Insight of the α-Amylase Inhibitory Activity of Plant Lipid Transfer Proteins.

We previously characterized the inhibitory activity of human salivary alpha-amylase (HSA) and Calloso-bruchus maculatus intestinal alpha-amylases by the plant lipid transfer protein from Vigna unguiculata (Vu-LTP). Herein, we further study this inhibitory activity. First by an analysis of protein alpha-amylase inhibitors complexed with alpha-amylase, we find that positively charged amino acids of inhibitors interact with the active site of alpha-amylases and we know that Vu-LTP is rich in positively charged amino acid residues. For this reason, we model Vu-LTP, and based on its three-dimensional structure, we choose five peptides to be synthesized. Herein, we report that two peptides of Vu-LTP are responsible for HSA inhibition. A comparison of primary and tertiary structures of LTPs with and without inhibitory activity against alpha-amylase, superimposed with the sequence of Vu-LTP mapped for HSA inhibition, reinforces our suggestion that positively charged amino acids in loops are responsible for the inhibition. To prove our observation, one modified peptide is synthesized in which Arg(39) is replaced by Gln. This modified peptide loses the HSA inhibitory property presented by the unmodified peptide. Therefore, we describe a new biological active for Vu-LTP, i.e. the alpha-amylase inhibitory activity that is not a fortuitous biological activity and probably has evolved to perform a biological function which is still unknown. A good candidate should be defense against insects. The results of this study also expand the possible biotechnological applications of LTPs.