叶酸的光降解和皮肤颜色的关系

　　作者：吕小云 张同喜 韩建国 谈笑玲 作者单位：青海大学医学院

　　【摘要】 目的 了解叶酸的光降解和皮肤颜色的关系。方法 用荧光光谱仪记录和分析叶酸和维生素B2的光降解过程，用Sigmaplot软件描绘紫外线在不同地区对不同肤色的皮肤的穿透程度。结果 维生素B2的加入可使叶酸在UVA和蓝色光下发生光降解。紫外线穿透肤色类型2最深，类型4最浅.穿透程度随地方的不同而变化。结论 叶酸在紫外线和可见光照射发生降解的事实进一步给“皮肤颜色理论”的可靠性提供了依据。 Lü Xiaoyun(1970～)，Male，Tu，Lecturer,Master

　　【关键词】 叶酸 黑色素 光降解 皮肤类型

　　1。Introduction

　　The skin colour is determined by the amount of melanin in the skin. People use skin types 1-4 to represent different skin colours. Type 1 is white, type 2 is less white, type 3 is brown and type 4 is black. Melanin is synthesized from tyrosine in melanocytes and localized in melanosomes. In lightly pigmented skin that has not been exposed to UVB, the melanosomes are found mostly in the basal layer[1]. Melanosomes have relatively high refractive indices (1.55-1.65) compared to the surrounding skin tissue (1.35-1.4) and are therefore important epidermal scatterers. Light with a long wavelength is more strongly scattered from it. Reflectance spectra of skin in the UV spectral region reveal that strongly pigmented skin reflects and scatters more UVB light than the lightly pigmented skin [2]. In response to exposure to UV radiation, melanocytes in the skin can synthesize the dark-brown pigment melanin to prevent UV-induced photolysis of folate and other nutrients [3，4]. Melanin absorbs UV radiation, reduces formation of harmful chemicals called free radicals that form in skin under exposure to UV radiation[5]. The correlation between skin color and latitude is believed to be related to the balancing needs of folate protection and vitamin D production [6].

　　A synthetic form of folate, folic acid (FA), is sensitive to UV radiation [7，8]. The most common folate derivative in human blood, 5-methyltetrahydrofolate (5MTHF), is very sensitive to UVB radiation [8]. But UVB penetrates the skin poorly and is not efficient in degrading folate in blood in vivo. UVA cannot degrade folate at all. This fact is unfavorable to skin color theory. However, our investigation reveals that when other vitamin riboflavin is presented, folate can be degraded quickly. There are a lot of vitamins in human skin; the result thus favored the skin color theory again.

　　Ultraviolet radiation is traditionally divided into three different regions: UVC (100 - 280), UVB (280 - 320) and UVA (320 - 400). The sun emits electromagnetic radiation from X-rays to radio waves. Due to the ozone layer, dangerous radiation shorter than 290 nm does not reach the surface of the earth. UVB, UVA, visible wavelengths and infrared, are of greatest biological significance. Figure 1 shows the estimated spectral irradiance at noon, midsummer with clear sky and normal ozone: in Oslo, Equator and Lhasa at the Tibetan Plateau. Because of the low latitude, high altitude and low total ozone amount, the Tibetan Plateau is the place with highest UVR on earth [9]. The spectra are calculated by Arne Dahlback, the model used has been compared with measurements and the deviation is within 5 per cent [10，11].

　　Fig. 1. Spectral irradiance in Oslo (-), Equator (- -) and Lhasa (……). These spectra are calculated by

　　Arne Dahlback, Department of Physics, University of Oslo.

　　Materials and Method

　　Chemicals: 5-methyltetrahydrofolate (5MTHF) calcium salt from Schircks Laboratories (Jona, Switzerland); Riboflavin from Sigma-Aldrich Chemie (Germany); Dulbecco's phosphate buffered saline (PBS) from PAA laboratories (Austria);

　　5-methyltetrahydrofolate (5MTHF) was dissolved in PBS to a concentration of 50 μM, snap frozen in liquid nitrogen and stored in 1 ml tubes at -80℃. For each experiment, 1 ml of the solution (one tube) was diluted in PBS (pH 7.4) to the needed concentration. Because of the instability of 5MTHF, the solutions were always kept on ice before use. Riboflavin were also dissolved in PBS to a concentration of 50 μM and wrapped with aluminium foil and stored in refrigerator. They were diluted into the needed concentration in PBS in each experiment. The temperature of the samples was approximately 25℃. All the experiments were done in dim light to avoid external light exposure.

　　Instruments: A Perkin-Elmer UV/VIS spectrophotometer (Lambda 40) and a Perkin-Elmer luminescence spectrometer (LS45); Blue light and UVA radiation source; Quartz suprasil cuvettes, 1.0 × 0.4 cm, 1 mL.

　　Software: SigmaPlot 10.0.

　　Results and discussion

　　Blue light and UVA can not degrade 5MTHF, see figure 2, the absorption spectra remain the same when exposure to blue light and UVA. Riboflavin, however, can be degraded by blue light and UVA. There is an obvious decay at 445 nm.

　　Fig. 2. Absorption spectra of 5MTHF and RF alone by blue light exposure. pH 7.4.

　　Figure 3 showed the absorption spectra of the mixed solution of 5MTHF and Riboflavin. After subtracting the spectra of riboflavin, we can see that there is an obvious increase at 250nm. As the characteristic spectra of the product of photodegradation of 5MTHF, 5-methyldihydrofolate (5MDHF), the maximum at 250 nm indicate that although 5MTHF is not easily degraded by blue light and UVA, but in the presence of riboflavin, it is degraded and oxidized to 5MDHF quickly.

　　Fig. 3. Absorption spectra of 5MTHF + RF by blue light exposure. The absorption spectra of

　　5MTHFobtained by subtracting corresponding RF spectra are shown in the right. pH 7.4.

　　We have plotted the penetration fluence of different layer of different skin types in figure 4 (a, b, c) according the calculation data from Kristian P.Nielsen et al. [1] It is seen from the graph that light penetrates the lighter pigmented skin deeper than the stronger pigmented skin. Take the different solar spectrum of different places (Equator, Oslo and Lhasa) into account, see figure 2, multiply the solar spectrum of these places with the fluence through epidermis (0.05 mm) (red in the figure), we get part d in figure 4.

　　Fig. 4. Different light fluence within different skin type (figure a, b, c). Figure d (1: Oslo;

　　2: Equator; 3: Lhasa. Skin type 2 (-); Skin type 3 (-); Skin type 4 (-)

　　It was seen from figure 4 that light can reach around 0.3 mm deep in the skin. The deeper in the skin, the less of light reaches. UVA (320 - 400) penetrates more than UVB (280 - 320); UVC (100 - 280) can hardly penetrate the skin. And the deeper the skin colour, the less of light penetrates in the skin. The result also indicates that the altitude and latitude of place have influence to the penetration of light in the skin. High altitude place has the most fluence while the lower latitude place has the lest fluence of penetration in the skin.

　　UVC generally does not penetrate the earth's atmosphere. UVB can degrade folate but only arrives to epidermis. Now we have shown that folate can not only be degraded by UVA irradiation which penetrates skin more deeply, but also by visible light that penetrates skin deeper than UVA in the presence of other vitamins like riboflavin. This may be a reason why people living under high solar irradiation need to develop something like melanin to protect this important vitamin. Our investigation thus continued to favour the human skin evolution theory suggested by Branda and Eaton [3], and developed further by Jablonski and Chaplin [6]

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