State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences , Guiyang 550002, P. R. China.
Environ Sci Technol. 2014 Jul 15;48(14):7974-81. doi: 10.1021/es502000d. Epub 2014 Jun 23.
Cultivation of paddy rice for human consumption is a dominant agricultural activity throughout Asia. High levels of mercury (Hg) in rice grain pose a potential threat to human health, although the extent of risk is dependent on the chemical speciation of Hg inside the grain. We have investigated the speciation and localization of Hg in three fractions of rice grain (hull, bran, and white rice) collected from a Hg-contaminated region in China. On a mass basis, the majority of inorganic mercury (IHg) in a rice grain is found in hull and bran. However, the majority of the more toxic species methyl mercury (MeHg) is found in edible white rice. Our data show that during grain processing, most of the IHg (∼78%) is eliminated, but the majority of the MeHg remains in the food product (∼80%). Synchrotron radiation microscopic X-ray fluorescence (SR-μXRF) mapping shows strong localization of Hg at the surface of brown rice grains, corresponding to the pericarp and aleurone layer. We infer that this Hg is predominantly IHg absorbed from the atmosphere. Based on X-ray absorption near-edge spectroscopy (XANES) data we propose that IHg in bran is primarily bound to cysteine, and is associated with phytochelatins. Consequently, IHg is largely immobile and restricted to the outer layers of rice grain. MeHg in bran is primarily bound to cysteine and is associated with proteins. However, this MeHg-cysteine association behaves like a mobile nutrient and is actively transported to the endosperm during seed ripening. Concentration of MeHg-cysteine in white rice has implications for public health. There is growing evidence for Hg contamination of rice throughout Asia due to point and diffuse sources of Hg pollution. The magnitude of the associated risk must be quantified through better understanding of the localization and speciation of mercury in rice. Our work makes an effort to contribute to this understanding.
水稻种植是亚洲地区主要的农业活动。稻米中汞（Hg）含量过高会对人类健康构成潜在威胁，尽管风险程度取决于谷物内 Hg 的化学形态。我们研究了从中国 Hg 污染区采集的三种稻米（稻壳、米糠和白米）中 Hg 的形态和定位。从质量上看，稻米中的大部分无机汞（IHg）存在于稻壳和米糠中。然而，毒性更大的甲基汞（MeHg）的大部分则存在于可食用的白米中。我们的数据表明，在谷物加工过程中，大部分 IHg（78%）被去除，但大部分 MeHg仍残留在食品中（80%）。同步辐射微区 X 射线荧光（SR-μXRF）图谱显示，Hg 强烈定位于糙米表面，对应于种皮和糊粉层。我们推断，这些 Hg 主要是从大气中吸收的 IHg。基于 X 射线吸收近边谱（XANES）数据，我们提出米糠中的 IHg 主要与半胱氨酸结合，并与植物螯合肽相关。因此，IHg 大部分是不移动的，局限于稻米的外层。米糠中的 MeHg 主要与半胱氨酸结合，并与蛋白质相关。然而，这种 MeHg-半胱氨酸结合物具有移动养分的特性，在种子成熟过程中会被主动转运到胚乳。白米中 MeHg-半胱氨酸的浓度对公共健康有影响。由于 Hg 污染的点源和弥散源，亚洲各地的稻米受到 Hg 污染的情况越来越严重。必须通过更好地了解稻米中汞的定位和形态来量化相关风险。我们的工作为这项理解做出了努力。
