Biochemical Characterization of a Novel Whey Protein

An electrophoretic variant of the major whey protein of murine milk has been uncovered in the YBR strain of mice. Both normal and variant forms of this protein, designated whey acidic protein (WAP), constitute a minimum of 2.4% of total mouse milk protein, display an acidic isoelectric point, exhibit a molecular weight of 14,000 after both denaturing and nondenaturing gel electrophoresis, and lack tyrosine and histidine as determined by amino acid analysis. Combined isotope incorporation and immunochemical studies show that WAP i s synthesized by the mammary gland and not by the liver. WAP appears to be unique to mouse milk since no milk protein of similar properties has been described in other species and since goat anti-mouseWAP antiserum does not cross-react with components of bovine, human, or rat milk. However, WAP does exhibit a structural similarity to certain apo-C lipoproteins of human serum very light density lipoproteins. Amino acid and tryptic peptide analyses suggest that the variant form of WAP (WAP-B) contains one more arginine and one l ss cysteine than normal WAP (WAPA). Genetic studies to be reported elsewhere show this difference to be under the control f a single Mendelian autosomal gene with alleles expressed in a co-dominant manner. The designation Wap is proposed for this gene with WapA and WapB as the wild type and mutant alleles, respectively.