Inhibition of two monomeric butyrylcholinesterases from rabbit liver by chlorpromazine and other drugs.

A new form of cholinesterases has been discovered in rabbit liver; the new enzymes are monomeric butyrylcholinesterases (EC 3.1.1.8), mBuChE I and mBuChE II. These enzymes are inhibited reversibly by chlorpromazine in the pharmacologically active concentration range and they exhibit mixed competitive-noncompetitive inhibition patterns. The apparent competitive inhibition constants, Ki with chlorpromazine, are 1.8 x 10(-6) M for mBuChE I and 7.6 x 10(-6) M for mBuChE II, whereas the noncompetitive inhibition constant is 1.1 x 10(-5) M for mBuChE II as determined by spectrophotometric assay with n-butyrylthiocholine iodide substrate. Although inhibition of mBuChE I also exhibited noncompetitive behavior, a binding constant could not be determined. Human serum oligometric butyrylcholinesterase (oBuChE) was employed as a control cholinesterase and also demonstrated mixed inhibition kinetics. The competitive inhibition constant for the oBuChE was 5.5 x 10(-7) M in the low substrate region, whereas the apparent noncompetitive binding constant was 1.6 x 10(-5) M in the activated higher substrate region with n-butyrylthiocholine iodide as the substrate and chlorpromazine as the reversible inhibitor. The presence of a noncompetitive binding component indicates the presence of an operative modifier or allosteric site binding the inhibitor on both mBuChEs and the oBuChE. The inhibition constants were calculated assuming that the enzymes followed simple Michaelian kinetics.