Investigation of the slow inhibition of almond beta-glucosidase and yeast isomaltase by 1-azasugar inhibitors: evidence for the 'direct binding' model

(-)-1-Azafagomine [(3R,4R,5R)-4,5-dihydroxy-3-hydroxymethylhexahydropyridazine; inhibitor 1] is a potent glycosidase inhibitor designed to mimic the transition state of a substrate undergoing glycoside cleavage. The inhibition of glycosidases by inhbitor 1 and analogues has been found to be a relatively slow process. This 'slow inhibition' process was investigated in the inhibition of almond beta-glucosidase and yeast isomaltase by inhibitor 1 and analogues. Progress-curve experiments established that the time-dependent inhibition of both enzymes by inhibitor 1 was a consequence of relatively slow dissociation and association of the inhibitor from and to the enzyme, and not a result of slow interchanges between protein conformations. A number of hydrazine-containing analogues of inhibitor 1 also inhibited beta-glucosidase and isomaltase slowly, while the amine isofagomine [(3R,4R,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine; inhibitor 5] only inhibited beta-glucosidase slowly. Inhibitor 1 and related inhibitors were found to leave almond beta-glucosidase with almost identical rate constants, so that the difference in K(i) values depended almost entirely on changes in the binding rate constant, k(on). The same trend was observed for the inhibition of yeast isomaltase by inhibitor 1 and a related inhibitor. The values of the rate constants were obtained at 25 degrees C and at pH 6.8.