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-Casein Interactions in the Suspension of the Two Major Calcium-Sensitive Human ß-Caseins

S. M. Sood^*, G. Erickson^* and C. W. Slattery^*,

^* Biochemistry Division, Department of Biochemistry and Microbiology
Department of Pediatrics, School of Medicine, Loma Linda University, Loma Linda, CA 92350

The possible effects of both the ß-casein (ß-CN) phosphorylation level and the -CN glycosylation level on micelle formation were studied using the doubly-phosphorylated form (ß-CN-2P) and the quadruply-phosphorylated form (ß-CN-4P) of human ß-CN, along with bovine -CN to compare with previous studies using the more highly glycosylated human -CN. Addition of bovine -CN to human ß-CN-2P, ß-CN-4P, or a 1/1 (wt/wt) mixture of the two was at /ß molar ratios from 0.0 to ~0.6 and micelles were reconstituted by addition of Ca⁺² either directly at 37°C for determination of the fraction suspended or at an initial temperature of 4° that was gradually increased to 37°C with the change in particle size monitored by turbidity measurements. Analysis of the data indicates that the 4P form requires more -CN for stabilization than the 2P form but that the mixture of the two is more like the 4P form in that lateral - interactions may enhance ß- interactions and micelle formation. Above a /ß molar ratio of about 0.2, the caseins were fully suspended into reconstituted micelles. However, micelle size decreased at a higher ratio, indicating that the -CN probably occupies a surface position and may regulate micelle size by its relative abundance. A comparison with published results suggests that the higher glycosylation level of human -CN may protect a larger surface area and result in smaller micelles. Changes in reconstituted micelle size with pH indicate that positively charged groups in the -CN may interact with the negatively charged phosphate esters in the ß-CN moieties in addition to -ß hydrophobic interactions.

Key Words: bovine -casein • human ß-casein • reconstituted milk micelles • protein-ion interactions • protein-protein interactions

Abbreviation key: ß-CN-0P to ß-CN-5P = phosphorylation level of human ß-CN ranging from 0.0 to 5.0 as indicated by number preceding P