Quantitative characterization of bovine plasminogen binding to caseins.

The binding of bovine plasminogen to whole casein, alpha s-casein, beta-casein, and kappa-casein is responsible for the progressive proteolysis of milk and dairy products. A sensitive and accurate microparticle-enhanced nephelometric immunoassay was developed to measure free plasminogen after interaction between bovine plasminogen and caseins and the quantitative parameters of plasminogen/casein binding were established. Two classes of binding sites for plasminogen were found in this study on each of the investigated caseins. Their dissociation constants (Kd) were determined by varying the plasminogen concentration at pH 6.6 and performing Scatchard analysis. The two binding sites appeared to be one of high affinity (Kd = 32 nM) and the other of lower affinity (Kd > 370 nM). The number of both binding sites per casein monomer was low (0.04 to 0.53). The great propensity of casein monomers to self-associate in homopolymers where plasminogen binding sites could be hidden and in copolymers present in bovine milk in the form of whole casein micelles accounts for calculated binding sites < 1 per monomer.