Korytowski W, Sarna T
Biophysics Section, Medical College of Wisconsin, Milwaukee 53226.
J Biol Chem. 1990 Jul 25;265(21):12410-6.
Bleaching of eumelanin has been studied in model systems consisting of synthetic dopa-melanin and various concentrations of hydrogen peroxide, molecular oxygen, and copper(II) ions at neutral and alkaline pH. The data show that at neutral pH, in the dark, metal-ion-free melanin is very resistant to oxidation by hydrogen peroxide. However, the rate of bleaching of melanin, induced by H2O2 is significantly accelerated by illumination from UVA-visible light. Bound-to-melanin copper(II) also accelerates the bleaching of melanin with the efficiency dependent on concentration of H2O2 and oxygen. It suggests possible involvement of melanin-copper complexes in Fenton-like processes. The formation of hydroxyl radicals during melanin bleaching has been concluded on the basis of the electrochemical detection of hydroxylation products of salicylate used as OH scavenger. Redox conversion of bound-to-melanin copper ions was monitored by EPR spectroscopy and direct measurement of melanin-Cu(II) complexes. It has been shown that melanin-copper(I) complexes were readily oxidized by either oxygen or hydrogen peroxide. The data indicate that bleaching of melanin is a complex process with two distinct stages, reversible oxidation of the hydroquinone moieties of melanin followed by irreversible reactions of the monomers that lead to degradation of the melanin polymer.
在由合成多巴黑色素以及中性和碱性pH条件下的各种浓度过氧化氢、分子氧和铜（II）离子组成的模型系统中，对真黑素的漂白进行了研究。数据表明，在中性pH条件下，在黑暗中，不含金属离子的黑色素对过氧化氢的氧化具有很强的抗性。然而，由过氧化氢诱导的黑色素漂白速率在UVA-可见光照射下会显著加快。与黑色素结合的铜（II）也会加速黑色素的漂白，其效率取决于过氧化氢和氧气的浓度。这表明黑色素-铜络合物可能参与类芬顿过程。基于对用作羟基清除剂的水杨酸羟基化产物的电化学检测，得出了黑色素漂白过程中羟基自由基形成的结论。通过电子顺磁共振光谱和对黑色素-Cu（II）络合物的直接测量监测了与黑色素结合的铜离子的氧化还原转化。结果表明，黑色素-铜（I）络合物很容易被氧气或过氧化氢氧化。数据表明，黑色素的漂白是一个复杂的过程，有两个不同的阶段，即黑色素对苯二酚部分的可逆氧化，随后是单体的不可逆反应，导致黑色素聚合物降解。
