However, measuring o-quinone formation could not be useful for steady-state studies. Therefore, several methods for following tyrosinase activity are proposed, and a kinetic characterization of the enzyme's action on these substrates is made.
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Affiliation(s)
- J L Muñoz
- Grupo de Investigación de Enzimología (GENZ), Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, E-30100 Espinardo, Murcia, Spain
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Sarangarajan R, Apte SP. Melanin Aggregation and Polymerization: Possible Implications in Age-Related Macular Degeneration. Ophthalmic Res 2005; 37:136-41. [PMID: 15867475 DOI: 10.1159/000085533] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2004] [Accepted: 01/18/2005] [Indexed: 11/19/2022]
Abstract
The state of aggregation of the polymer melanin may determine its propensity to act either as an antioxidant or as a pro-oxidant. Age-related alterations in its state of aggregation are suggested to alter the degree of polymerization so as to confer increased pro-oxidant propensity to the melanin polymer. Degradative processes in/of melanosomes and lysosomes in the retinal pigment epithelium (RPE) appear to be intimately connected so that they may involve exchange of contents between these two organelles. An increased pro-oxidant environment inside lysosomes has been associated with preventing the digestion of cellular components including photoreceptor outer rod segments partly by altering function of lysosomal hydrolases. It is speculated that age-related accumulation of low-molecular-weight phototoxic pro-oxidant melanin oligomers within lysosomes in the RPE may be partly responsible for decreasing the digestive rate of incorporated cellular components (including photoreceptor outer rod segments) which may lead to lipofuscin formation. More work is required to definitively refute or support such a hypothesis.
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Affiliation(s)
- Rangaprasad Sarangarajan
- Department of Phamaceutical Sciences, Massachusetts College of Pharmacy and Health Sciences, Worcester, MA, USA
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Fenoll LG, Peñalver MJ, Rodríguez-López JN, Varón R, García-Cánovas F, Tudela J. Tyrosinase kinetics: discrimination between two models to explain the oxidation mechanism of monophenol and diphenol substrates. Int J Biochem Cell Biol 2004; 36:235-46. [PMID: 14643889 DOI: 10.1016/s1357-2725(03)00234-6] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The kinetic behaviour of tyrosinase is very complex because the enzymatic oxidation of monophenol and o-diphenol to o-quinones occurs simultaneously with the coupled non-enzymatic reactions of the latter. Both reaction types are included in the kinetic mechanism proposed for tyrosinase (Mechanism I [J. Biol. Chem. 267 (1992) 3801-3810]). We previously confirmed the validity of the rate equations by the oxidation of numerous monophenols and o-diphenols catalysed by tyrosinase from different fruits and vegetables. Other authors have proposed a simplified reaction mechanism for tyrosinase (Mechanism II [Theor. Biol. 203 (2000) 1-12]), although without deducing the rate equations. In this paper, we report new experimental work that provides the lag period value, the steady-state rate, o-diphenol concentration released to the reaction medium. The contrast between these experimental data and the respective numerical simulations of both mechanisms demonstrates the feasibility of Mechanism I. The need for the steps omitted from Mechanism II to interpret the experimental data for tyrosinase, based on the rate equations previously deduced for Mechanism I is explained.
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Affiliation(s)
- L G Fenoll
- GENZ-Grupo de Investigación Enzimologi;a, Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, A. Correos 4021 E-30080, Murcia, Spain
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Kim YJ, Chung JE, Kurisawa M, Uyama H, Kobayashi S. New Tyrosinase Inhibitors, (+)-Catechin−Aldehyde Polycondensates. Biomacromolecules 2004; 5:474-9. [PMID: 15003008 DOI: 10.1021/bm034320x] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
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.
