Human Casein. II. Isolation of Human β-Casein Fraction and Human β-Casein B

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 fl-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 aerylamide gel analysis, had a high carbohydrate and cystine content, stabilized Fraction 2, cow a s-and flcasein in the presence of 20 mY CaC12. 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 mY CaC12. 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 c-caseinlike fraction and the other is a cow-~casein-like fraction. Human fl-easein B is a major component of human casein. Isolated fl-casein B was nearly honmgeneous and its physicochenfical properties, such as calcium sensitivity, carbohydrate, and phosphorous content and amino acid composition resembled those of Fraction 2 and cow fl-casein.