Reversible inhibition of cholinesterases by salts of pyrilium, thiopyrilium and selenopyrilium derivatives

Salts of pyrilium, thiopyrilium and selenopyrilium derivatives at pH 7.5 and temperature of 25 degrees C are studied for their effect on the catalytic activity of acetyl cholinesterase (EC 3.1.1.7) of human blood erythrocytes and butyryl cholinesterase (EC 3.1.1.8) of horse blood serum which is measured by the method of potentiometric titration. All enumerated salts are established to be strong reversible inhibitors of mixed-type cholinesterases, that is testified by small values of the inhibitory constants: competitive Ki, noncompetitive K'i and generalized K epsilon. Pyrilium and selenopyrilium salts inhibit acetyl cholinesterase of human blood erythrocytes to a higher extent than butyryl cholinesterase of horse blood serum, and thiopyrilium salts inhibit the latter to the highest extent. By the value of the inhibitory effect on acetyl cholinesterase of human blood erythrocytes thiopyrilium salts exceed the analogous pyrilium salts, whereas in experiments with butyl cholinesterase of horse blood serum there is an opposite dependence.