Butyrylcholinesterase (BChE), a major detoxification enzyme found abundantly in many tissues and organisms, constitutes the first line defense in the serum of higher organisms and is a marker for toxic exposure. In this study, the interaction of two plant growth regulators, indole-3-acetic acid (IAA) and chlorogenic acid (CA) with purified human and horse serum BChE is investigated. The time dependent interaction of IAA with the two enzyme species was concentration dependent and rapid. Through kinetic studies, IAA was found to be linear-mixed type inhibitor for human serum BChE, and uncompetitive inhibitor for the horse serum enzyme. For the human BChE, a and the K-i value was calculated as 2.15 +/- 1.09 and 3.09 +/- 0.98 mM, respectively, and for the horse enzyme the Ki value was calculated as 1.05 +/- 0.09 mM. The time dependent interaction of CA with the two enzyme species was biphasic. At low CA concentrations, CA stimulated the activities of both enzyme species whereas at high CA concentrations, inhibition was observed. At high concentrations, the inhibition kinetics for both enzymes fitted the non-competitive inhibition model. The Ki values calculated for human and horse BChE were 2.75 +/- 0.14 and 0.96 +/- 0.07 mM, respectively. The differences in the interaction of these two growth regulators with the two enzymes species arises from the structural differences between the human and horse serum BChE which can be considered as a triple human mutant BChE. (C) 2003 Elsevier Inc. All rights reserved.