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Affiliation(s)
- Young-Jin Kim
- Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
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Zhang F, Bi S, Liu J, Wang X, Yang X, Yang L, Yu Q, Hu J, Bai Z. ELECTROCHEMICAL AND SPECTROMETRIC STUDIES ON THE PRINCIPLE OF INDIRECT DETER-MINATION OF ALUMINUM USINGl-DOPA AS AN ELECTROACTIVE COMPLEXING LIGAND. ANAL LETT 2002. [DOI: 10.1081/al-120002367] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
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Gidanian S, Farmer PJ. Redox behavior of melanins: direct electrochemistry of dihydroxyindole-melanin and its Cu and Zn adducts. J Inorg Biochem 2002; 89:54-60. [PMID: 11931963 DOI: 10.1016/s0162-0134(01)00405-6] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Synthetic melanin films, formed on electrode surfaces by oxidative polymerization of 5,6-dihydroxyindole solution, were used to directly measure the chromophore's redox reactivity. Films on optically transparent indium-tin oxide (ITO) electrodes allow correlation of spectral changes with electrochemical potential. Spectroelectrochemical titrations show an initial reversible transformation that is ascribed to formation of a unique quinone-imine chromophore. The apparent E(1/2) for maximum quinone-imine formation is approximately 125 mV (vs. Ag/AgCl) but at potentials higher than 100 mV, an irreversible bleaching is evident. Correlation of the current with the monomer concentration implies that only one in six monomers is oxidized to the quinone-imine before the irreversible bleaching occurs. Films pretreated with CuCl(2) and Zn(CH(3)COO)(2) show elevated quinone-imine absorbances, even under reducing conditions, indicating a preferential stabilization of this state by coordination to the metals.
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Affiliation(s)
- Shirley Gidanian
- Department of Chemistry, University of California, Irvine 92697-2025, USA
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Szpoganicz B, Gidanian S, Kong P, Farmer P. Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions. J Inorg Biochem 2002; 89:45-53. [PMID: 11931962 DOI: 10.1016/s0162-0134(01)00406-8] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Melanins are colloidal pigments known to have a high affinity for metal ions. In this work, the nature of the metal-binding sites are determined and the binding affinities are quantified. Initial potentiometric titrations have been performed on synthetic dihydroxyindole (DHI) melanin solutions to determine the chemical speciation of quinole/quinone subunits. Two types of acidic functionalities are assignable: catechol groups, with pK(a) between 9 and 13, and quinone imines (QI), with pK(a) of 6.3. The presence of the quinone-imine tautomer has, to our knowledge, never been assessed in polymeric melanins. Melanin solutions obtained from N-methylated DHI lack the pK(a) 6.3 buffer, consistent with its inability to form the quinone-imine tautomer. EPR spectroscopy of the DHI-melanin samples demonstrates that the semiquinone radical is in too low a concentration to contribute to the bulk binding of metals. Changes in the titration curves after addition of Cu(II) and Zn(II) ions were analyzed to obtain the binding constants and stoichiometry of the metal-melanin complexes, using the BEST7 program. UV-Vis spectra at neutral and high pH are used to identify absorbances due to Cu-bound quinone imine and catechol groups. The derived binding constants were used to determine speciation of the Cu(II) and Zn(II) ions coordinated to the quinone imine and catechol groups at various pH. The mixed complexes, Zn(QI)(Cat)(-) and Cu(QI)(Cat)(-) are shown to dominate at physiological pH.
