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Changes in Isoelectric Focusing Patterns of Milk Proteins from Storage in Aqueous Urea-Mercapthoethanol Solution.

CSIRO, Division of Food Research, Dairy Research Laboratory, P. O. Box 20 Highett, Victoria, Australia

ABSTRACT

Patterns of isoelectric focusing of ß-lactoglobuiin, -lactalbumin, and bovine serum albumin changed when these proteins were stored in aqueous 6M urea solutions containing mercaptoethanol. These changes, which were not detected by electrophoresis, involved progressive loss of some bands observed on focusing of freshly prepared solutions. They occurred rapidly (less than 15 min) at pH 7 and 9 but required 2 to 3 days at pH 5.7. Isoelectric focusing patterns were not altered on storage at pH 5.0 or 2.0, or if the urea concentration was reduced substantially below 6M, or in the absence of mercaptoethanol. The half cystine residues in the proteins were implicated in changes in isoelectric focusing pattern as the changes were not evident on storage of _S- or ß-casein (free of cystine residues) and as reduction and blockage of these residues in ß-lactoglobulin rendered the patterns stable on storage.

These changes were not accompanied to any considerable extent by aggregation. It is likely that these changes were the result of unfolding of the proteins under the influence of the 6M urea, followed by -SS/-SH interchange involving mercaptoethanol.