Radioprotectors with few side effects are useful for carbon-ion therapy, which directly induces clustering damage in DNA. With the aim of finding the most effective radioprotector, we investigated the effects of selected amino acids which might have chemical DNA-repair functions against therapeutic carbon ions. In the current study, we employed five amino acids: tryptophan (Trp), cysteine (Cys), methionine (Met), valine (Val) and alanine (Ala). Samples of supercoiled pBR322 plasmid DNA with a 17 mM amino acid were prepared in TE buffer (10 mM Tris, 1 mM ethylenediaminetetraacetic acid, pH 7.5). Phosphate buffered saline (PBS) was also used in assays of the 0.17 mM amino acid. The samples were irradiated with carbon-ion beams (290 MeV/u) on 6 cm spread-out Bragg peak at the National Institute of Radiological Sciences and Heavy Ion Medical Accelerator in Chiba, Japan. Breaks in the DNA were detected as changes in the plasmids and quantified by subsequent electrophoresis on agarose gels. DNA damage yields and protection factors for each amino acid were calculated as ratios relative to reagent-free controls. Trp and Cys showed radioprotective effects against plasmid DNA damage induced by carbon-ion beam, both in PBS and TE buffer, comparable to those of Met. The double-strand break (DSB) yields and protective effects of Trp were comparable to those of Cys. The yields of both single-strand breaks and DSBs correlated with the scavenging capacity of hydroxyl radicals (rate constant for scavenging hydroxyl radicals multiplied by the amino acid concentration) in bulk solution. These data indicate that the radioprotective effects of amino acids against plasmid DNA damage induced by carbon ions could be explained primarily by the scavenging capacity of hydroxyl radicals. These findings suggest that some amino acids, such as Trp, Cys and Met, have good potential as radioprotectors for preventing DNA damage in normal tissues in carbon-ion therapy.

副作用少的辐射防护剂可用于碳离子疗法，直接诱导 DNA 的聚集损伤。为了找到最有效的辐射防护剂，我们研究了可能具有化学 DNA 修复功能的选定氨基酸对治疗性碳离子的影响。在目前的研究中，我们使用了五种氨基酸：色氨酸 (Trp)、半胱氨酸 (Cys)、蛋氨酸 (Met)、缬氨酸 (Val) 和丙氨酸 (Ala)。在 TE 缓冲液（10 m M Tris，1 m M乙二胺四乙酸，pH 7.5）中制备具有 17 m M氨基酸的超螺旋 pBR322 质粒 DNA 样品。磷酸盐缓冲盐水 (PBS) 也用于测定 0.17 m M氨基酸。在日本千叶的国家放射科学研究所和重离子医学加速器的 6 厘米展开布拉格峰上，用碳离子束 (290 MeV/u) 照射样品。DNA 的断裂被检测为质粒的变化，并通过随后的琼脂糖凝胶电泳定量。每个氨基酸的 DNA 损伤产量和保护因子计算为相对于无试剂对照的比率。Trp 和 Cys 在 PBS 和 TE 缓冲液中对碳离子束诱导的质粒 DNA 损伤显示出辐射保护作用，与 Met 相当。Trp 的双链断裂 (DSB) 产量和保护作用与 Cys 相当。单链断裂和 DSB 的产量与散装溶液中羟基自由基的清除能力（清除羟基自由基的速率常数乘以氨基酸浓度）相关。这些数据表明氨基酸对由碳离子引起的质粒 DNA 损伤的辐射保护作用可以主要通过羟基自由基的清除能力来解释。这些发现表明，某些氨基酸，如 Trp、Cys 和 Met，在碳离子治疗中具有作为防止正常组织 DNA 损伤的放射保护剂的良好潜力。