Organophosphate inhibition of human heart muscle cholinesterase isoenzymes

The rate of acetylcholine hydrolysis of mammalian heart muscle influences c ardiac responses to vagal innervation. We characterized cholinesterases of human left ventricular heart muscle with respect to both substrate specific ity and irreversible inhibition kinetics with the organophosphorus inhibito r N,N'-di-isopropylphosphorodiamidic fluoride (mipafox). Specimens were obt ained postmortem from three men and four women (61 +/- 5 years) with no his tory of cardiovascular disease. Myocardial choline ester hydrolyzing activi ty was determined with acetylthiocholine (ASCh; 1.25 mM), acetyl-beta-methy lthiocholine (A beta MSCh; 2.0 mM), and butyrylthiocholine (BSCh; 30 mM). A fter irreversible and covalent inhibition (60 min; 25 degrees C) with a wid e range of mipafox concentrations (50 nM-5 mM), residual choline ester hydr olyzing activities were fitted to a sum of up to five exponentials using we ighted least-squares non-linear curve fitting. In each ease, quality of cur ve fitting reached its optimum on the basis of a four component model. Fina l classification of heart muscle cholinesterases was achieved according to substrate hydrolysis patterns (nmol/min per g wet weight) and to second-ord er organophosphate inhibition rate constants k(2) (1/mol per min); one chol ine ester hydrolyzing enzyme was identified as acetylcholinesterase (AChE; k(2)/mipafox = 6.1 (+/- 0.8) x 10(2)), and one as butyrylcholinesterase (BC hE; k(2)/mipafox = 5.3 (+/- 1.1) x 10(3)). An enzyme exhibiting both ChE-li ke substrate specificity and relative resistance to mipafox inhibition (k(2 )/mipafox = 5.2 (+/- 1.0) x 10(-1)) was classified as atypical cholinestera se. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.