O'Brien E M, Dostert P, Tipton K F
Department of Biochemistry, Trinity College, Dublin, Republic of Ireland.
Biochem Pharmacol. 1995 Jul 31;50(3):317-24. doi: 10.1016/0006-2952(95)00145-p.
Oxidation of the anticonvulsant drug milacemide [2-n-(pentylamino)acetamide] by monoamine oxidase-B (MAO-B) has been reported to be important in terminating its activity. Comparison of the oxidation of this compound by MAO-B preparations from ox and rat liver showed the former enzyme to have a significantly higher Km value towards this substrate. In keeping with this, the Ki values for milacemide acting as a competitive inhibitor of these enzymes showed it to have a lower affinity for ox liver MAO-B. Comparative studies on the time-dependent inhibition of the two enzymes also showed a lower sensitivity of that from the ox liver. Studies with a series of analogues involving replacement of pentylamino group of milacemide showed marked differences between the sensitivities of the two enzymes. The largest differences were shown by the compound 2(4-(3-chlorobenzoxy)phenethylamino)acetamide which gave IC50 values of 0.051 +/- 0.008 and 4.1 +/- 0.8 microM with the rat and ox enzymes, respectively, when activities were assayed without prior enzyme-inhibitor preincubation. When the enzyme and inhibitor were incubated for 60 min at 37 degrees before assay these values fell to 0.027 +/- 0.002 and 3.5 +/- 0.4 microM, respectively. These marked differences prompted a study of the inhibition of MAO-A and MAO-B from human liver and brain, mouse brain and rat brain as well as MAO-B from ox liver by milacemide and alpha-methylmilacemide. There were no significant differences in the sensitivities of any of the mitochondrial MAO-A preparations studied towards these compounds. However, MAO-B from human brain and liver mitochondrial resembled that from ox liver in being less sensitive to inhibition than the rat and mouse enzymes. Purification of the ox liver MAO-B did not significantly affect its interactions with milacemide and alpha-methylmilacemide. The marked species differences reported here raise questions concerning the validity of rodent model systems, that have frequently been used for assessing the in vivo and in vitro actions of milacemide and its analogues, for the situation in the human.
据报道，抗惊厥药物米拉醋胺[2 - N -（戊基氨基）乙酰胺]被单胺氧化酶 - B（MAO - B）氧化在终止其活性方面很重要。比较牛和大鼠肝脏来源的MAO - B制剂对该化合物的氧化作用，发现前者对该底物的Km值显著更高。与此一致的是，米拉醋胺作为这些酶的竞争性抑制剂的Ki值表明其对牛肝脏MAO - B的亲和力较低。对这两种酶的时间依赖性抑制的比较研究也表明牛肝脏来源的酶敏感性较低。对一系列涉及取代米拉醋胺戊基氨基的类似物的研究表明，这两种酶的敏感性存在显著差异。化合物2(4-(3 - 氯苄氧基)苯乙氨基)乙酰胺表现出的差异最大，在未经酶 - 抑制剂预孵育测定活性时，其对大鼠和牛酶的IC50值分别为0.051±0.008和4.1±0.8 microM。当在测定前将酶和抑制剂在37℃孵育60分钟时，这些值分别降至0.027±0.002和3.5±0.4 microM。这些显著差异促使研究米拉醋胺和α - 甲基米拉醋胺对人肝脏和大脑、小鼠大脑和大鼠大脑中的MAO - A和MAO - B以及牛肝脏中的MAO - B的抑制作用。所研究的任何线粒体MAO - A制剂对这些化合物的敏感性均无显著差异。然而，人肝脏和脑线粒体中的MAO - B与牛肝脏中的MAO - B相似，对抑制的敏感性低于大鼠和小鼠的酶。牛肝脏MAO - B的纯化并未显著影响其与米拉醋胺和α - 甲基米拉醋胺的相互作用。此处报道的显著种属差异引发了关于啮齿动物模型系统有效性的问题，该模型系统经常被用于评估米拉醋胺及其类似物在体内和体外的作用，而这些作用与人的情况相关。
