K-casein and its Interaction in Human Milk Micelles

<p>Human milk K-casein was isolated from the acid-precipitated casein fraction on Sephadex G-200 and Mono Q HPLC. Samples of K-casein purified from different donors were found similar in both amino acid and carbohydrate composition. A molar extinction coefficient of 11.2 was determined on the basis of amino acid analysis with a norleucine internal standard. Tracer K-casein for human micelle studies was ³H-labeled in the sialic acid moiety of its carbohydrate. The human micelle system was investigated in its native form at 37°C. Micelle reformation by re-equilibration in skimmed milk at 4°C (overnight) followed by 3 h at 37°C was studied with and without the labeled K-casein tracer. After the micelles were fractionated by ultracentrifugation into sized pellets, the mole ratio of K/p-casein was determined by reverse phase HPLC and the six forms of phosphorylated p-casein were quantitated by anion exchange HPLC. In all systems, the relative amount of K-casein increased inversely with the micelle size, suggesting a surface location for K-casein in the micelles.</p> <p>Support for the surface location also came from the reequilibrated 3H-K-casein micelle study, where the labeled K-casein increased linearly with the micelle surface area/volume ratio. Of the p-caseins, 0-P and 1-P showed greatest variability with micelle size. The proportion of 0-P within the (3-casein fraction decreased with decreasing micelle size but to a greater extent in the re-equilibrated system compared with the native. This indicates that the lack of ability to form Ca++ ion bridges permitted 0-P p-casein to dissociate at low temperatures but hampered reassociation at 37°C. These observations suggest the biosynthetic process is not as simple as component p-Casein with 1-P appeared to increase as the micelle surface area increased. aggregation from whey solution. supporting a surface position for 1-P and its potential to stabilize micelles against precipitation by Ca++. Collectively these findings support a surface position for K-casein in agreement with the Slattery-Evard model for bovine milk micelles. However, the presence of p-caseins with different properties, due to levels of phosphorylation, makes the human micelle more complex.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Biochemistry

Year

1990

Author dc:creator

• Dev, Barbara C.

Contributors dc:contributor

• Charles W. Slattery
• E. Clifford Herrmann
• George T. Javor
• Subburaman Mohan
• R. Bruce Wilcox

• Biochemistry
• Milk, Human -- chemistry; Micelles; Caseins -- analysis

Statement dc:rights

• This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.

Language dc:language

English

Repository record dc:identifier

https://scholarsrepository.llu.edu/etd/601

OAI identifier oai:identifier

oai:scholarsrepository.llu.edu:etd-1715