Casanova M, Bell D A, Heck H D
Chemical Industry Institute of Toxicology, Research Triangle Park, North Carolina 27709, USA.
Fundam Appl Toxicol. 1997 Jun;37(2):168-80.
Metabolism of dichloromethane (DCM) to formaldehyde (HCHO) via a glutathione S-transferase (GST) pathway is thought to be required for its carcinogenic effects in B6C3F1 mice. In humans, this reaction is catalyzed primarily by the protein product of the gene GSTT1, a member of the Theta class of GST, and perhaps to a small extent by the protein product of the gene GSTM1. Humans are polymorphic with respect to both genes. Since HCHO may bind to both DNA and RNA forming DNA-protein crosslinks (DPX) and RNA-formaldehyde adducts (RFA), respectively, these products were determined in isolated hepatocytes from B6C3F1 mice, F344 rats, Syrian golden hamsters, and humans to compare species with respect to the production of HCHO from DCM and its reaction with nucleic acids. Only mouse hepatocytes formed detectable amounts of DPX, the quantities of which corresponded well with quantities of DPX formed in the livers of mice exposed to DCM in vivo [Casanova, M., Conolly, R.B., and Heck, H. d'A. (1996). Fundam. Appl. Toxicol. 31, 103-116]. Hepatocytes from all rodent species and from humans with functional GSTT1 and GSTM1 genes formed RFA. No RFA were detected in human cells lacking these genes. Yields of RFA in hepatocytes of mice were 4-fold higher than in those of rats, 7-fold higher than in those of humans, and 14-fold higher than in those of hamsters. The RFA:DPX ratio in mouse hepatocytes incubated with DCM was approximately 9.0 +/- 1.4, but it was 1.1 +/- 0.3 when HCHO was added directly to the medium, indicating that HCHO generated internally from DCM is not equivalent to that added externally to cells and that it may occupy separate pools. DPX were not detected in human hepatocytes even at concentrations equivalent to an in vivo exposure of 10,000 ppm; however, the possibility that very small amounts of DPX were produced from DCM cannot be excluded, since HCHO was formed in human cells. Maximal amounts of DPXliver that might be formed in humans were predicted from the amounts in mice and the relative amounts of RFA in hepatocytes of both species. With predicted DPXliver as the dosimeter, the unit risk, the upper 95% confidence limit on the cancer risk, and the margin of exposure were calculated at several concentrations using the linearized multistage and benchmark dose methods. Since the actual delivered dose is smaller than that predicted, the results suggest that DCM poses at most a very low risk of liver cancer to humans.
二氯甲烷（DCM）通过谷胱甘肽S-转移酶（GST）途径代谢为甲醛（HCHO）被认为是其对B6C3F1小鼠产生致癌作用所必需的。在人类中，该反应主要由GST基因家族中Theta类的GSTT1基因的蛋白质产物催化，可能在较小程度上由GSTM1基因的蛋白质产物催化。人类在这两个基因方面存在多态性。由于HCHO可能分别与DNA和RNA结合形成DNA-蛋白质交联（DPX）和RNA-甲醛加合物（RFA），因此在来自B6C3F1小鼠、F344大鼠、叙利亚金黄地鼠和人类的分离肝细胞中测定了这些产物，以比较不同物种在DCM产生HCHO及其与核酸反应方面的情况。只有小鼠肝细胞形成了可检测量的DPX，其数量与体内暴露于DCM的小鼠肝脏中形成的DPX数量非常吻合[卡萨诺瓦，M.，康诺利，R.B.，和赫克，H.d'A.（1996年）。基础与应用毒理学。31，103 - 116]。来自所有啮齿动物物种以及具有功能性GSTT1和GSTM1基因的人类的肝细胞形成了RFA。在缺乏这些基因的人类细胞中未检测到RFA。小鼠肝细胞中RFA的产量比大鼠高4倍，比人类高7倍，比地鼠高14倍。用DCM孵育的小鼠肝细胞中RFA:DPX的比率约为9.0±1.4，但直接向培养基中添加HCHO时该比率为1.1±0.3，这表明由DCM在内部产生的HCHO与外部添加到细胞中的HCHO不等同，并且它可能占据不同的池。即使在相当于体内暴露10,000 ppm的浓度下，人类肝细胞中也未检测到DPX；然而，由于人类细胞中形成了HCHO，不能排除由DCM产生极少量DPX的可能性。根据小鼠中的含量以及两个物种肝细胞中RFA的相对含量预测了人类中可能形成的最大DPXliver量。以预测的DPXliver作为剂量计，使用线性化多阶段和基准剂量方法在几个浓度下计算了单位风险、癌症风险的95%上置信限和暴露边际。由于实际递送的剂量小于预测剂量，结果表明DCM对人类造成肝癌的风险至多非常低。
