Structure primaire de la caséine β bovine

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Abstract
In order to determine the primary structure of the bovine β‐caseins, the A2 variant was first cleaved with trypsin and cyanogen bromide (CNBr).The tryptic hydrolyzate was separated by column chromatography on Dowex‐50. After additional purifications, 13 peptides numbered T1 to T13 were isolated. The 14th tryptic peptide, T14, not eluted from the column, was directly obtained from the hydrolyzate by preparative paper electrophoresis. Amino‐acid analyses were carried out on all these peptides. Peptide T1 contains 2 Arginyl residues, one of which is NH2‐terminal. This peptide represents the NH2‐terminal end of the β‐casein. Its composition is identical to that reported by Peterson et al. in 1958 [1] for a phosphopeptide isolated from the β‐casein. Peptide T11 contains 2 Lysyl residues, one of which is NH2‐terminal. The COOH‐terminal peptide was identified with peptide T4, devoid of basic amino‐acids. The 14 tryptic peptides account for all the residues of the β‐casein A2 when compared with the already published amino‐acid analyses of the whole protein [2,3]. Several other peptides, originating from incomplete or non‐specific cleavages, were also obtained.Similarly, 7 peptides accounting for the 6 methionyl residues of the protein were obtained from the same variant of β‐casein after cleavage with cyanogen bromide. Six of these peptides were separated on Sephadex G‐50. The seventh peptide was obtained by chromatography on Dowex‐50 of the whole CNBr‐digest. After additional purifications, the amino‐acid compositions of the 7 pure peptides were determined. Peptide CN1 contains peptide T1. It represents the NH2‐terminal end of β‐casein. The COOH‐terminal end of β‐casein was identified with peptide CN7. As in the case of the tryptic peptides, the 7 CNBr‐peptides account for all of the amino‐acid residues of β‐casein A2.β‐casein A2 has 208 amino‐acid residues. Its molecular weight is very close to 24000.

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