Nguyen Bach-Cuc, Kochevar Irene E
Wellman Laboratories of Photomedicine, Department of Dermatology, Massachusetts General Hospital, Harvard Medical School, Boston 02114, USA.
J Invest Dermatol. 2003 Apr;120(4):655-61. doi: 10.1046/j.1523-1747.2003.12089.x.
Dihydroxyacetone, the browning ingredient in sunless tanning formulations, reacts with amino acids in the outer stratum corneum to form a mixture of high molecular weight pigments. Our initial observations indicated that high hydration of dihydroxyacetone-treated skin completely inhibited development of pigmentation. To investigate the mechanism underlying this effect, studies were carried out in isolated murine epidermis, polyvinyl alcohol/lysine films, and lysine in glycerol/water solvent. Murine epidermis treated with dihydroxyacetone showed a biphasic dependence on relative humidity: maximum pigmentation developed at 84% relative humidity and minimum pigmentation at 0% and 100% relative humidity. Filaggrin proteolysis, which shows a similar dependence on relative humidity and provides free amino acids in the outer stratum corneum, did not account for the relative humidity dependence of dihydroxyacetone pigmentation. A similar biphasic pigmentation response was obtained when polyvinyl alcohol film containing lysine was treated with dihydroxyacetone and incubated at various relative humidities, indicating that the structure of the stratum corneum was not a major factor. To remove the influence of the matrix, the reaction of dihydroxyacetone with lysine was followed at varying concentrations of water in mixed glycerol/buffer solvent. Again, greater pigment formation was found at an intermediate level of water (6% vol/vol) and little pigmentation at 0% and 100% water content. These results are consistent with a requirement for water at low relative humidity, which facilitates formation of free amine groups needed for the initial reaction with dihydroxyacetone, and with inhibition of the dehydration reactions by water through the law of mass action at high relative humidity.
二羟基丙酮是免晒美黑配方中的褐变成分，它与角质层外层的氨基酸发生反应，形成高分子量色素混合物。我们最初的观察结果表明，经二羟基丙酮处理的皮肤高度水合会完全抑制色素沉着的形成。为了研究这种效应背后的机制，我们在分离的小鼠表皮、聚乙烯醇/赖氨酸薄膜以及甘油/水溶剂中的赖氨酸中进行了研究。用二羟基丙酮处理的小鼠表皮对相对湿度呈现双相依赖性：在相对湿度为84%时色素沉着最大，在0%和100%相对湿度时色素沉着最小。角蛋白聚集素蛋白水解对相对湿度也有类似的依赖性，并在角质层外层提供游离氨基酸，但这并不能解释二羟基丙酮色素沉着对相对湿度的依赖性。当用二羟基丙酮处理含有赖氨酸的聚乙烯醇薄膜并在不同相对湿度下孵育时，也获得了类似的双相色素沉着反应，这表明角质层的结构不是主要因素。为了消除基质的影响，我们在甘油/缓冲液混合溶剂中不同水浓度下跟踪二羟基丙酮与赖氨酸的反应。同样，在中间水含量水平（6%体积/体积）时发现色素形成更多，而在水含量为0%和100%时色素沉着很少。这些结果与低相对湿度下需要水相一致，水有助于形成与二羟基丙酮初始反应所需的游离胺基团，并且与高相对湿度下通过质量作用定律水对脱水反应的抑制作用相一致。
