Some Properties of Desulfurized k-Casein

Abstract

The disulfide groups in k-casein were modifed by desulfurization with Raney nickel under atmosphere of hydrogen and by alkylation with iodoacetamide. Approximately 50% of the SS groups were removed by the desulfurization for 48hours. An almost fully desulfurized fraction was further separated by gel filtration chromatography on Sepharose 6B. Decrease in sedimentation coefficient and increases in elution volume of Sepharose gel chromatography and gel electrophoretic mobility demonstrated the unfolding of k-casein molecules by desulfurization. The desulfurized k-casein retained the original stabilizing capacity for Ca-αsl-ca-seinate. Upon heating for 30minutes in boiling water, both the desulfurized and the S-carboamidomethyl k-caseins decreased the αsl-ca-sein stabilizing ability by 40 to 50% while no significant change was observed for the control k-casein. Aggregated peaks appeared when the heated modified k-caseins were chromatographed on Sepharose 2B at pH 7.6, whereas no aggregation was observed for heated unmodified k-casein. Using Sepharose gel chromatography and polyacrylamide gel electrophoresis, the interaction with /Mactoglobulin which occurs upon heating was not detected for the modified k-caseins. These results indicate the importance of SS groups in k-casein in maintaining a structural integrity which controls heat stability of the molecules.