Moore D E, Sik R H, Bilski P, Chignell C F, Reszka K J
Laboratory of Molecular Biophysics, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.
Photochem Photobiol. 1994 Dec;60(6):574-81. doi: 10.1111/j.1751-1097.1994.tb05150.x.
Sunlight has been implicated in the high incidence of skin cancer found in patients receiving 6-mercaptopurine (PSH) in the form of its pro-drug azathioprine. In this study we have used EPR spectroscopy in conjunction with the spin-trapping technique to determine whether PSH and its metabolic or photochemical oxidation products generate highly reactive free radicals upon UV irradiation. When an aqueous anaerobic solution (pH 5 or 9) of PSH (pKa = 7.7) and either 2-methyl-2-nitrosopropane (MNP) or nitromethane (NM) were irradiated (lambda > 300 nm) with a Xe arc lamp, the corresponding purine-6-thiyl (PS.) radical adduct and the reduced form of the spin trap (MNP/H. or CH3NO2.-) were observed. However, no radical adducts were detected when PSH and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were irradiated (lambda = 320 nm) in oxygen-free buffer. These findings suggest that PSH does not photoionize but that instead MNP and NM are reduced by direct electron transfer from excited state PSH, 1.3(PSH). In aerobic solution, oxygen can act as an electron acceptor and the O2.- and PS. radicals are formed and trapped by DMPO. 6-Mercaptopurine did photoionize when irradiated with a Nd:YAG laser at 355 nm as evidenced by the appearance of the DMPO/H.(eq- + H+) adduct, which decreased in intensity in the presence of N2O. 1.3(6-Mercaptopurine) oxidized ascorbate, formate and reduced glutathione to the corresponding ascorbyl, CO2.- or glutathiyl radicals. The photochemical behavior of 6-thioxanthine and 6-thiouric acid was similar to PSH. However, the excited states of these metabolic oxidation products exhibited stronger reducing properties than 1.3(PSH)*.(ABSTRACT TRUNCATED AT 250 WORDS)
阳光被认为与接受前药硫唑嘌呤形式的6-巯基嘌呤（PSH）的患者皮肤癌高发有关。在本研究中，我们使用电子顺磁共振光谱结合自旋捕获技术来确定PSH及其代谢或光化学氧化产物在紫外线照射下是否会产生高活性自由基。当用氙弧灯照射（λ>300nm）PSH（pKa = 7.7）与2-甲基-2-亚硝基丙烷（MNP）或硝基甲烷（NM）的厌氧水溶液（pH 5或9）时，观察到相应的嘌呤-6-硫基（PS.）自由基加合物和自旋捕获剂的还原形式（MNP/H.或CH3NO2.-）。然而，当在无氧缓冲液中照射（λ = 320nm）PSH和5,5-二甲基-1-吡咯啉-N-氧化物（DMPO）时，未检测到自由基加合物。这些发现表明PSH不会发生光离子化，而是MNP和NM通过来自激发态PSH，1.3(PSH)*的直接电子转移而被还原。在有氧溶液中，氧气可作为电子受体，O2.-和PS.自由基形成并被DMPO捕获。当用355nm的Nd:YAG激光照射时，6-巯基嘌呤确实发生了光离子化，DMPO/H.(eq- + H+)加合物的出现证明了这一点，在N2O存在下其强度降低。1.3(6-巯基嘌呤)*将抗坏血酸、甲酸和还原型谷胱甘肽氧化为相应的抗坏血酰、CO2.-或谷胱甘肽基自由基。6-硫代黄嘌呤和6-硫代尿酸的光化学行为与PSH相似。然而，这些代谢氧化产物的激发态表现出比1.3(PSH)*更强的还原性质。（摘要截短于250字）
