ENVIRONMENTAL-INFLUENCES ON PURIFIED KAPPA-CASEIN - DISULFIDE INTERACTIONS

Bovine kappa-casein, the stabilizing protein of the colloidal milk pro tein complex, has a unique pattern of disulfide bonding. The protein e xhibits varying molecular sizes on SDS-PAGE (sodium dodecyl sulfate-po lyacrylamide gel electrophoresis), ranging from monomer to octamer and above in the absence of reducing agents. Heat treatment of the sample s with SDS prior to electrophoresis caused an apparent decrease in pol ymeric distribution: up to 60% became monomers after 30 min at 90 degr ees C as estimated by densitometry of SDS-PAGE. In contrast, heat trea tment of the samples without detergent at 90 or 37 degrees C significa ntly increased in high molecular weight polymers, as judged by electro phoresis and analytical ultracentrifugation. In 6 M urea, the protein could be completely reduced by dithiothreitol, but, upon dialysis, var ying degrees of polymer reformation occurred, depending on the dialysi s 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 kappa-casein and on its resultant physical chemical properties .