Portal de Periódicos da CAPES

The Caseins of Milk as Calcium-Binding Proteins

2003; Humana Press; Linguagem: Inglês

10.1385/1-59259-183-3

Harold M. Farrell, Thomas F. Kumosinski, Edyth L. Malin, Eleanor M. Brown,

Infant Nutrition and Health

The virtual image of milk, which would be constructed by most people, is that of a creamy white fluid rich in calcium. The lubricity and taste of milk are related to this perception and are based upon three unique biological structures: the colloidal calcium-protein complexes (the casein micelles), the milk-fat globules with their limiting membrane, and the milk sugar:lactose (1). The complexity of these structures is necessitated by the fact that milk is, in essence, predominantly water. It is the accommodation of these ingredients to an aqueous environment that forms the basis for the structure of milk at the molecular level. Adaptation of milk components to their ultimate aqueous environment begins during secretion. Lipid and protein synthesis are partitioned from the start. Amino acids and their metabolic precursors are actively transported into the secretory epithelial cells and assembled into proteins on the ribosomes of the highly developed rough endoplasmic reticulum. All proteins of mammary origin have conserved leader sequences which cause insertion of the nascent proteins into the lumen of the endoplasmic reticulum (see Fig. 1). The proteins are then transported through the Golgi apparatus, as shown in Fig. 1; presumably the globular proteins of milk are folded during this period. In the Golgi apparatus, the caseins, which are the major milk proteins in most species, are phosphorylated to begin the process of calcium transport. In general, when milks that contain>2% protein are analyzed, the accompanying inorganic phosphate and calcium levels yield insoluble precipitates (apatite or brushite).