A determination of the positions of disulphide bonds in Paim I, α-amylase inhibitor fromStreptomyces corchorushii, using fast atom bombardment mass spectrometry

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Abstract
Paim I, a protein α‐amylase inhibitor, is a single‐chain polypeptide which consists of 73 amino acids, including 4 half‐cystine residues. The positions of disulphide bonds in Paim I have been determined with the combination of enzymatic digestion and fast atom bombardment (FAB) mass spectrometry. Denatured Paim I was digested to peptides with Staphylococcus aureus V8 protease. These peptides were subjected to FAB mass spectrometry, with or without isolation by high‐performance liquid chromatography. The positions of disulphide bonds in Paim I were determined from the relative molecular masses of the peptides containing a disulphide bond and by the enzyme specificity of S. aureus V8 protease. It is deduced that Paim I has two disulphide bridges at Cys(8)‐Cys(24) and Cys(42)‐Cys(70).