A secondary isotope effect study of equine serum butyrylcholinesterase-catalyzed hydrolysis of acetylthiocholine

β-Secondary deuterium isotope effects have been measured for equine serum butyrylcholinesterase-catalyzed hydrolysis of acetyl-L3-thiocholine (L = H or 2H). The dependencies of initial rates on isotopic substrate concentrations show close adherence to Michaelis–Menten kinetics, and yield the following isotope effects: D3kcat/Km = 0.98 ± 0.02 and D3kcat = 1.10 ± 0.02. The modestly inverse isotope effect on kcat/Km is consistent with partial rate limitation by a step that converts the sp2-hybridized ester carbonyl of the E + A reactant state into a quasi-tetrahedral transition state in the acylation stage of catalysis. On the other hand, the markedly normal isotope effect on kcat indicates that the Michaelis complex that accumulates at substrate saturation of the active site during catalytic turnover is a tetrahedral intermediate, whose decomposition is the rate-limiting step. These results compliment a previous report [J.R. Tormos et al., J. Am. Chem. Soc. 127 (2005) 14538–14539] that showed that substrate-activated hydrolysis of acetylthiocholine (ATCh), catalyzed by recombinant human butyrylcholinesterase, is also rate limited by decomposition of an accumulating tetrahedral intermediate.