Environmental Influences on Purified κ-Casein: Disulfide Interactions

Bovine k-casein, the stabilizing protein of the colloidal milk protein complex, has a unique pattern of disulfide bonding. The protein exhibits varying molecular sizes on SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis), ranging from monomer to octamer and above in the absence of reducing agents. Heat treatment of the samples with SDS prior to electrophoresis caused an apparent decrease in polymeric distribution: up to 60% became monomers after 30 min at 90C as estimated by densitometry of SDS-PAGE. In contrast, heat treatment of the samples without detergent at 90 or 37C significantly increased in high molecular weight polymers, as judged by electrophoresis and analytical ultracentrifugation. In 6 M urea, the protein could be completely reduced by dithiothreitol, but, upon dialysis, varying degrees of polymer reformation occurred, depending on the dialysis conditions. Spontaneous reoxidation to polymeric forms is favored at low pH (<5.15) and low ionic strength. The results are discussed with respect to the influence of the method of preparation on the polymer size of k-casein and on its resultant physical chemical properties.