κ-Casein and β-Caseins in Human-Milk Micelles: Structural Studies

The function of human milk micelles is to deliver nutrients, particularly insoluble minerals such as the necessary calcium phosphate, to the child in a readily ingested suspension that meets the special requirements of the human infant digestive system. The micelle structure which fulfills that function is not known. However, the development of ion-exchange and reverse-phase HPLC methods for the purification and quantitation of the κ- and β-caseins, along with tritium labeling of the carbohydrate of the κ-casein in human milk to aid in its detection, provided the tools for probing micelle structure by examining the composition of micelles fractionated according to size by differential centrifugation. The relative amount of κ-casein increased as micelle size decreased, and thus as surface area/volume increased. Since κ-casein also stabilizes the micelles against precipitation by Ca2+ ions, a surface position for most of the κ-casein is implied. The relative amounts of the various phosphorylation levels of the human β-caseins remained essentially constant except for the nonphosphorylated (0-P) form, which apparently decreased as micelle size decreased. These data suggest that the β-caseins are linked in the micelles partially through electrostatic interactions involving the organic phosphoryl groups, lacking in the 0-P, so that 0-P can dissociate into the whey. That further implies that the complete surface is not protected by the κ-casein but that the β-caseins are also accessible to the solution.