Effects of Chemical Modification with 2-Phenyl-1,4-Dibromoacetoin on αs1-Casein

Reaction of a~-casein B with 2-phenyl-1,4-dibromo-acetoin (PDA) for 26 hours resulted in the loss of approximately two residues each of histidine and methionine per 27,000 g of a~l-casein. The modified protein was 90% soluble in 8m~ CaC12 and quantitatively precipitated at 13m~[ while the control precipitated at 8m~. The modified protein was unable to penetrate pores of polyacrylamide elcetrophoretic gels in 4.4~ urea and 2-mercaptoethanol and revealed an aggregated peak at the void volume on Sephadex G-200. PDA-asl-casein showed reduced ability to interact with Kcasein to form micelles resistant to precipitation with calcium. The calcium binding ability of the modified protein decreased from 12.4 Ca ++ to 4.5 per asl-easein monomer. During the reaction of PDA with asl-casein , it was found by ~'-bromosuceinimide (NBS) titration in 6~ urea-acetate-formate, pH 4.0, that 2.0 residues of tryptophan per monomer were detected at zero reaction time, which was increased to 3.2 residues after 26 hours reaction with PDA.