Role of Tyrosine Residues in Interactions between Casein Components

Abstract

It was indicated from ultraviolet difference spectra and ultracentrifugal experiments that associations occurred between two casein components (α_s and, κ-caseins, β- and κ-caseins and α_s- and β-caseins) at lower CaCl₂ concentrations (2_??_3mM) and that aromatic amino acid residues participated in the associations. Chemical modification studies with 2-hydroxy-5-nitrobenzylbromide indicated that tryptophane residues of each casein component were not essential for these associations. It was also demonstrated by nitration of tyrosine residues with tetranitromethane that tyrosine residues of k-casein were essential for α_s•κ-association and for κ-association and that tyrosine residues of as casein were important to α_s•βassociation.
Interactions between casein components were also studied at higher CaCl₂ concentration (10mM) which is enough for micelle formation. It was found that tyrosine residues of _κ-casein played an important role for the stabilization of as and β-caseins. Properties of the nitrated-β-casein were almost the same as that of the native β-casein except the absorption spectrum. _??__s•β-Interaction in the presence of 10mM CaCl₂ was investigated by use of the nitrated-β-casein instead of the native β-casein. It was proved that α_s-casein was stabilized by the nitrated-β-casein and that precipitation of the nitrated-β-casein increased in the presence of α_s casein.
The mechanism of interactions between casein components at higher CaCl₂ concentration (10mM) are discussed in connection with the associations at lower CaCl₂ concentrations (2_??_3mM).