Purification and properties of two forms of rat alpha-lactalbumin

Two species of alpha-lactalbumin, alpha-lactalbumin1 and alpha-lactalbumin2, were separated from rat milk and purified to homogeneity by gel filtration, followed by the DEAE-cellulose chromatography. alpha-Lactalbumin1 is a bigger molecule in contrast to other known alpha-lactalbumins, and has a molecular weight of 21,500 as determined by sedimentation equilibrium and sodium dodecyl sulfate-polyacrylamide gel analysis. alpha-Lactalbumin2 has a molecular weight of 16,000 measured by sedimentation analysis. alpha-Lactalbumin2, however, exhibits abnormally high molecular weight of 22,500 on sodium dodecyl sulfate polyacrylamide gels. Both alpha-lactalbumins are active in lactose synthase assay and are glycoproteins containing 7 to 9% carbohydrate. Antiserum raised against alpha-lactalbumin1 cannot discriminate between the two species in a radioimmunoassay.