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10th
International Inter University Scientific Meeting
Academy of Studenica PERSPECTIVES IN MELANOMA MANAGEMENT & NANOTECHNOLOGY IN BIOMEDICINE Organizers: Institute of Oncology Sremska Kamenica; Union of Cancer Prevention Societies of Vojvodina, Novi Sad; Clinic of Oncology, Nis; Institute for Oncology and Radiology of Serbia, Belgrade Center for Bioengineering, Faculty of Mechanical Engineering, University of Belgrade President: Vladimir Baltic Vice-presidents: Zlata Janjic, Radan Dzodic, Borislava Nikolin; Djuro Koruga |
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ISSN
1450-708
Content
7 /2004
Content
7 /2004
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SYNTHESIS
OF MELANIN
Andrijeviæ Lj. Institute of Oncology Sremska Kamenica, Clinic for internal oncology, Sremska kamenica, Serbia and Montenegro |
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| ABSTRACT The aromatic amino acids are phenylalanine, tyrosine and, tryptophan. Phenylalanine contains a benzene ring, tyrosine a phenol group, and tryptophan the hetrocyclic structure, indole. In each case the aromatic moiety is attached to the ?-carbon through a methylene (-CH2-) carbon. Phenylalanine and tyrosine are discussed together, since tyrosine results from hydroxylation of phenylalanine and is the first product in phenylalanine degradation. Because of this, tyrosine is not usually considered to be essential, whereas phenylalanine is. Three- quarters of ingested phenylalanine is hydroxylated to tyrosine. This is catalyzed by phenylalanine hydoxylase, wich is tetrabiopterin dependent. Dihydrobiopterin form is produced during oxidation of aromatic amino acids and is then reduced to the tetrahydrobiopterin form by a dehydrogenase using NADH and H+. The reaction occurs only in the direction of tyrosine formation, and phenylalanine cannot be sythesized from tyrosine. Tyrosine metabolism produces fumarate and acetoacetate - the first step in metabolism of tyrosine is transamination by tyrosine aminotransferase to p - hydroxyphenylpyruvate. The enzime is inducible , its synthesis being increased by glucocorticoids and dietary tyrosine. p- Hydroxyphenylpyruvate oxidase produces homogentisic acid . This comlex reaction involves decarboxylation, oxidation, migration of the carbon side chain, and hydroxylation. Ascorbic acid is required for at least one of these activities, but all four are catalyzed by the one enzyme. The aromatic ring in next cleaved by the iron- containing homogentisate oxidase to maleyl acetoacetate, in a reaction catalyzed by maleylacetoacetate isomerase, and enzyme that seems to require glutathione for activity. Fumarylacetoacetate is then cleaved to fumarate and acetoacetate. Fumarate can be further utilized in the TCA cycle (tricarboxylic acid cycle - Krebs cycle) for energy or for gluconeogenesis. Acetoacetate can be used, as acetyl CoA, for lipid synthesis or energy. Tyrosine is required for synthesis of melanin - Conversion of tyrosine to melanin requires tyrosinase, a copper - containing protein. The two - step raction uses DOPA as a cofactor internal to the reaction and produces dopaquinone. During melanogenesis, following exposure to UVB light, tyrosinase and a protein called tyrosinase - related protein, which may function in posttransalational modification of tyrosinase, are induced. A lack of tyrosinase activity produces albinism. There are various types of melanin. All are aromatic quinones and the conjugated bond system gives rise to color. The dark pigment that is usually associated with melanin is eumelanin, from the Greek for "good melanin". Other melanins are yellow or colorless. |
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©
Academy of Studenica, 2004