Interaction of the bean (Phaseolus vulgaris) alpha-amylase inhibitors withhuman alpha-amylases: structural and functional aspects

Kidney bean (Phaseolus vulgaris L. cv. Tendergreen) seed contains two isofo rms (alpha AI1, alpha AI') of alpha-amylase inhibitor which strongly inhibi t both human salivary (HSA) and pancreatic (HPA) alpha-amylases. These two isoforms differ in their isoelectric points and neutral sugar contents but possess similar molecular masses (43 kDa). X-ray analysis of the HPA-alpha AI1 complex, molecular modelling of the HSA-alpha AI1 interaction, and inhi bition of enzymatic reactions show that aAI1, in its native dimeric form, f orms a 1.2 stoichiometric complex with alpha-amylases and exhibits a mixed non competitive mechanism of inhibition. The kinetics parameters and optima l conditions for inhibition were determined for both proteins. Given their low concentration in seeds and the acidic optimum pHs found for both isofor ms, inhibitor-containing diets should not cause harmful effects in humans. However, residual activity at neutral pH and a high resistance to proteolys is could allow alpha AI to be used as a starch blocker to control postprand ial glucose release in humans who suffer from non insulin-dependent diabete s mellitus.