Cholinesterases of heart muscle. Characterization of multiple enzymes using kinetics of irreversible organophosphorus inhibition.

ABSTRACT

Cholinesterases of porcine left ventricular heart muscle were characterized with respect to substrate specificity and inhibition kinetics with organophosphorus inhibitors N,N'-di-isopropyl-phosphorodiamidic fluoride (Mipafox), di-isopropylphosphorofluoridate (DFP), and diethyl p-nitro-phenyl phosphate (Paraoxon). Total myocardial choline ester hydrolysing activity (234 nmol/min/g wet wt with 1.5 mM acetylthiocholine, ASCh; 216 nmol/min/g with 30 mM butyrylthiocholine, BSCh) was irreversibly and covalently inhibited by a wide range of inhibitor concentrations and, using weighted least-squares non-linear curve fitting, residual activities as determined with four different substrates in each case were fitted to a sum of up to four exponential functions. Quality of curve fitting as assessed by the sum of squares reached its optimum on the basis of a three component model, thus, indicating the presence of three different enzymes taking part in choline ester hydrolysis. Final classification of heart muscle cholinesterases was obtained according to both substrate hydrolysis patterns with ASCh, BSCh, acetyl-beta-methylthiocholine and propionylthiocholine, and second-order rate constants for the reaction with organophosphorus inhibitors Mipafox, DFP, and Paraoxon. One choline ester-hydrolysing enzyme was identified as acetylcholinesterase (EC 3.1.1.7), and one as butyrylcholinesterase (EC 3.1.1.8). The third enzyme with relative resistance to organophosphorus inhibition was classified as atypical cholinesterase.