Heat-induced association of beta-lactoglobulin (beta-lg) and casein micelles, mainly at 85--90 degrees C, was studied by means of preparative ultracentrifugation, using [3H]-labelled beta-lg. Qualitative aspects were studied by gel electrophoresis and electron microscopy. In this association, the formation of intermolecular S-S bonds between beta-lg and kappa-casein plays a role, but hydrophobic bonds are also involved. After heating mixtures containing 25 g/kg casein and 4 g/kg beta-lg at 90 degrees C for 20 min, the amount of beta-lg sedimented with the casein micelles by ultracentrifugation decreased by approximately 30% when the milk salt buffer system was reduced to 0.25 of its normal concentration; it decreased by about 20% when the pH was increased from 6.8 to 7.3 and increased by 15% when the pH was reduced from 6.8 to 5.8. When the beta-lg concentration was decreased from 4 to 2 g/kg, the amount of sedimented beta-lg decreased by 25% after heating at 90 degrees C for 6 min. After heating in milk salt buffer, the amount of sedimented beta-lg was 15% higher than after heating in salt solution which contained neither Ca nor citrate ions. Although alpha-lactalbumin (alpha-la) is involved in the heat-association of beta-lg and casein micelles, no influence of alpha-la on the amount of associated beta-lg was found. In addition to the heat-association product of beta-lg and whole casein micelles, an association product consisting mainly of beta-lg and kappa-casein was also formed. This product was observed by electron microscopy as noodle-like particles. The size of these particles depended on the beta-lg/kappa-casein ratio.

The phosphatidylcholine exchange protein from bovine liver consists of a single polypeptide chain and has a blocked N terminus. The protein contains an estimated 244 amino acid residues in accordance with a determined molecular weight of 28000. The protease from mouse submaxillaris gland cleaved the citraconylated and S‐carboxymethylated derivative of the exchange protein at one specific site (Arg14‐Glu15) close to the N terminus. Analysis of the two resulting peptides showed that N‐acetyl‐methionine was the N‐terminal residue and gave the sequence of the first 41 residues.The modified protein was also fragmented with the protease from Staphylococcus aureus. The peptides isolated represented 88% of the protein; their sequences were determined by manual and automated Edman degradation. Alignment of a number of these peptides gave the complete sequence of the N‐terminal half up to position 122.