Disulfide Bonds of Purothionin, a Lethal Toxin for Yeasts

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Abstract
Purothionin isolated from commercial wheat flour contained several components and two of them (A-I and A-II) were isolated in pure form by CM-52 column chromatography. Each component contained 45 amino acid residues with 4 disulfide bonds. Purothionin A-II was digested with trypsin and thermolysin to isolate cystine peptides. These were separated and purified by chromatography on an SP-Sephadex column, and paper electrophoresis and chromatography. A peptide containing a -Cys-Cys- sequence was hydrolyzed with 10N sulfuric acid. Amino acid compositions and partial sequence studies of the cystine peptides and their performic acid-oxidized peptides revealed the positions of all 4 disulfide bonds in purothionin A-II. They were formed between residues 3 and 39, 4 and 31, 12 and 29, and 16 and 25. The results of a partial study of purothionin A-I are also presented.