Environmental effects on disulfide bonding patterns of bovine kappa-casein

Bovine kappa-casein, the stabilizing protein of the colloidal milk protein complex, has a unique disulfide bonding pattern. The protein exhibits varying molecular sizes on SDS-PAGE ranging from monomer to octamer and above in the absence of reducing agents. Heating the samples with SDS prior to electrophoresis caused an apparent decrease in polymeric distribution: up to 60% monomer after 30 min at 90 degrees C as estimated by densitometry of SDS-PAGE. In contrast, heating the samples without detergent at 90 or 37 degrees C caused a significant increase in high-molecular-weight polymers as judged by electrophoresis and analytical ultracentrifugation. In 6 M urea, the protein could be completely reduced, 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 kappa-caseins and on their resultant physical chemical properties.