Mortensen S R, Brimijoin S, Hooper M J, Padilla S
Cellular and Molecular Toxicology Branch, US Environmental Protection Agency, Research Triangle Park, North Carolina 27711, USA.
Toxicol Appl Pharmacol. 1998 Jan;148(1):46-9. doi: 10.1006/taap.1997.8287.
The toxicological literature is replete with studies which have attempted to correlate differences in in vivo sensitivity to anticholinesterases with a common in vitro measure: acetylcholinesterase (AChE) IC50 values. Generally, it is assumed that these IC50 values reflect the intrinsic sensitivity of the AChE molecule to the inhibitor. Our goal was to ascertain whether differences in AChe sensitivity to an organophosphate (i.e., IC50 values) are due to varying properties of the enzyme molecule (i.e., present assumption) or to extrinsic factors. Tissue samples were obtained from immature and adult Long-Evans rats. AChE IC50 values were determined by incubating tissue homogenates with chlorpyrifos-oxon (active metabolite of chlorpyrifos, a common organophosphate insecticide) for 30 min at 26 degrees C, and then measuring residual AChE activity. The following IC50 values were noted for postnatal day 4 and adult animals, respectively: brain, 10 nM for both ages; liver, 96 and 527 nM; plasma, 18 and 326 nm. Thus, the "apparent" sensitivity of AChe was prone to vary dramatically with age and tissue type. In contrast, when AChE was isolated from the same tissues by immunoprecipitation, there were no age- or tissue- related differences (IC50 approximately equal to 3 nM in every case). These data show clearly that IC50 values from a crude homogenate do not measure the true sensitivity of AChE to the inhibitor. Presumably, for chlorpyrifos-oxon, at least, the tissue IC50 values depend greatly on a tissue's propensity to sequester or hydrolyze chlorpyrifos-oxon.
毒理学文献中充斥着大量研究，这些研究试图将体内对抗胆碱酯酶敏感性的差异与一种常见的体外测量指标——乙酰胆碱酯酶（AChE）的半数抑制浓度（IC50）值联系起来。一般认为，这些IC50值反映了AChE分子对抑制剂的内在敏感性。我们的目标是确定AChE对有机磷酸酯的敏感性差异（即IC50值）是由于酶分子的不同特性（即目前的假设）还是外部因素所致。从幼年和成年的Long-Evans大鼠获取组织样本。通过将组织匀浆与毒死蜱氧磷（毒死蜱的活性代谢产物，一种常见的有机磷酸酯杀虫剂）在26℃孵育30分钟，然后测量残余的AChE活性，来确定AChE的IC50值。分别记录出生后第4天和成年动物的以下IC50值：脑，两个年龄段均为10 nM；肝脏，分别为96 nM和527 nM；血浆，分别为18 nM和326 nM。因此，AChE的“表观”敏感性容易随年龄和组织类型而显著变化。相比之下，当通过免疫沉淀从相同组织中分离出AChE时，不存在与年龄或组织相关的差异（每种情况下IC50约等于3 nM）。这些数据清楚地表明，粗匀浆的IC50值并不能衡量AChE对抑制剂的真正敏感性。据推测，至少对于毒死蜱氧磷来说，组织IC50值在很大程度上取决于组织对毒死蜱氧磷的隔离或水解倾向。
