The inhibition of horse serum butyrylcholinesterase (EC 3.1.1.8) by sodium arsenite (NaAsO2) and zinc chloride (ZnCl2) was studied at 37 degrees C in a TRIS buffer, pH 7.5, with a flow microcalorimeter operating in the mixing mode. The dose-response curves were analyzed with the Hill equation, as deduced from the "fractal kinetic" model of Savageau by considering an exponential distribution of reaction rate constants. The C-50 values were in the millimolar range and zinc chloride was a better inhibitor than sodium arsenite. This was confirmed by the values of the apparent rate constants. The Hill exponent, interpreted as a fractional reaction order, was also higher for zinc chloride. According to the statistical interpretation, the heterogeneity of binding reactions was slightly higher for Zn, in agreement with a speciation study which showed the existence of several Zn species in solution, with the free Zn2+ ion accounting for about 10% of the total. (C) 2013 Elsevier B.V. All rights reserved.