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Affiliation(s)
- Bruno Szpoganicz
- Departamento de Química, Universidade Federal de Santa Catarina, SC 88040-900, Florianopolis, Brazil
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Rodríguez-López JN, Fenoll LG, Peñalver MJ, García-Ruiz PA, Varón R, Martínez-Ortíz F, García-Cánovas F, Tudela J. Tyrosinase action on monophenols: evidence for direct enzymatic release of o-diphenol. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1548:238-56. [PMID: 11513969 DOI: 10.1016/s0167-4838(01)00237-0] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
Using gas chromatography-mass spectrometry, the direct enzymatic release of o-diphenol (4-tert-butylcatechol) during the action of tyrosinase on a monophenol (4-tert-butylphenol) has been demonstrated for the first time in the literature. The findings confirm the previously proposed mechanism to explain the action of tyrosinase on monophenols (J.N. Rodríguez-López, J. Tudela, R. Varón, F. García-Carmona, F. García-Cánovas, J. Biol. Chem. 267 (1992)). Oxytyrosinase, the oxidized form of the enzyme with a peroxide group, is the only form capable of catalysing the transformation of monophenols into diphenols, giving rise to an enzyme-substrate complex in the process. The o-diphenol formed is then released from the enzyme-substrate complex or oxidized to the corresponding o-quinone. In order to detect the enzymatic release of o-diphenol, the non-enzymatic evolution of the o-quinone to generate o-diphenol by weak nucleophilic attack reactions and subsequent oxidation-reduction was blocked by the nucleophilic attack of an excess of cysteine. Furthermore, the addition of catalytic quantities of an auxiliary o-diphenol (e.g. catechol) considerably increases the accumulation of 4-tert-butylcatechol. The enzyme acting on 4-tert-butylphenol generates the enzyme-4-tert-butylcatechol complex and 4-tert-butylcatechol is then released (with k(-2)) generating mettyrosinase. The auxiliary o-diphenol added (catechol) and the 4-tert-butylcatechol generated by the enzyme then enter into competition. When [catechol] >> [4-tert-butylcatechol], the enzyme preferentially binds with the catechol to close the catalytic cycle, while 4-tert-butylcatechol is accumulated in the medium. In conclusion, we demonstrate that the enzyme produces 4-tert-butylcatechol from 4-tert-butylphenol, the concentration of which increases considerably in the presence of an auxiliary o-diphenol such as catechol.
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Affiliation(s)
- J N Rodríguez-López
- Grupo Investigación Enzimologia, Departmento de Bioquímica y Biologia Molecular-A, Facultad de Biologia, Univesidad de Murcia, Spain
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Fenoll LG, Rodríguez-López JN, García-Sevilla F, García-Ruiz PA, Varón R, García-Cánovas F, Tudela J. Analysis and interpretation of the action mechanism of mushroom tyrosinase on monophenols and diphenols generating highly unstable o-quinones. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1548:1-22. [PMID: 11451433 DOI: 10.1016/s0167-4838(01)00207-2] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Tyrosinase can act on monophenols because of the mixture of met- (E(m)) and oxy-tyrosinase (E(ox)) which exists in the native form of the enzyme. The latter form is active on monophenols, while the former is not. However, the kinetics are complicated because monophenols can bind to both enzyme forms. This situation becomes even more complex since the products of the enzymatic reaction, the o-quinones, are unstable and continue evolving to generate o-diphenols in the medium. In the case of substrates such as L-tyrosine, tyrosinase generates very unstable o-quinones, in which a process of cyclation and subsequent oxidation-reduction generates o-diphenol through non-enzymatic reactions. However, the release of o-diphenol through the action of the enzyme on the monophenol contributes to the concentration of o-diphenol in the first pseudo-steady-state [D(0)](ss). Hence, the system reaches an initial pseudo-steady state when t-->0 and undergoes a transition phase (lag period) until a final steady state is reached when the concentration of o-diphenol in the medium reaches the concentration of the final steady state [D(f)](ss). These results can be explained by taking into account the kinetic and structural mechanism of the enzyme. In this, tyrosinase hydroxylates the monophenols to o-diphenols, generating an intermediate, E(m)D, which may oxidise the o-diphenol or release it directly to the medium. We surmise that the intermediate generated during the action of E(ox) on monophenols, E(m)D, has axial and equatorial bonds between the o-diphenol and copper atoms of the active site. Since the orbitals are not coplanar, the concerted oxidation-reduction reaction cannot occur. Instead, a bond, probably that of C-4, is broken to achieve coplanarity, producing a more labile intermediate that will then release the o-diphenol to the medium or reunite it diaxially, involving oxidation to o-quinone. The non-enzymatic evolution of the o-quinone would generate the o-diphenol ([D(f)](ss)) necessary for the final steady state to be reached after the lag period.
