Characterization of the third α-amylase inhibitor, αAI-3, in the common bean (Phaseolus vulgaris L.)

Seeds of the common bean (Phaseolus vulgaris L.) contain proteinous alpha-amylase inhibitor (alpha AI) that inhibits insect and mammalian alpha-amylase enzymes. Based on the specific inhibition of alpha-amylase enzymes, common beans were classified into four different alpha AI types, alpha AI-1, 2, 3, and null type. This trait is controlled by a single locus, and alleles encoding the three alpha AI variants, alpha AI-1, 2, and -3, are codominant. Two different inhibitor proteins were purified from a common bean cultivar containing alpha AI-3. The two proteins are encoded at the same locus and designated alpha AI-3a and alpha AI-3b. aAI-3a inhibited the activity of porcine pancreatic alpha-amylase, while alpha AI-3b inhibited the activity of Mexican bean weevil larvae. N-terminal sequences and the specificity of alpha AI-3a and alpha AI-3b were very similar to alpha AI-1 and alpha AI-2, respectively, indicating that the common bean classified as alpha AI-3 type contains both analogs of alpha AI-1 and alpha AI- 2.