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Reductive Methylation of ß-Lactoglobulin

Department of Food Science, University of Wisconsin, Madison 53706

ABSTRACT

The effect of reductive methylation on the properties of ß-lactoglobulin was studied with preparations in which 6, 32, or 80% of the amino groups were methylated. Content of sulfhydryl groups was not changed and little difference in electrophoretic mobility of the protein was evident on polyacrylamide gel electrophoresis. Thin-layer isoelectric focusing from pH 4 to 6 indicated that protein in which 80% of the amino groups were modified showed an increase in isoelectric point of no more than .1 pH unit for each electrophoretic component, compared with untreated ß-lactoglobulin (A plus B variants). Reductive methylation with [carbon-14] formaldehyde at low reagent concentration yielded a product which contained approximately one mole of [carbon-14] methyl groups for each mole of ß-lactoglobulin. Tracer [carbon-14]ß-lactoglobulin added to skim milk exhibited a heat-denaturation curve similar to that described for unmodified ß-lactoglobulin. Such a radio-actively labeled derivative will be useful for studying protein interactions in milk.