Shenouda Josephine, Green Paula, Sultatos Lester
Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey, Newark, NJ 07103, USA.
Toxicol Appl Pharmacol. 2009 Dec 1;241(2):135-42. doi: 10.1016/j.taap.2009.08.014. Epub 2009 Aug 19.
Acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase (EC 3.1.1.8) are enzymes that belong to the superfamily of alpha/beta-hydrolase fold proteins. While they share many characteristics, they also possess many important differences. For example, whereas they have about 54% amino acid sequence identity, the active site gorge of acetylcholinesterase is considerably smaller than that of butyrylcholinesterase. Moreover, both have been shown to display simple and complex kinetic mechanisms, depending on the particular substrate examined, the substrate concentration, and incubation conditions. In the current study, incubation of butyrylthiocholine in a concentration range of 0.005-3.0 mM, with 317 pM human butyrylcholinesterase in vitro, resulted in rates of production of thiocholine that were accurately described by simple Michaelis-Menten kinetics, with a K(m) of 0.10 mM. Similarly, the inhibition of butyrylcholinesterase in vitro by the organophosphate chlorpyrifos oxon was described by simple Michaelis-Menten kinetics, with a k(i) of 3048 nM(-1) h(-1), and a K(D) of 2.02 nM. In contrast to inhibition of butyrylcholinesterase, inhibition of human acetylcholinesterase by chlorpyrifos oxon in vitro followed concentration-dependent inhibition kinetics, with the k(i) increasing as the inhibitor concentration decreased. Chlorpyrifos oxon concentrations of 10 and 0.3 nM gave k(i)s of 1.2 and 19.3 nM(-1) h(-1), respectively. Although the mechanism of concentration-dependent inhibition kinetics is not known, the much smaller, more restrictive active site gorge of acetylcholinesterase almost certainly plays a role. Similarly, the much larger active site gorge of butyrylcholinesterase likely contributes to its much greater reactivity towards chlorpyrifos oxon, compared to acetylcholinesterase.
乙酰胆碱酯酶（EC 3.1.1.7）和丁酰胆碱酯酶（EC 3.1.1.8）属于α/β-水解酶折叠蛋白超家族的酶。虽然它们有许多共同特征，但也有许多重要差异。例如，它们的氨基酸序列同一性约为54%，但乙酰胆碱酯酶的活性位点峡谷比丁酰胆碱酯酶的活性位点峡谷小得多。此外，根据所检测的特定底物、底物浓度和孵育条件，两者都已显示出简单和复杂的动力学机制。在当前研究中，在体外将丁酰硫代胆碱在0.005 - 3.0 mM的浓度范围内与317 pM的人丁酰胆碱酯酶孵育，硫代胆碱的生成速率可用简单的米氏动力学准确描述，K(m)为0.10 mM。同样，有机磷毒死蜱氧磷在体外对丁酰胆碱酯酶的抑制作用可用简单的米氏动力学描述，k(i)为3048 nM⁻¹ h⁻¹，K(D)为2.02 nM。与丁酰胆碱酯酶的抑制作用相反，毒死蜱氧磷在体外对人乙酰胆碱酯酶的抑制遵循浓度依赖性抑制动力学，k(i)随着抑制剂浓度降低而增加。10 nM和0.3 nM的毒死蜱氧磷浓度分别给出k(i)为1.2和19.3 nM⁻¹ h⁻¹。虽然浓度依赖性抑制动力学的机制尚不清楚，但乙酰胆碱酯酶小得多、限制性更强的活性位点峡谷几乎肯定起了作用。同样，与乙酰胆碱酯酶相比，丁酰胆碱酯酶大得多的活性位点峡谷可能导致其对毒死蜱氧磷的反应性更高。
