The Terminal Amino Acids of Wheat Gliadin

Wheat gliadin has been found by two different methods to contain three N-terminal histidine residues for each molecular weight of 27,000. Trace amounts of N-terminal aspartic acid, glutamic acid, alanine, valine, and serine were also detected in the preparation used. Hydrolysis in boiling hydrochloric acid partially destroyed the di-2,4-dinitrophenyl derivative of histidine. Losses of from 5% t o 25% occurred depending upon the time and conditions of hydrolysis. Carboxypeptidase did not release free amino acids from wheat gliadin but qualitative evidence for the occurrence of C-terminal glutamic acid and C-terminal "leucine" was obtained. I t has been reported by Koros' (9) that wheat gliadin contains three free amino groups and that these belong to the three N-terminal histidine residues, Although doubt exists that gliadin is composed of a homogeneous and unique molecular species the observations of KoroS would point to the existence of a t least one characteristic feature in gliadin. Because details of KoroS's work were unavailable independent experiments were done to determine the nature of the N-, and also of the C-, terminal amino acids of gliadin. Since the completion of the present work KoroS's results have been questioned (6) and i t was reported that Sanger's method revealed one N-terminal tyrosine residue in gliadin from Triticum durum, and two N-terminal residues, one each of glutamic acid and tyrosine, in gliadin from Triticum vulgare; no N-terminal histidine was detected. Results recorded in the present communication support the earlier report (9) that histidine is the major N-terminal amino acid in gliadin. In addition, evidence presented indicates tha t glutamic acid and "leucine" occupy the C-terminal positions.