Reductive Methylation of β-Lactoglobulin

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Abstract
The effect of reductive methylation on the properties of .beta.-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 4-6 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 .beta.-lactoglobulin (A plus B variants). Reductive methylation with [14C] formaldehyde at low reagent concentration yielded a product which contained approximately 1 mol or [14C]methyl groups for each mol of .beta.-lactoglobulin. Tracer [14C] .beta.-lactoglobulin added to skim milk exhibited a heat-denaturation curve similar to that described for unmodified .beta.-lactoglobulin. Such a radioactively labeled derivative will be useful for studying protein interactions in milk.

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