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Human Casein, II. Isolation of Human ß-Casein Fraction and Human ß-Casein B

Central Research Laboratory, Morinaga Milk Industry Company, Ltd., Tokyo, Japan

ABSTRACT

Human casein was separated into three fractions (Fractions 1, 2, and 3) by Sephadex G-150 gel filtration at 10 C without urea and then human ß-casein B was obtained by diethylaminoethyl (DEAE) cellulose column chromatography of Fraction 2. The percentage yields of Fractions 1, 2, and 3 were approximately 27, 64, and 9%, respectively. Fraction 1 showed a spreading zone in acrylamide gel analysis, had a high carbohydrate and cystine content, stabilized Fraction 2, cow _s- and ß-casein in the presence of 20 mm CaCl₂. Fraction 2, in contrast, showed the characteristic band pattern of whole casein, lacking a spreading zone in acrylamide gel analysis, had a low carbohydrate and cystine content, and high proline and leucine content. Furthermore, Fraction 2 was precipitated at normal temperature (20 to 40 C), but was soluble at low temperature (0 to 10 C) in the presence of 20 mM CaCl₂. Fraction 3 showed fast-moving bands and had a low carbohydrate and a high phosphorus content. These results suggest that human casein is composed of two main fractions, one is a cow -casein-like fraction and the other is a cow-ß-casein-like fraction. Human ß-casein B is a major component of human casein. Isolated ß-casein B was nearly homogeneous and its physicochemical properties, such as calcium sensitivity, carbohydrate, and phosphorous content and amino acid composition resembled those of Fraction 2 and cow ß-casein.