Disulfide bridges in an alpha-amylase inhibitor from wheat kernel.

An alpha-amylase inhibitor (0.53-inhibitor) was digested with pepsin and the cystine-containing peptides were isolated by SP-Sephadex C-25 column chromatography and high voltage paper electrophoresis. Amino acid compositions and partial sequences of these peptides and amino acid compositions of the peptides separated after performic acid oxidation revealed the presence of 4 disulfide bonds in the inhibitor. They were formed between residues 6 and 115, 20 and either 41 or 42, 99 and either 41 or 42, and 28 and 83. The disulfide bond partners of residues 20 and 99 were not conclusively determined because of the difficulty of cleavage of the Cys-Cys bond at residues 41 and 42. Among 9 cysteines in this inhibitor Cys-54 must be in a free form, since no evidence was obtained for its contribution to disulfide bond formation.