Fuller R W, Hemrick-Luecke S K, Molloy B B
Biochem Pharmacol. 1983 Apr 1;32(7):1243-9. doi: 10.1016/0006-2952(83)90278-2.
The effects of N-[2-(o-iodophenoxy)ethyl]cyclopropylamine hydrochloride (LY121768) on types A and B monoamine oxidase (MAO) assayed with radiocarbon-labeled serotonin and phenylethylamine, respectively, were studied in vitro and in vivo. Type A MAO from rat brain was inhibited in vitro by LY121768 with an IC50 of 4 x 10(-10) M, whereas 2500 times higher concentrations of LY121768 (IC50 = 1 x 10(-6) M) were required to inhibit type B MAO. The inhibition of type A MAO increased with time of incubation of LY121768 with enzyme prior to substrate addition and persisted after dialysis, indicative of irreversible inhibition. The irreversible inactivation was prevented by harmaline, a reversible, competitive inhibitor of type A MAO, indicating a requirement for catalytic activity of MAO in the time-dependent inactivation by LY121768. In rats, LY121768 selectively inhibited type A MAO in brain and in liver at low doses. The inhibition of type A MAO persisted for several days after a single 10 mg/kg i.p. dose of LY121768 and was associated with a significant increase in brain dopamine, norepinephrine and epinephrine concentrations and a significant decrease in the concentration of the dopamine metabolites, homovanillic acid and 3,4-dihydroxyphenylacetic acid. The inactivation of type A MAO by LY121768 in vivo was prevented by co-administration of harmaline, indicating a similar mechanism for the in vivo inactivation as for the in vitro inactivation of MAO by LY121768. A reasonable inference from these data and from previous literature is that LY121768 acts as a "suicide substrate" for MAO and inactivates the enzyme by formation of a reactive intermediate which binds covalently to the enzyme. The presence of iodine in the LY121768 molecule not only confers high selectivity for type A MAO but offers a site for radionuclide introduction that might be a useful means of labeling type A MAO in vitro or in vivo for various purposes.
分别用放射性碳标记的血清素和苯乙胺测定了盐酸N-[2-(邻碘苯氧基)乙基]环丙胺(LY121768)对A型和B型单胺氧化酶(MAO)的作用，进行了体外和体内研究。LY121768在体外对大鼠脑A型MAO有抑制作用，IC50为4×10(-10)M，而抑制B型MAO则需要浓度高2500倍的LY121768(IC50=1×10(-6)M)。在加入底物之前，LY121768与酶孵育的时间越长，对A型MAO的抑制作用越强，且透析后仍持续存在，表明是不可逆抑制。A型MAO的可逆竞争性抑制剂哈马灵可防止这种不可逆失活，这表明在LY121768的时间依赖性失活过程中需要MAO的催化活性。在大鼠中，低剂量的LY121768可选择性抑制脑和肝脏中的A型MAO。单次腹腔注射10mg/kg LY121768后，对A型MAO的抑制作用持续数天，同时脑多巴胺、去甲肾上腺素和肾上腺素浓度显著升高，多巴胺代谢产物高香草酸和3,4-二羟基苯乙酸浓度显著降低。同时给予哈马灵可防止LYl21768在体内使A型MAO失活，这表明LY121768在体内使MAO失活的机制与体外相似。从这些数据和以前的文献中可以合理推断，LY121768作为MAO的“自杀底物”，通过形成与酶共价结合的反应性中间体使酶失活。LY121768分子中碘的存在不仅赋予了对A型MAO的高选择性，还提供了一个引入放射性核素的位点，这可能是在体外或体内以各种目的标记A型MAO的有用方法。
