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Interactions of doubly phosphorylated human β‐casein with native and reduced, carboxymethylated bovine κ‐casein leading to micelle formation: Role of inorganic phosphate
Author(s) -
Sood Satish M,
Booth Cassie,
Jhawar Harbir S,
Farrell Harold M,
Slattery Charles W
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a617-c
Subject(s) - chemistry , micelle , casein , pi , phosphate , colloid , imidazole , precipitation , inorganic phosphate , ammonium , nuclear chemistry , chromatography , analytical chemistry (journal) , crystallography , stereochemistry , aqueous solution , biochemistry , organic chemistry , physics , meteorology
Milk micelles are largely colloidal complexes of casein (CN) phosphoproteins. The reconstituted micelle system in the absence of inorganic phosphate (P i ) is quite temperature dependent, forming at higher temperatures but disappearing upon cooling. In the present study, β‐CN‐2P (~30% of the total β‐CN) at 3 mg/ml in 0.02 M NaCl, 0.01 M imidazole, pH 7 was mixed with native (N‐κ) or reduced, carboxymethylated (RCM‐κ) bovine κ‐CN at κ/β (w:w) ratios from 1:3 to 1:6. Turbidity (OD 400 nm ) and a lack of precipitation as temperature (T) increased from 4 to 37°C were used as an index of stability and micelle formation. Analytical ultracentrifuge measurements at different temperatures were also carried out with added Ca +2 and inorganic phosphate (P i ). At 10°C in the absence of Ca +2 and P i , N‐κ plus β‐CN‐2P (1:3) showed two peaks of ~2 and 4.6 S which became a single peak of ~30S at 37°C. With 2.5 mM Ca +2 and 2 mM P i , a single peak with ~42S was obtained at 37°C. The S value increased up to 6 mM P i and then decreased as material precipitated at higher P i . RCM‐κ alone also showed two peaks at 10°C of ~7 and 46S, and at 37°C, two skewed peaks of ~14 and 53S were obtained. For RCM‐κ and β‐CN‐2P at a ratio of 1:6 (w:w),very turbid solutions were seen with increases in P i . The stabilizing action of RCM‐κ was dependent on its concentration. These studies suggest that interactions dictated by the sulfhydryl groups of κ‐CN are less important than other interactions.