Natural plant enzyme inhibitors. Isolation and characterisation of two alpha-amylase inhibitors from Colocasia antiquorum tubers

Two α‐amylase inhibitors I‐1 and I‐2, were purified to homogeneity from Colocasia antiquorum tubers by extraction with 0.02m phosphate buffer pH 7.6, treatment with diethyl amino ethyl‐cellulose, ammonium sulphate fractionation, chromatography on Sephadex G‐50 and chromatography on phenyl sepharose CL‐4B. Both the inhibitors were basic proteins. I‐1 alone contained carbohydrates to the extent of 5.4%. The molecular weights of I‐1 and I‐2 were found to be about 14 300 and 12 500, respectively, by sodium dodecyl sulphate‐polyacrylamide gel electrophoresis. The inhibitors inactivated human salivary, human pancreatic and hog pancreatic amylases but they had no action on Bacillus subtilis and Aspergillus oryzae amylases and resisted attack by pepsin, trypsin, chymotrypsin and pronase. Chemical modification of amino groups and guanidino groups of I‐1 and I‐2 resulted in loss of inhibitory activities. Formation of enzyme inhibitor complexes between hog pancreatic amylase, I‐1 and I‐2 were demonstrated by gel chromatography on Sephadex G‐100. Total amylase inhibitory activity in Colocasia tubers decreased gradually during plant growth.