Booth G M, Lee A H
Bull World Health Organ. 1971;44(1-3):91-8.
The study of toxicology and other related fields has been largely based on in vitro techniques. These methods have provided quantitative information on the effects of inhibitors on enzymes, but none on the localized effects of inhibitors on selected sites of action within the animal. Histochemical study of frozen sections does provide data on the site of action of toxicants. The utility of histochemistry in conjunction with in vitro methods is discussed.The substrates acetylthiocholine and phenyl thioacetate were utilized in demonstrating cholinesterase. Neither substrate penetrated well into freshly dissected nerve cord preparations, but both compounds were hydrolysed by sectioned tissue. The leaving group of phenyl thioacetate was demonstrated to be benzenethiol. In general, acetylthiocholine was hydrolysed slightly more rapidly by insect cholinesterases. A unique cholinesterase was found in motor end-plates of cricket muscle, which hydrolyses acetylthiocholine and which was inhibited by physostigmine. No other insect muscle preparation showed this activity. Topical application of insecticides showed that a vital site of action in flies is the peripheral area of the thoracic ganglia and that in crickets the brain and nerve cord are involved at knock-down. Kinetic data indicate that acetylthiocholine has a greater affinity than does phenyl thioacetate for a variety of enzyme sources. Ultrastructural evidence shows that cholinesterases that hydrolyse acetylthiocholine are membrane-bound. Phenyl thioacetate was found to be useful as a model in designing new insecticides.
毒理学及其他相关领域的研究很大程度上基于体外技术。这些方法提供了关于抑制剂对酶作用的定量信息，但没有关于抑制剂在动物体内特定作用部位的局部效应的信息。冷冻切片的组织化学研究确实提供了关于毒物作用部位的数据。本文讨论了组织化学与体外方法结合的实用性。利用底物乙酰硫代胆碱和苯硫乙酸来证明胆碱酯酶的存在。两种底物都不能很好地渗透到刚解剖的神经索制剂中，但两种化合物都能被切片组织水解。已证明苯硫乙酸的离去基团是苯硫酚。一般来说，昆虫胆碱酯酶对乙酰硫代胆碱的水解速度略快一些。在蟋蟀肌肉的运动终板中发现了一种独特的胆碱酯酶，它能水解乙酰硫代胆碱，并被毒扁豆碱抑制。没有其他昆虫肌肉制剂显示出这种活性。杀虫剂的局部应用表明，苍蝇的一个重要作用部位是胸神经节的周边区域，而在蟋蟀中，击倒时脑和神经索会受到影响。动力学数据表明，乙酰硫代胆碱对多种酶源的亲和力比苯硫乙酸大。超微结构证据表明，水解乙酰硫代胆碱的胆碱酯酶是膜结合的。发现苯硫乙酸在设计新型杀虫剂方面可作为一种模型。
