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A study of the properties of dissociated bovine casein micelles

Published online by Cambridge University Press:  01 June 2009

L. K. Creamer  and
Gillian P. Berry
L. K. Creamer
Affiliation:
New Zealand Dairy Research Institute, Private Bag, Palmerston North, New Zealand
Gillian P. Berry
Affiliation:
New Zealand Dairy Research Institute, Private Bag, Palmerston North, New Zealand
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Summary

Although casein micelles are disrupted by removal of Ca, individual caseins remain aggregated in sub-micellar casein aggregates or sub-units. These sub-units have been studied by: (1) the use of gel filtration on Sepharose 4B at 6, 20 and 37°C at pH 6·7 and 0·1 ionic strength, (2) ultracentrifugation and (3) electron microscopy. At 37°C the protein composition of the sub-units varied across the gel-filtration peak, with κ-casein being eluted towards the leading edge and the ratio of αs1- to β-casein being almost constant across the peak. Re-chromatography of the protein from the leading edge of this peak gave a new wide peak with the κ-casein again being eluted towards the leading edge. However, αs1-casein was eluted before β-casein in the leading edge of the new peak. Prior treatment of the casein micelles by dispersion with 6 m-urea solution, precipitation with acid or reduction with 2-mercaptoethanol did not alter the gel-filtration pattern. An examination of the purified casein components and their mixtures showed that a 1:1 ratio mixture of αs1- and β-casein had the same peak maximum elution volume as casein micelle sub-units. κ-Casein by itself eluted at the void volume of the gel-filtration column, but after admixture with a sample of small micelles it eluted at the leading edge of the sub-unit peak and was indistinguishable from the κ-casein normally present. These results suggest that the sub-units are in equilibrium with their component caseins and that their size distribution is determined by only those factors (such as protein concentration, pH, temperature and ionic strength) which determine the strength of association between the casein components. The results from electron microscopy and ultracentrifugation support these conclusions.

Type
Research Article
Information
Journal of Dairy Research , Volume 42 , Issue 1 , February 1975 , pp. 169 - 183
Copyright
Copyright © Proprietors of Journal of Dairy Research 1975

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