Amorphous calcium phosphate in casein micelles of bovine milk

Abstract

The calcium phosphate remaining after hydrazine deproteination of casein micelles isolated from bulk skim milk exhibits under the electron microscope a very fine and uniform granularity being formed by small subunits with a true diameter of approximately 2.5 nm. This material, which is about 10 percent by weight citrate, termed calcium phosphate citrate (CPC) complex, also contains Mg and Zn at molar ratios of 0.03 and 0.003 respectively. Radial distribution function (RDF) and infrared analyses show that CPC is a Mg-containing amorphous calcium phosphate (ACP) similar to synthetic and cytoplasmic ACP. presence of CPC in casein micelles as an amorphous colloid bonded with phosphoproteins provides the means for storing in milk large amounts of Ca (16 mM) and Pi (10 mM) in a readily utilizable form but at a higher ion concentration than found in biological solutions.

Microporous Ultrafiltration of Skim Milk

“…Micelle fractionation according to size has been achieved by permeation chromatography using controlled-pore glass columns (McGann et al, 1979;Ekstrand et al, 1981;Donnelly et al, 1984) and by ultracentrifugation (Davies and Law, 1983) for the study of composition and physical properties of various sized micelles. Casein composition has been reported to vary with micelle size (Rose and Colvin, 1966;McGann et al, 1980) in agreement with reports that a higher k-casein content accompanies a decrease in micellar size. However, considerable variation was reported (McGann et al, 1980) for the content of the remaining caseins in micelles of different size ranges.…”

Analysis and Molecular Modeling of the Formation, Structure, and Activity of the Phosphatidylserine-Calcium-Phosphate Complex Associated with Biomineralization

“…Interaction of Ca 2ϩ with PS in the absence of P i leads to the formation of 2:1 PS-Ca 2ϩ complexes that chelate Ca 2ϩ and have no nucleational activity (7,30). Addition of high concentrations of Ca 2ϩ to P i -rich solutions causes the formation of amorphous calcium phosphate (ACP), an ephemeral mineral phase (31,32) that rapidly and spontaneously converts to HA unless stabilized by various agents, such as Mg 2ϩ (33,34), certain proteins (35)(36)(37), and acidic lipids such as PS (7). Interaction with PS during formation of ACP leads to production of PS-CPLX (38).…”

Physicochemical Characterization of Casein Phosphopeptide-Amorphous Calcium Phosphate Nanocomplexes

“…Although the structural details are still being elucidated, the casein micelles are believed to be roughly spherical particles with a radius of ϳ100 nm, dispersed in a continuous phase of water, salt, lactose, and whey proteins (4). The calcium phosphate isolated after exhaustive hydrazine deproteination of micelles has been reported to exhibit a fine and uniform granularity under the electron microscope with the particles consisting of small subunits of 2.5-nm diameter (5,6). The calcium phosphate, present as nanometer-sized ion clusters, and caseins are not covalently bound; hence the casein micelle is known as an association colloid (7).…”