Chemnitius J M, Haselmeyer K H, Gonska B D, Kreuzer H, Zech R
Department of Cardiology, Georg-August University, Göttingen, Germany.
Pharmacol Res. 1996 Jul-Aug;34(1-2):65-72. doi: 10.1006/phrs.1996.9999.
Inhibitory effects of the dopamine D2-receptor antagonistic benzamide compound metoclopramide (MCP) on acetylcholinesterase (AChE; EC 3.1.1.7) isoenzymes of both erythrocytes and human caudate nucleus and on human serum cholinesterase (ChE; EC 3.1.1.8) were studied in vitro using a spectrophotometric assay with acetylthiocholine (ASCh) as substrate. MCP concentrations in the assays varied from 0.30 microM to 0.15 mM. All isoenzymes studied were inhibited by metoclopramide in a concentration-dependent manner. MCP inhibition of AChE and ChE isoenzymes was not time-dependent and of the reversible type. Double reciprocal plots of the reaction velocity against varying ASCh concentrations revealed that, for AChE isoenzymes of erythrocytes and of the caudate nucleus, MCP reduced both maximal reaction velocity (Vmax) and substrate affinity (apparent Michaelis constant, KM, increased). Thus, MCP inhibition of both AChE isoenzymes was of mixed competitive/non-competitive type. MCP constants for reversible competitive (Ki) and non-competitive (Ki) inhibition could be determined for erythrocyte AChE (Ki = 10 microM; Ki = 70 microM) and caudate nucleus AChE (Ki = 9.3 microM; Ki = 82 microM). In contrast to MCP inhibition of AChE isoenzymes, the type of reversible MCP inhibition of human serum ChE depended on substrate concentration. If substrate concentration exceeded 0.2 mM, MCP inhibition was of mixed competitive/non-competitive type (Ki = 0.19 microM; Ki = 1.4 microM). MCP inhibition was of uncompetitive type, if substrate concentration was below 0.2 mM (Ki(u) = 1.0 microM). The mixed-type MCP inhibition of cholinesterase isoenzymes, because of its non-competitive component, can only partially be overcome by increased concentrations of the cholinergic transmitter acetylcholine (ACh). Since, with intravenous infusions, peak MCP plasma concentrations in humans reach 4 microM, MCP inhibition of ACh hydrolysis in vivo may contribute both to prokinetic and anti-emetic actions of the substance and to its extrapyramidal side effects.
采用以乙酰硫代胆碱（ASCh）为底物的分光光度法，在体外研究了多巴胺D2受体拮抗剂苯甲酰胺化合物甲氧氯普胺（MCP）对红细胞和人尾状核乙酰胆碱酯酶（AChE；EC 3.1.1.7）同工酶以及人血清胆碱酯酶（ChE；EC 3.1.1.8）的抑制作用。实验中MCP的浓度范围为0.30微摩尔/升至0.15毫摩尔/升。所研究的所有同工酶均受到甲氧氯普胺的浓度依赖性抑制。MCP对AChE和ChE同工酶的抑制作用不具有时间依赖性，且为可逆型。以不同ASCh浓度为横坐标、反应速度为纵坐标绘制的双倒数图显示，对于红细胞和尾状核的AChE同工酶，MCP降低了最大反应速度（Vmax）并降低了底物亲和力（表观米氏常数KM增大）。因此，MCP对两种AChE同工酶的抑制作用属于混合竞争性/非竞争性类型。对于红细胞AChE（Ki = 10微摩尔/升；Ki = 70微摩尔/升）和尾状核AChE（Ki = 9.3微摩尔/升；Ki = 82微摩尔/升），可以确定可逆竞争性（Ki）和非竞争性（Ki）抑制的MCP常数。与MCP对AChE同工酶的抑制作用不同，MCP对人血清ChE的可逆抑制类型取决于底物浓度。如果底物浓度超过0.2毫摩尔/升，MCP抑制作用为混合竞争性/非竞争性类型（Ki = 0.19微摩尔/升；Ki = 1.4微摩尔/升）。如果底物浓度低于0.2毫摩尔/升，MCP抑制作用为非竞争性类型（Ki(u) = 1.0微摩尔/升）。由于其非竞争性成分，MCP对胆碱酯酶同工酶的混合型抑制作用只能通过增加胆碱能递质乙酰胆碱（ACh）的浓度来部分克服。由于静脉输注时，人体中MCP的血浆峰值浓度可达4微摩尔/升，MCP在体内对ACh水解的抑制作用可能既有助于该物质的促动力和止吐作用，也有助于其锥体外系副作用。
