An evaluation of the inhibition of human butyrylcholinesterase and acetylcholinesterase by the organophosphate chlorpyrifos oxon.

Acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase (EC 3.1.1.8) are enzymes that belong to the superfamily of α/β-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 mM – 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 Km 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 ki of 3,048 nM−1h−1, and a KD 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 ki increasing as the inhibitor concentration decreased. Chlorpyrifos oxon concentrations of 10 nM and 0.3 nM gave kis of 1.2 nM−1h−1 and 19.3 nM−1h−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）是属于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，但也存在许多重要差异。例如，尽管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要小得多。此外，根据所检测的特定底物、底物浓度和孵育条件，两者都已被证明具有简单和复杂的动力学机制。在当前研究中，在0.005 mM - 3.0 mM的浓度范围内，将丁硫胆碱与317 pM的人丁酰胆碱酯酶在体外进行孵育，产生硫胆碱的速率可由简单的米氏动力学准确描述，Km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也可用简单的米氏动力学描述，ki为3048 nM⁻¹h⁻¹，KD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，随着抑制剂浓度降低，ki增大。毒死蜱氧磷浓度为10 nM和0.3 nM时，ki分别为1.2 nM⁻¹h⁻¹和19.3 nM⁻¹h⁻¹。尽管浓度依赖性抑制动力学的机制尚不清楚，但乙酰胆碱酯酶更小、更具限制性的活性位点峡谷几乎肯定起到了一定作用。同样，与乙酰胆碱酯酶相比，丁酰胆碱酯酶更大的活性位点峡谷可能导致其对毒死蜱氧磷具有更高的反应性。