engleski

Peripheral binding of ethopropazine to horse serum butyrylcholinesterase

The inhibition of purified horse serum butyrylcholinesterase (BChE) with ethopropazine (0.25-20 uM) was studied in order to evaluate the binding site(s) on the enzyme. Activities were measured spectrophotometrically with acetylthiocholine (ATCh ; 0.05-80 mM) as substrate at 37º C in 0.1 M phosphate buffer pH=7.4. The pS-curve for the ATCh hydrolysis fitted well the Webb equation: Ks and Kss = 0.25 and 2.0 mM respectively, beta=3.2. This equation assumes two binding sites for the substrate on the enzyme, catalytic and peripheral, and the beta-value above unity indicates apparent substrate activation. Inhibition of BChE with ethopropazine was non-competitive at substrate concentrations up to 1.0 mM. The enzyme-inhibitor dissociation constant was 0.81 uM. Competition between ATCh and ethopropazine occured at substrate concentrations above the Kss value for ATCh. Such inhibition pattern indicates binding of ethopropazine to the peripheral, non-productive, site on the enzyme

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