This study is the first report on the effectiveness and specificity of glucopyranosylidene-spiro-thiohydantoin (G-TH) inhibitor on the 2-chloro-4-nitrophenyl-4-O-beta-D-galactopyranosyl-maltoside (GalG(2)CNP) hydrolysis catalysed by human salivary alpha-amylase (HSA). The inhibition of hydrolysis is a mixed-noncompetitive type. In any case, only one molecule of inhibitor binds to HSA. Since our substrate and inhibitor are small molecules the long enough active site facilitates accommodating both of them simultaneously. However, the product formation can be excluded from enzyme-substrate-inhibitor complex (ESI) since Dixon plots are linear. Kinetic constants calculated from secondary plots and nonlinear regression are almost entirely equal, confirming the fidelity of the suggested model. Kinetic constants (K-1i = 7.3 mM, L-1i = 2.84 mM) show that G-TH is not such a potent inhibitor of HSA as acarbose and indicate higher stability for ESI than for enzyme-inhibitor complex. (C) 2003 Elsevier Inc. All rights reserved.