Mehra R K, Tran K, Scott G W, Mulchandani P, Saini S S
Environmental Toxicology Graduate Program, Boyce Hall, University of California, Riverside 92521, USA.
J Inorg Biochem. 1996 Feb;61(2):125-42. doi: 10.1016/0162-0134(95)00046-1.
Phytochelatins (PCs) are glutathione-derived peptides with the general structure (gamma-Glu-Cys)nGly, where n varies from 2 to 11. A variety of metal ions such as Cu(II), Cd(II), Pb(II), Zn(II), and Ag(I) induce PC synthesis in plants and some yeasts. It has generally been assumed that the inducer metals also bind PCs. However, very little information is available on the binding of metals other than Cu(I) and Cd(II) to PCs. In this paper, we describe the Ag(I)-binding characteristics of PCs with the structure (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly, and (gamma-Glu-Cys)4Gly. The Ag(I)-binding stoichiometries of these three peptides were determined by (i) UV/VIS spectrophotometry, (ii) luminescence spectroscopy at 77 K, and (iii) reverse-phase HPLC. The three techniques yielded similar results. ApoPCs exhibit featureless absorption in the 220-340 nm range. The binding of Ag(I) to PCs induced the appearance of specific absorption shoulders. The titration end point was indicated by the flattening of the characteristic absorption shoulders. Similarly, luminescence at 77 K due to Ag(I)-thiolate clusters increased with the addition of graded Ag(I) equivalents. The luminescence declined when Ag(I) equivalents in excess of the saturating amounts were added to the peptides. At neutral pH, (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly, and (gamma-Glu-Cys)4Gly bind 1.0, 1.5, and 4.0 equivalents of Ag(I), respectively. The Ag(I)-binding capacity of (gamma-Glu-Cys)2Gly and (gamma-Glu-Cys)3Gly was increased at pH 5.0 and below so that Ag(I)/-SH ratio approached 1.0. A similar pH-dependent binding of Ag(I) to glutathione was also observed. The increased Ag(I)-binding to PCs at lower pH is of physiological significance as these peptides accumulate in acidic vacuoles. We also report lifetime data on Ag(I)-PCs. The relatively long decay-times (approximately 0.1-0.3 msec) accompanied with a large Stokes shift in the emission band are indicative of spin-forbidden phosphorescence.
植物螯合肽（PCs）是一类由谷胱甘肽衍生而来的肽，其一般结构为（γ-谷氨酰-半胱氨酸）n甘氨酸，其中n在2到11之间变化。多种金属离子，如铜（II）、镉（II）、铅（II）、锌（II）和银（I），可诱导植物和一些酵母中PCs的合成。一般认为，诱导金属离子也会与PCs结合。然而，关于除铜（I）和镉（II）之外的其他金属离子与PCs结合的信息非常少。在本文中，我们描述了具有（γ-谷氨酰-半胱氨酸）2甘氨酸、（γ-谷氨酰-半胱氨酸）3甘氨酸和（γ-谷氨酰-半胱氨酸）4甘氨酸结构的PCs与银（I）的结合特性。通过以下方法测定了这三种肽与银（I）的结合化学计量比：（i）紫外/可见分光光度法，（ii）77K下的发光光谱法，以及（iii）反相高效液相色谱法。这三种技术得到了相似的结果。脱金属植物螯合肽（ApoPCs）在220 - 340nm范围内表现出无特征吸收。银（I）与PCs的结合诱导了特定吸收峰的出现。滴定终点由特征吸收峰的变平指示。同样，由于银（I）-硫醇盐簇导致的77K下的发光随着分级加入银（I）当量而增加。当向肽中加入超过饱和量的银（I）当量时，发光下降。在中性pH下，（γ-谷氨酰-半胱氨酸）2甘氨酸、（γ-谷氨酰-半胱氨酸）3甘氨酸和（γ-谷氨酰-半胱氨酸）4甘氨酸分别结合1.0、1.5和4.0当量的银（I）。（γ-谷氨酰-半胱氨酸）2甘氨酸和（γ-谷氨酰-半胱氨酸）3甘氨酸在pH 5.0及以下时与银（I）的结合能力增加，使得银（I）/ - SH比值接近1.0。也观察到了银（I）与谷胱甘肽类似的pH依赖性结合。在较低pH下银（I）与PCs结合的增加具有生理意义，因为这些肽在酸性液泡中积累。我们还报告了银（I）-PCs的寿命数据。相对较长的衰减时间（约0.1 - 0.3毫秒）以及发射带中较大的斯托克斯位移表明是自旋禁阻磷光。
