Studies of Casein. I. Some Observations on the Heterogeneity of Casein Fractions

The heterogeneity of casein is discussed in the light of methods currently used for the fractionation of casein. In particular, the possible heterogeneity of certain preparations of α-casein is considered. This is important because it has been generally considered that α-casein is the protective colloid which is altered when the enzyme, rennin, acts on casein micelles. Recently, Waugh and von Hippel (1956) have suggested that their new component x-casein, and not α-casein, is the protective colloid. These two viewpoints could be reconciled if α-casein samples previously examined contained x-casein as well. In the present work, a study is made of filter paper electrophoresis, micelle-forming properties, and sedimentation of casein fractions. It is shown that x-casein is concentrated with α-casein in fraction A during the alcohol fractionation method of Hipp et al. (1952). On the other hand fraction B contains α-casein essentially free of x-casein. The a-casein obtained in the urea fractionation method of Hipp et al. also contains x-casein. Thus only alcohol fraction B is a suitable source of pure α-casein. During the paper electrophoretic examination of casein fractions a number of minor protein components are observed. A component moving more slowly than γ-casein is present in acid casein, second-cycle casein-fraction P, and an alcohol fraction. This component was first observed in the latter fraction by Hipp et al. (1952) when preparing γ-casein. Electropherograms of second-cycle casein-fraction S indicate the presence of x-, β-, and γ-casein, and two minor components moving between x- and β The way in which these components arise is briefly discussed.