Primary structure of bovine alpha S1 casein. Chaining of peptides obtained by the action of cyanogen bromide and peptides resulting from trypsin hydrolysis of maleylated alpha S1 casein

Abstract

Our previous studies [1, 2] on maleyl bovine αs1‐B casein were concerned with the isolation and amino acid composition of the tryptic peptides designated as Tm peptides. The sequence of the COOH‐terminal Tm peptide containing 48 residues was also reported.In the present study, we report the overlaps of all the cyanogen bromide peptides as well as the Tm peptides from αs1‐B casein.The αs1‐B casein which contains five methionyl residues was cleaved with cyanogen bromide. The six expected peptides obtained by this treatment and designated as CN peptides, were separated initially on Dowex‐50 X 2 and further purified by Sephadex column chromatography. The amino acid composition of these six purified CN peptides was determined. The results were in perfect agreement with the amino acid composition of αs1‐B casein deduced from the analysis of Tm peptides.The three Tm peptides from αs1‐B casein which contained methionyl residues were also cleaved by cyanogen bromide. The fragments obtained were purified by either paper chromatography or Sephadex column chromatography and the amino acid composition of these purified peptides was determined. Of the eight peptides thus obtained, three were found identical with the CN peptides obtained directly by BrCN treatment of the αs1‐B casein. The remaining three CN peptides from αs1‐B casein for which identical fragments could not be located in the Tm peptides, were digested with trypsin (in two cases after maleylation) and the resulting fragments were purified by Sephadex column chromaography. These tryptic peptides provided the remaining gaps. This permitted us to locate the relative position of all the CN peptides in the αs1‐B casein molecule. These results also indicate the location of all the Tm peptides except 2 in the αs1‐B casein molecule. Further, the chymotryptic peptides from one of the CN fragments of αs1‐B casein provided the missing overlaps.