An example of enzyme hysteresis. The slow and tight interaction of some fully competitive inhibitors with small intestinal sucrase.

Of the fully competitive inhibitors of small intestinal sucrase investigated in this or in other papers, acarbose, nojirimycin, and deoxynojirimycin (Fig. 2) have the highest affinity for the enzyme, their Ki values being in the 10(-7)-10(-8) M range. Furthermore, thier interaction with the enzyme is slow, the steady state being reached in their presence in a matter of minutes. Their overall "on" and "off" constants are small, which indicates that a conformational change accompanies the interaction of these substances with the active site of intestinal sucrase. The structure of these inhibitors, as well as the pH dependence of their Ki values, agrees with and allows additions to be made to the catalytic mechanism earlier suggested for this enzyme (Cogoli, A., and Semenza, G. (1975) J. Biol. Chem. 250, 7802-7809). None of these inhibitors of sucrase has any sizeable effect on the small intestinal Na+-dependent D-glucose transport system.