Tyrosinases, copper mammalian

Oxidations now known to be catalyzed by copper-containing enzymes were noticed over a century ago, when Schoenbein observed that oxidation of natural substrates resulted in pigment formation in mushrooms. Individual enzymes were gradually identified laccase by Yoshida in 1883 and tyrosinase by Bertrand in 1896. However, it was not imtil potato polyphenol oxidase was isolated in 1937 by Kubowitz that the role of copper was defined. The family of copper oxidases includes a number of enzymes of both plant and animal origin that may very probably be found to react through similar mechanisms, but which exhibit a number of individual characteristics. The enzymes to be described in this section include potato phenol oxidase, mushroom polyphenol oxidase (tyrosinase), laccase, mammalian and insect tyrosinase, and ascorbic acid oxidase. Each of these differs in certain respects from the others, and undoubtedly other related enzymes will be described from other sources that resemble these, but also display individualities. In these cases, identities in nomenclature must not be extended to imply identities in enzyme structure or activity. 

Polyphenol oxidase occurs within certain mammalian tissues as well as both lower (46,47) and higher (48-55) plants. In mammalian systems, the enzyme as tyrosinase (56) plays a significant role in melanin synthesis. The PPO complex of higher plants consists of a cresolase, a cate-cholase and a laccase. These copper metalloproteins catalyze the one and two electron oxidations of phenols to quinones at the expense of 02. Polyphenol oxidase also occurs in certain fungi where it is involved in the metabolism of certain tree-synthesized phenolic compounds that have been implicated in disease resistance, wound healing, and anti-nutrative modification of plant proteins to discourage herbivory (53,55). This protocol presents the Triton X-114-mediated solubilization of Vida faba chloroplast polyphenol oxidase as performed by Hutcheson and Buchanan (57). 

Mammalian Tyrosinase. Another copper-containing tyrosinase has been found in cytoplasmic particles of melanocytes of mammalian skin. This enzyme attacks n-tyrosine, but not D-tyrosine, after a lag period. With dopa as substrate, there is no lag. The product is a polymerized oxidation product, a highly colored melanin. The enzyme is more stable than the plant tyrosinases, and can be assayed both manometrically and by estimation of the product. C Mabeled tyrosine has been used to produce labeled, insoluble melanin. 

Copper is present in a number of mammalian proteins and the characteristics of these have been summarized by Scheinberg and Sternlieb [3]. Amongst the copper proteins identified in man, but of unknown function, are cerebro-cuprein I, a protein extracted from normal human brain by Porter and Ainsworth [4], liver copper-protein [5], and erythrocuprein [6]. Tyrosinase, a protein of 0.25% copper content is to be found wherever melanin is present in the body, it catalyses the oxidation of tyrosine to dopa and accelerates the conversion of dopa to dopa quinone, the initicJ stages in the conversion of tyrosine to melanin. Lack of this enzyme is not, however, associated with deficient production of pressor amines, another pathway being present in the adreneil gletnd for the hydroxylation of tyrosine [7]. 

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