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Affiliation(s)
- L G Fenoll
- GENZ: Grupo de Investigación Enzimologia, Departmento de Bioquímica y Biologia Molecular-A, Facultad de Biologia, Universidad de Murcia, Spain
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Fenoll LG, Rodríguez-López JN, García-Sevilla F, Tudela J, García-Ruiz PA, Varón R, García-Cánovas F. Oxidation by mushroom tyrosinase of monophenols generating slightly unstable o-quinones. EUROPEAN JOURNAL OF BIOCHEMISTRY 2000; 267:5865-78. [PMID: 10998046 DOI: 10.1046/j.1432-1327.2000.01572.x] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Tyrosinase can act on monophenols because of the mixture of mettyrosinase (Em) and oxytyrosinase (Eox) that exists in the native form of the enzyme. The latter form is active on monophenols although the former is not. However, the kinetics are complicated because monophenols can bind to both enzyme forms. This situation becomes even more complex as the products of the enzymatic reaction, the o-quinones, are unstable and continue evolving to generate o-diphenols in the medium. In the case of substrates such as 4-methoxyphenol, 4-ethoxyphenol and 4-tert-butylphenol, tyrosinase generates o-quinones which become unstable with small constants of approximately < 10-3 s-1. The system evolves from an initial steady state, reached when t-->0, through a transition state towards a final steady state, which is never reached because the substrate is largely consumed. The mechanisms proposed to explain the enzyme's action can be differentiated by the kinetics of the first steady state. The results suggest that tyrosinase hydroxylates monophenols to o-diphenols, generating an intermediate Em-diphenol in the process, which may oxidize the o-diphenol or release it directly into the medium. In the case of o-quinone formation, its slow instability generates o-diphenol which activates the enzymatic system yielding parabolic time recordings.
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Affiliation(s)
- L G Fenoll
- GENZ:Grupo de Investigación Enzimología, Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, Murcia, Spain
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Bridelli MG. Self-assembly of melanin studied by laser light scattering. Biophys Chem 1998; 73:227-39. [PMID: 17029729 DOI: 10.1016/s0301-4622(98)00148-3] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/1997] [Revised: 03/24/1998] [Accepted: 03/24/1998] [Indexed: 11/24/2022]
Abstract
The unknown molecular weight and chemical structure of melanin place the study of these pigments outside the range of the classical biochemical techniques; thus in this paper the problem of characterizing these heterogeneous biopolymers was approached by means of light scattering techniques, static and dynamic. The static technique allowed us to identify the macromolecular properties (MW and R(g)(2)(1/2)) of melanin extracted from sepia inksac and of two synthetic analogues: L-Dopa melanin obtained by autooxidation and by enzymatic oxidation by Tyrosinase. By dynamic light scattering (DLS), the hydrodynamic radius R(h) was measured to monitor the temporal behaviour of the polymerization and aggregation processes and R(h) variation by changing the chemical constraints of the polymerization medium, such as pH and ionic strength. The fractal dimension d of the aggregates of melanin, both natural and synthetic, in the past only recognized during the aggregation of the synthetic one by lowering the pH of the medium, was a useful parameter to further investigate and compare the structure of melanin granules of differing origins, revealing for the natural sample, a structure with clusters that are spherical, not largely hydrated and self-assembled, following a reaction limited aggregation kinetics (d=2.38).
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Affiliation(s)
- M G Bridelli
- INFM and Department of Physics, University of Parma, Viale delle Scienze, 43100 Parma, Italy.
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Espín JC, García-Ruiz PA, Tudela J, García-Cánovas F. Study of stereospecificity in mushroom tyrosinase. Biochem J 1998; 331 ( Pt 2):547-51. [PMID: 9531496 PMCID: PMC1219387 DOI: 10.1042/bj3310547] [Citation(s) in RCA: 91] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
This paper reports experiments on the stereospecificity observed in the monophenolase and diphenolase activities of mushroom tyrosinase. Several enantiomorphs of monophenols and o-diphenols were assayed: L-tyrosine, D,L-tyrosine, D-tyrosine; L-alpha-methyltyrosine, D,L-alpha-methyltyrosine; L-dopa, D,L-dopa, D-dopa; L-alpha-methyldopa, D,L-alpha-methyldopa; L-isoprenaline, D,L-isoprenaline and D-isoprenaline. The Vmax values obtained for each series were the same. The electronic densities on the carbon atoms in the meta (C-3) and the para (C-4) positions of the benzene ring were determined by NMR assays. This value is related to the nucleophilic power of the oxygen atom belonging to the hydroxy group, which could explain the Vmax values experimentally obtained for the monophenolase and diphenolase activities of mushroom tyrosinase. The spatial orientation of the ring substituents led to lower Km values for L-isomers than for D-isomers. However, the Vmax values were the same for each series of isomers because spatial orientation did not affect the NMR value of C-4. Therefore mushroom tyrosinase showed stereospecificity in its affinity towards its substrates (Km) but not in the transformation reaction rate (Vmax) of these substrates.
