Porcine beta-lactoglobulin A and C. Occurrence, isolation and chemical properties.

The occurrence of the dominant 'whey' protein in samples of milk from 1180 sows is examined. It exhibits genetic polymorphism with some unusual features. Although immunologically different from bovine beta-lactoglobulin, it is shown by chemical studies of the isolated protein to be a beta-lactoglobulin. Two homozygous genetic variants, designated porcine beta-lactoglobulin A and C, are isolated and their amino acid compositions and peptide maps compared. It is shown that the C variant has +1 His, -1 Gln, and +1 Asp, -1 Glu, with respect to the A variant. These variants, containing ca. 162 residues per molecule, are considered in relationship to porcine beta-lactoglobulins isolated by other workers. The sequence of the first 50 residues is determined and compared with the sequence of the bovine protein. The sequences of ca. 70% of the remaining residues is proposed on the basis of the composition of tryptic peptides and assumed homology.