Jiménez M, Garcia-Canovas F, Garcia-Carmona F, Tudela J, Iborra J L
Int J Biochem. 1986;18(1):39-47. doi: 10.1016/0020-711x(86)90006-6.
The pathway for alpha-methyldopa oxidation to alpha-methyldopachrome, by mushroom tyrosinase, is proposed. Characterization of intermediates in this oxidative reaction and stoichiometry determination have both been undertaken. The steps for alpha-methyldopa transformation into its aminochrome would be: alpha-methyldopa----o-alpha-methyldopaquinone-H+----o-alpha- methyldopaquinone----leuko-alpha-methyldopachrome----alpha- methyldopachrome. The stoichiometry for this conversion corresponded to the equation: 2 o-alpha-methyldopaquinone-H+----alpha-methyldopa + alpha-methyldopachrome. At very acid pH values, another route implying the addition of water to the quinonic ring, competes with the first one. Two chemical pathways can be proposed from alpha-methyldopaquinone-H+, the relative importance of which is determined by the pH. A theoretical and experimental kinetic approach was applied to this oxidative reaction. Rate constants and thermodynamic activation parameters of the chemical steps, have been evaluated. The results obtained confirmed that alpha-methyldopa oxidation by tyrosinase followed a scheme similar to that established for L-dopa and alpha-methylnoradrenaline.
本文提出了蘑菇酪氨酸酶将α-甲基多巴氧化为α-甲基多巴色素的途径。对该氧化反应中的中间体进行了表征，并确定了化学计量关系。α-甲基多巴转化为其氨基色素的步骤为：α-甲基多巴→邻-α-甲基多巴醌-H⁺→邻-α-甲基多巴醌→白细胞-α-甲基多巴色素→α-甲基多巴色素。该转化的化学计量关系符合以下方程式：2邻-α-甲基多巴醌-H⁺→α-甲基多巴 + α-甲基多巴色素。在非常酸性的pH值下，另一条涉及向醌环加水的途径与第一条途径竞争。从邻-α-甲基多巴醌-H⁺可以提出两条化学途径，其相对重要性由pH值决定。对该氧化反应应用了理论和实验动力学方法。评估了化学步骤的速率常数和热力学活化参数。所得结果证实，酪氨酸酶催化的α-甲基多巴氧化遵循与L-多巴和α-甲基去甲肾上腺素所确立的方案相似的方案。
