CHYMOSIN-CATALYZED HYDROLYSIS OF GLYCOSYLATED AND NONGLYCOSYLATED BOVINE KAPPA-CASEIN ADSORBED ON LATEX-PARTICLES

Bovine kappa-casein A has been fractionated into its nonglycosylated a nd variously glycosylated forms, The nonglycosylated fraction and a hi ghly glycosylated fraction were allowed to adsorb to negatively charge d polystyrene latex particles, and the hydrodynamic thickness of the p rotein layers was determined by photon correlation spectrometry. After the coated latex was washed, the proteolytic enzyme chymosin, which c leaves kappa-casein at a specific peptide bond, was added. The diamete r of the coated particles decreased due to the release of the negative ly charged caseinomacropeptide part of the protein, leaving the hydrop hobic para-kappa-casein still adsorbed to the latex. In the presence o f calcium ions, this decrease in diameter was followed by an increase as the latex particles aggregated. The rate of the aggregation was dep endent on the concentration of calcium ions, with the aggregation of t he nonglycosylated kappa-casein-coated latex being significantly more sensitive. Since the caseinomacropeptide portion of the protein molecu le is the part that contains the glycosylation sites, it would appear that the difference is due to formation of calcium bridges between the unhydrolyzed molecules remaining on the latex surface. The results ar e discussed in terms of the suitability of this system as a model for the chymosin-catalyzed aggregation of casein micelles in milk, the fir st step in the cheesemaking process. (C) 1996 Academic Press. Inc.