engleski

Interaction of enantiomers of quinuclidin-3-yl acetates with cholinesterases

We studied the reactions of human erythrocyte acetylcholinesterase (AChE ; EC 3.1.1.7) and human serum butyrylcholinesterase (BChE ; EC 3.1.1.8) with (R)- and (S)-enantiomers of N-methyl-3-acetoxy-quinuclidinium iodide (MetQAc) and N-benzyl-3-acetoxy-quinuclidinium bromide (BzQAc). The hydrolysis of (R)- and (S)-enantiomers of MetQAc and BzQAc, and for comparison hydrolysis of acetylcholine (ACh) by AChE and BChE, was measured in 0.9 % sodium chloride by pH-stat method at pH=7.4 and 37oC. The product of hydrolysis of the compounds, acetic acid, was titrated with 5 mM sodium hydroxide for 5-10 min. The concentrations of the acetates were 0.5 - 20 mM. The maximum activities of AChE were 2.6, 4.3 and 6.4 &micro ; ; ; ; ; ; mol min– 1 ml-1, while that of BChE were 0.64, 3.3 and 5.6 &micro ; ; ; ; ; ; mol min– 1 ml-1 for (R)-BzQAc, (R)-MetQAc and ACh, respectively. (S)-enantiomer of quinuclidin-3-yl acetates were not hydrolysed by neither AChE nor BChE in observation time of 10 minutes. To check whether (S)-enantiomers react with AChE or BChE at all, hydrolysis of acetylthiocholine (0.1 - 1.0 mM) by the enzymes was measured in the presence of these compounds as enzyme inhibitors. Acetylthiocholine hydrolysis was measured spectrophotometricaly with DTNB as a thiole reagent at 412 nm in 0.1 M phosphate buffer (pH=7.4) at 37oC. Dissociation constants of enzyme-inhibitor complexes (Ki) were determined from the degree of inhibition of acetylthiocholine hydrolysis. For AChE Ki constants were 1.8 and 0.07 mM, and for BChE 0.61 and 0.14 mM for (S)-MetQAc and (S)-BzQAc, respectively. Both cholinesterases showed a high degree of stereoselectivity revealing (R)- over (S)- preference as far as hydrolysis of the enantiomers is concerned. However, (S)-acetates acted as inhibitors of acethylthiocholine hydrolysis. Quinuclidinium compounds were synthesised by Mislav Oršulić, Ines Primožič and Srđanka Tomić at Department of Chemistry, Faculty of Science, University of Zagreb. This study was supported by the Ministry of Science and Technology of the Republic of Croatia.

quinuclidin-3-yl acetates; enantiomers; cholinesterase; interaction

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