DIFFERENCES IN ALLELIC PROTEIN EXPRESSION IN THE MILK OF HETEROZYGOUS KAPPA-CASEIN COWS

The objective of this study was to determine whether heterozygous kappa-casein AB cows express the A and B proteins in equal quantities in their milk. Milk samples were collected from 148 heterozygous kappa-casein cows. Chymosin was added to casein isolated from the milk samples to liberate the kappa-casein A and B macropeptides. Alkaline urea PAGE was used to separate and resolve the A and B macropeptide bands. These bands were quantified using densitometric tracings of the gel. Densitometric values were verified by quantification of the bands using amino acid analyses. There was a greater proportion of the total kappa-casein macropeptide located in the protein band derived from the kappa-casein B allele, suggesting that this allele does have an elevated level of protein expression in the bovine mammary gland. This supports previous findings that the proportion of kappa-casein in milk is related to the kappa-casein genotype BB > AB > AA. It would be of interest to know the cause of this increased expression because of the importance of the kappa-casein protein on the manufacturing properties of milk.