Cereal Chem 54:511 - 523.  |  VIEW ARTICLE
Amino Acid Sequences of the Two Alpha -Purothionins of Hexaploid Wheat.

B. L. Jones and A. S. Mak. Copyright 1977 by the American Association of Cereal Chemists, Inc. 

Alpha-purothionin extracted from flour milled from hexaploid wheat was separated by ion-exchange chromatography into two pure proteins. The two proteins, alpha1- and alpha2-purothionins, were present in approximately equal amounts and differed in amino acid composition by eight residues. Native and reduced and pyridylethylated alpha1- and alpha1-purothionins were hydrolyzed with chymotrypsin to obtain peptides suitable for amino acid sequence studies. The complete amino acid sequences of the two alpha- purothionins were determined. The alpha1-and alpha2-purothionin molecules each consist of 45 amino acid residues. Their amino acid sequences differ at six positions. Three of the differences are substitutions of chemically similar amino acids. None of the substitutions alters the net charge of the proteins. The alpha1- purothionin contains isoleucine at position 33, where alpha2-purothionin has leucine; this accounts for the often-observed fact that alpha-purothionin as extracted contains only a half residue of isoleucine. The alpha1- and alpha2-purothionin sequences differ from that of beta-purothionin at 5 and 6 positions, respectively, with one amino acid change in each case (glycine-aspartic acid, residue 42) which alters the net charge on the alpha-proteins as compared with the beta-purothionin. The amino acid sequences of the alpha-purothionins show considerable homology to those of the viscotoxins of mistletoe, as has already been shown for beta-purothionin.