Abstract

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.

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.

LLU Discipline

Biochemistry

Department

Biochemistry

School

Graduate School

First Advisor

Charles W. Slattery

Second Advisor

E. Clifford Herrmann

Third Advisor

George T. Javor

Fourth Advisor

Subburaman Mohan

Fifth Advisor

R. Bruce Wilcox

Degree Name

Doctor of Philosophy (PhD)

Degree Level

Ph.D.

Year Degree Awarded

1990

Date (Title Page)

8-1990

Language

English

Library of Congress/MESH Subject Headings

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

Type

Dissertation

Page Count

xiii; 171

Digital Format

PDF

Digital Publisher

Loma Linda University Libraries

Usage 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.

Collection

Loma Linda University Electronic Theses and Dissertations

Collection Website

http://scholarsrepository.llu.edu/etd/

Repository

Loma Linda University. Del E. Webb Memorial Library. University Archives

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Biochemistry Commons

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