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Affiliation(s)
- J C Espín
- GENZ: Grupo investigación Enzimología, Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, A. Correos 4021, E-30080 Espinardo, Murcia, Spain
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Involvement of lactones in the formation of 6-hydroxydopa and 6-hydroxyhydrocaffeic acid during oxidation of dopa and hydrocaffeic acid. J Electroanal Chem (Lausanne) 1996. [DOI: 10.1016/0022-0728(96)04602-5] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Prota G. The chemistry of melanins and melanogenesis. FORTSCHRITTE DER CHEMIE ORGANISCHER NATURSTOFFE = PROGRESS IN THE CHEMISTRY OF ORGANIC NATURAL PRODUCTS. PROGRES DANS LA CHIMIE DES SUBSTANCES ORGANIQUES NATURELLES 1995; 64:93-148. [PMID: 7782013 DOI: 10.1007/978-3-7091-9337-2_2] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Affiliation(s)
- G Prota
- Department of Organic and Biological Chemistry, University of Naples, Italy
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Garrido-del Solo C, García-Cánovas F, Havsteen BH, Valero E, Varón R. Kinetics of an enzyme reaction in which both the enzyme-substrate complex and the product are unstable or only the product is unstable. Biochem J 1994; 303 ( Pt 2):435-40. [PMID: 7980401 PMCID: PMC1137346 DOI: 10.1042/bj3030435] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
A kinetic analysis of the Michaelis-Menten mechanism has been made for the case in which both the enzyme-substrate complex and the product are unstable or only the product is unstable, either spontaneously or as the result of the addition of a reagent. This analysis allows the derivation of equations which under conditions of limiting enzyme concentration relate the concentration of all of the species to the time. A kinetic data analysis is suggested, which leads to the evaluation of the kinetic parameters involved in the reaction. The analysis is based on the equation which describes the formation of products with time and one's experimental progress curves. We demonstrate the method numerically by computer simulation of the reaction with added experimental errors and experimentally by the use of data from the kinetic study of the action of tyrosinase on dopamine.
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Affiliation(s)
- C Garrido-del Solo
- Departamento de Química-Física, Universidad de Castilla-La Mancha, Albacete, Spain
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García Moreno M, Varón R, Sánchez Gracia A, Tudela J, García Cánovas F. The effect of pH on the suicide inactivation of frog epidermis tyrosinase. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1205:282-8. [PMID: 8155710 DOI: 10.1016/0167-4838(94)90246-1] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
This paper presents a new reaction mechanism for the effect of the pH on the suicide inactivation of the diphenolase activity of tyrosinase. The applicability of the mechanism is supported by the experimental characterization of the kinetic behaviour of the frog epidermis enzyme acting on catechol, L-dopa and alpha-methyldopa at several pH values. Two enzyme froms 'met-' and 'oxy-' tyrosinase, but no their corresponding enzyme-diphenol complexes, present one ionizable group with very similar value of Ka which has been determined. The highest values of catalytic and inactivation efficiencies correspond to alpha-methyldopa and catechol, respectively. These kinetic studies have been carried out by using the transient phase approach previously developed, with negligible substrate consumption during the assay time. That illustrate the usefulness of the method for multisubstrate enzyme reactions.
