Structure primaire de la caséine α_S1 bovine

Abstract

This paper is the first one of a series devoted to the primary structure of the major protein of cow's milk, α S1 ‐casein, a single‐chain phosphoprotein, formerly reported to have arginine as an NH 2 ‐terminal, and Leu‐TrpOH as COOH‐terminal residues. Tryptic digestion of maleyl α S1 ‐casein (genetic variant B), in theory limited to arginyl bonds, yields eight peptidic fragments, Tm 1 to Tm8, which were fractionated by column chromatography and, in some cases, more thoroughly purified by preparative paper electrophoresis and/or chromatography. The amino acid composition and the phosphate content of these fragments were determined. The fragments Tm3 and Tm8 arise from a partial non specific cleavage of Tm5. thus, six fragments only–Tm1,2,4,5,6 and 7–arise from the specific cleavage of the α S1 ‐casein chain at arginyl bonds, a result in accordance with the reported number of six arginyl residues in the chain, one being NH 2 ‐terminal. The fragment Tm7, with two arginyl residues, and Tm2 devoid of arginine appear to represent respectively the NH 2 ‐ and COOH‐terminal parts of the α S1 ‐casein chain. These peptidic fragments–except Tm4 which is devoid of lysine–were further digested by trypsin at lysyl bonds, after eliminating the blocking maleyl groups. The resulting peptides were purified by the same methods as mentioned above, and the composition of these tryptic peptides was established. As a control, the tryptic peptides of α S1 ‐casein were simultaneously purified by the same methods from a hydrolysate of the protein which had not been treated with maleic anhydride. In summary, this first step of our work gives: (a) the composition of all the bovine α S1 ‐casein tryptic peptides; (b) partial information on the location of these peptides in the protein chain; (c) confirmation that the molecular weight of the bovine α S1 ‐casein monomer is approximately 23 600.