Casein and Casein Subunits in Preterm Milk, Colostrum, and Mature Human Milk

Human milk proteins have both nutritional and physiological roles for the breast-fed infant. While the biochemistry and developmental patterns for many whey proteins are well known, our knowledge of human casein and its subunits is still limited. We have recently developed a method to isolate casein from whey proteins in human milk and to separate the casein subunits by fast protein liquid chromatography. In this study we have applied this methodology to study the casein subunit pattern in pretern milk, colostrum, and mature milk. Casein concentration increased with lactation time, largely due to an increase in glycosylated forms of casein (k-caseins). Thus, the relative proportion of β-casein to k-casein decreased during the lactation period. The patterns of phos-phorylated and glycosylated casein subunits were found to vary during lactation, showing that both synthesis and posttranslational modification of β- and k-casein are regulated by different mechanisms.