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Affiliation(s)
- M García Moreno
- Departamento de Química-Física, E.U. Politécnica de Albacete, Universidad de Castilla-La Mancha, Spain
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Ros JR, Rodríguez-López JN, García-Cánovas F. Effect of L-ascorbic acid on the monophenolase activity of tyrosinase. Biochem J 1993; 295 ( Pt 1):309-12. [PMID: 8216233 PMCID: PMC1134854 DOI: 10.1042/bj2950309] [Citation(s) in RCA: 104] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The effect of ascorbic acid on the monophenolase activity of tyrosinase, using tyrosine as substrate, has been studied. Over the ranges of ascorbic acid concentration used, no direct effect on the enzyme is found. However, a shortening of the characteristic induction period of the hydroxylation reaction is observed. The evolution of the reaction is dependent on the concentration of ascorbic acid. Low concentrations permit the system to reach the steady state when all ascorbic acid is consumed, whereas high concentrations do not. In the light of these results it is proposed that the influence of ascorbic acid on the reaction is due to its ability to reduce the enzymically generated o-quinones. A relationship between the ascorbic acid concentration, and the induction period generated by it, with the diphenolase activity of tyrosinase is established, which can be used as a basis for the determination of trace amounts of this reducing agent.
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Affiliation(s)
- J R Ros
- Departamento de Bioquímica y Biología Molecular, Facultad de Biología, Universidad de Murcia, Spain
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Ros JR, Rodríguez-López JN, García-Cánovas F. Effect of ferrous ions on the monophenolase activity of tyrosinase. BIOCHIMICA ET BIOPHYSICA ACTA 1993; 1163:303-8. [PMID: 8507669 DOI: 10.1016/0167-4838(93)90166-o] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
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.
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Affiliation(s)
- J R Ros
- Departamento de Bioquímica y Biología Molecular, Facultad de Biología, Universidad de Murcia, Espinardo, Spain
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Li J, Christensen BM. Identification of Products and Intermediates During L-Dopa Oxidation to Dopachrome Using High Pressure Liquid Chromatography with Electrochemical Detection. ACTA ACUST UNITED AC 1993. [DOI: 10.1080/10826079308019575] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Rodríguez-López JN, Bañón-Arnao M, Martinez-Ortiz F, Tudela J, Acosta M, Varón R, García-Cánovas F. Catalytic oxidation of 2,4,5-trihydroxyphenylalanine by tyrosinase: identification and evolution of intermediates. BIOCHIMICA ET BIOPHYSICA ACTA 1992; 1160:221-8. [PMID: 1445949 DOI: 10.1016/0167-4838(92)90011-2] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The oxidation of 3,4-dihydroxyphenylalanine (dopa) by O2 catalyzed by tyrosinase yields 4-(2-carboxy-2-aminoethyl)-1,2-benzoquinone, with its amino group protonated (o-dopaquinone-H+). This evolves non-enzymatically through two branches (cyclization and/or hydroxylation), whose respective operations are determined by pH. The hydroxylation branch of o-dopaquinone-H+ only operates significantly at pH < or = 5.0 and involves the accumulation of 2,4,5-trihydroxyphenylalanine (topa), which has been detected by high-performance liquid chromatography (HPLC). This last compound is also a substrate of tyrosinase. The oxidation of topa by both tyrosinase and periodate yields 5-(2-carboxy-2-aminoethyl)-4-hydroxy-1,2-benzoquinone, with its amino group protonated (o-topaquinone-H+), which is red (RTQH) (lambda max 272-485 nm) at pH 7.0 and yellow (TTQH) (lambda max 265-390 nm) at pH 3.0. This is based on pKa 4.5 of the 2-OH group of the benzene ring of o-topaquinone-H+, as derived from spectrophotometric and HPLC assays. At physiological pH, RTQH undergoes deprotonation of the ammonium group of the side chain to yields RTQ, which cyclize into 2-carboxy-2,3-dihydroxyindolen-5,6-quinone (dopachrome), with a 1:1 stoichiometry and first-order kinetics. The evolution of RTQH has been analyzed by spectrophotometry, HPLC, cyclic voltammetry and constant potential electrolytic assays. From HPLC assays, the value of the first-order constant for the evolution of RTQH at pH 7.0 (kRTQHapp 4.83 x 10(-5) s-1), as well as of the rate constant for the cyclization step of RTQ (kRTQc 2.53 x 10(-3) s-1) were determined.
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Affiliation(s)
- J N Rodríguez-López
- Departamento de Química-Física, E.U. Politécnica de Albacete, Universidad de Castilla-La Mancha, Spain
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