Jimenez M, Garcia-Carmona F, Garcia-Canovas F, Iborra J L, Lozano J A, Martinez F
Arch Biochem Biophys. 1984 Dec;235(2):438-48. doi: 10.1016/0003-9861(84)90217-0.
A minor pathway for dopamine oxidation to dopaminochrome, by tyrosinase, is proposed. Characterization of intermediates in this oxidative reaction and stoichiometric determination have both been undertaken. After oxidizing dopamine with mushroom tyrosinase or sodium periodate in a pH range from 6.0 to 7.0, it was spectrophotometrically possible to detect o-dopaminoquinone-H+ as the first intermediate in this pathway. The steps for dopamine transformation to dopaminochrome are as follows: dopamine----o-dopaminequinone-H+----o-dopaminequinone---- leukodopaminochrome--- - dopaminochrome. No participation of oxygen was detected in the conversion of leukodopaminochrome to dopaminochrome. Scanning spectroscopy and graphical analysis of the obtained spectra also verified that dopaminequinone-H+ was transformed into aminochrome in a constant ratio. The stoichiometry equation for this conversion is 2 o-dopaminequinone-H+----dopamine + dopaminochrome. The pathway for dopamine oxidation to dopaminochrome by tyrosinase has been studied as a system of various chemical reactions coupled to an enzymatic reaction. A theoretical and experimental kinetic approach is proposed for such a system; this type of mechanism has been named "Enzymatic-chemical-chemical" (EzCC). Rate constants for the implied chemical steps at different pH and temperature values have been evaluated from the measurement of the lag period arising from the accumulation of dopaminochrome that took place when dopamine was oxidized at acid pH. The thermodynamic activation parameters of the chemical steps, the deprotonation of dopaminequinone-H+ to dopaminequinone, and the internal cyclization of dopaminequinone to leukodopaminochrome have been calculated.
本文提出了一条由酪氨酸酶催化多巴胺氧化生成多巴色素的次要途径。对该氧化反应中间体进行了表征，并进行了化学计量测定。在pH值为6.0至7.0的范围内，用蘑菇酪氨酸酶或高碘酸钠氧化多巴胺后，通过分光光度法可检测到邻多巴胺醌-H+作为该途径中的首个中间体。多巴胺转化为多巴色素的步骤如下：多巴胺→邻多巴胺醌-H+→邻多巴胺醌→无色多巴色素→多巴色素。在无色多巴色素转化为多巴色素的过程中未检测到氧气的参与。对所得光谱进行扫描光谱分析和图形分析，也证实了多巴胺醌-H+以恒定比例转化为氨基色素。该转化的化学计量方程为2邻多巴胺醌-H+→多巴胺+多巴色素。酪氨酸酶催化多巴胺氧化生成多巴色素的途径已作为一个与酶促反应偶联的各种化学反应体系进行了研究。针对该体系提出了一种理论和实验动力学方法；这种机制被命名为“酶-化学-化学”（EzCC）。通过测量在酸性pH下氧化多巴胺时由于多巴色素积累而产生的延迟期，评估了不同pH和温度值下隐含化学步骤的速率常数。计算了化学步骤、多巴胺醌-H+去质子化生成多巴胺醌以及多巴胺醌内环化生成无色多巴色素的热力学活化参数。
