Incorporation of individual casein constituents into casein aggregates cross-linked by colloidal calcium phosphate in artificial casein micelles

Abstract

Artificial casein micelles were prepared at a casein concentration of 2-5% with 10-40 niM-Ca, 12-27 mM-phosphate and 10 mM-citrate and cross-linking of casein by colloidal Ca phosphate (CCP) was examined. No casein aggregates crosslinked by CCP were formed at 10 mM-Ca, 12 mM-phosphate and 10 mM-citrate, which are the approximate concentrations found in the soluble phase of bovine milk. Although the amounts of casein aggregates cross-linked by CCP and of colloidal inorganic phosphate increased with increasing Ca and phosphate concentrations, the effect was not uniform. The incorporation rates of individual casein constituents into casein aggregates cross-linked by CCP were in the order a s2 -> a sl -> /?-casein. This was the reverse of the order of dissociation rates during dialysis of casein aggregates cross-linked by CCP reported previously.Bovine casein micelles are roughly spherical colloidal particles of 20-600 nm diam., which are heterogeneous in constitution (Schmidt, 1982). They are composed of 93 % casein and 7 % inorganic constituents. The main inorganic constituent of casein micelles is colloidal Ca phosphate (CCP). Direct experimental evidence for this has been obtained by Aoki et al. (1986), who isolated casein aggregates cross-linked by CCP from bovine casein micelles by high-performance gel chromatography on a TSK-GEL G4000SW column in the presence of 6 M-urea. It has also been shown that caseins are cross-linked through their ester phosphate groups by CCP (Aoki et al. 19876).Bovine caseins are synthesized in the mammary gland and micelle formation occurs in the Golgi vesicles. It is assumed that casein submicelles are aggregated into micelles by CCP formed by transformation of Ca, phosphate and other inorganic constituents from cytosol to the Golgi vesicles (Schmidt, 1982;Holt, 1985). However, the formation of cross-linkages between casein molecules and CCP in the bioassembly of casein micelles has not yet been elucidated sufficiently.In a previous study (Aoki et al. 1988) we examined the dissociation during dialysis of casein aggregates cross-linked by CCP in bovine micelles, and elucidated that the dissociation rates of individual casein constituents were in the order /?-> a sl -> a s2 -casein. The higher the ester phosphate content, the slower was the dissociation rate of individual casein constituents. These facts suggested that the strength of the bonding between casein molecules and CCP depended on the ester phosphate content.

Interference-blind microfluidic sensor for ascorbic acid determination by UV/vis spectroscopy

“…Thus, the mass transfer process of substrate inside the microfluidic channel can be accelerated to fully contact with the immobilized enzyme for efficient reaction. [26] Combining all the advantages, the enzyme-loaded serpentine microfluidic sensor can provide a very sensitive detection to its substrate.…”

Improvement of Functional Properties of Whey Protein Isolate Through Glycation and Phosphorylation by Dry Heating

“…The preparation of test solutions was conducted according to the procedures for artificial casein micelles (Aoki, 1989). Forty microliters of 1.0 M potassium citrate, 200 L of 0.2 M CaCl 2 , and 240 L of 0.2 M K 2 HPO 4 were added to 2 mL of 4% protein solution, followed by the addition of 200 L of 0.2 M CaCl 2 , and 100 L of 0.2 M K 2 HPO 4 .…”

“…The calcium and Pi in the supernatant were then determined, with the former being determined using a Hitachi Z-600 atomic absorp- All data shown are the mean value of the 2 determinations, with a deviation of <1%. tion spectrophotometer (Hitachi Ltd., Tokyo, Japan; Aoki, 1989).…”

Recent advances in phosphorylation of food proteins: A review

“…Among these modifications, phosphorylation has been proven as an important and efficient method for improving the functional properties of food proteins (Aoki et al, 1997;Matheis et al, 1983). Moreover, phosphoproteins show multiple physiological functions, such as enhancing calcium absorption (Aoki, 1989;Naito, Kawakami, & Imamura, 1972), an active antioxidant for lipids (Lu & Baker, 1986), and immunoregulatory function (Hata, Higashiyama, & Otani, 1998;Kitazawa et al, 1998;Kitazawa, Itoh, Tomioka, Mizugaki, & Yamaguchi, 1996;Otani et al, 2000). Accordingly, it is expected that besides the functional properties, some physiological functions of food proteins may be improved or given by phosphorylation.…”