Differences in Allelic Protein Expression in the Milk of Heterozygous -Casein Cows

¹ Department of Animal Science, University of California, Davis 95616-8521

The objective of this study was to determine whether heterozygous -casein AB cows express the A and B proteins in equal quantities in their milk. Milk samples were collected from 148 heterozygous -casein cows. Chymosin was added to casein isolated from the milk samples to liberate the -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 -casein macropeptide located in the protein band derived from the -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 -casein in milk is related to the -casein genotype BB > AB > AA. It would be of interest to know the cause of this increased expression because of the importance of the -casein protein on the manufacturing properties of milk.

Key Words: milk protein • -casein • expression

Submitted on August 27, 1990
Accepted on November 5, 1990

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