Abstract
Effects of calcium ion on bovine κ-casein components having different carbohydrate contents were investigated by means of ultraviolet absorption and fluorescence spectra. The blue shift of ultraviolet spectrum of each κ-casein component was observed by the addition of calcium ion and the difference spectrum indicated the absorption maximum at 282 nm and minima at 279 and 286 nm. All κ-casein components gave the same maximum or minima wavelengths in their difference spectra in spite of the difference of carbohydrate content. The extent of blue shift was proportional to the concentration of calcium ion. The effect of calcium ion on the κ-casein component having no carbohydrate was larger than other components which have carbohydrate. It was considered that the natural fluorescence of κ-casein was due to tryptophan residue. The maximum wavelength of fluorescence spectrum of κ-casein component containing most carbohydrates was shorter and the fluorescent intensity was smaller than those of other components. The red shift of the fluorescence spectra by the addition of calcium ion was observed for κ-casein components containing carbohydrates but not for the component having no car-bohydrate. It is considered from these facts that the hydrophobic region of the component containing carbohydrate is exposed to solvent in comparison to that of the component having no sugar, and that calcium ion induces the hydrophobic region to come to the surface of protein. The fluorescence maximum wavelength of unfractionated κ-casein was shorter than those of fractionated κ-casein components. This fact suggests that unfractionated κ-casein forms the stable complex as a result of gathering of κ-casein components having different carbohydrate contents. Furthermore, the following facts have been observed in our laboratory : a) unfractionated κ-casein forms the larger polymer than fractionated κ-casein components, b) the complex of unfractionated κ-casein with other casein components (αs1- and β-caseins) was larger than corresponding complexes of fractionated κ-casein components with other caseins, c) unfractionated κ-casein is most heat stable. These facts suggest that the heterogeneity of κ-casein is effective on the stability of casein complex and the sugar part of κ-casein takes part in the stability.
