[Effect of carbodiimides on the catalytic properties of cholinesterases]

The specific modifiers of the carboxylic groups - 1.3-dicyclohexylcarbodiimide (DCC) and 1-cyclohexyl-3-(2-morpholinyl-4-ethyl)carbodiimide methyl-p-toluenesulfonate (CMC) cause irreversible inhibition of acetylcholinesterase from human erythrocytes (AChE) and butyrycholinesterase from horse blood serum (BuChE). The rate of hydrolysis of both acetylcholine and non-charged substrates - phenylacetate and indophenylacetate - is thereby decreased. At initial steps of the reaction the irreversible inhibition obeys the kinetics of the pseudo-monomolecular reactions with the following values of bimolecular constants for the rates of DCC and CMC reactions: for AChE - 2.3 +/- 0.19; 5.1 +/- 0.23 M-1 min-1, for BuChE - 1.5 +/- 0.14 and (2.2 +/- 0.21) . 10(2) M-1 min-1 (25 degree, pH 7.5). The morpholinyl carbodiimide with a quaternary nitrogen. a more specific inhibitor of cholinesterases, which is of a mixed (Ki = (3.0 +/- 0.21) . 10(-4) M for AChE) and of competitive (Ki = (3.3 +/- 0.24) . 10(-5) M for BuChE) type. Under AChE modification by CMC at concentrations causing 50 - 70% inhibition, the substrate inhibition constants and the thermal stability of the non-inhibited part of the enzyme are increased. The role of functional groups of the catalytic and regulatory centers of cholinesterases in the chemical modification by carbodiimides is discussed.