Structure of the casein micelle. The accessibility of the subunits to various reagents

Abstract

Summary Carboxypeptidase A (CPA, mol. wt = 34600) and 1-dimethylaminonaphthalene-5-sulphonyl chloride (dansyl chloride, mol. wt = 270) were shown to react with the 3 casein subunits, α s1 -, β- and κ-caseins, which constitute the micelle. Virtually all of the C-terminal amino acids were released by CPA from these subunits without disrupting the micellar structure. It was verified that the rate of dissociation of the micelle into low molecular weight complexes is considerably slower than the rates at which these reagents act on micellar components. It is concluded that dansyl chloride, CPA, and probably also rennin, whose molecular weight is similar to that of CPA, are able to penetrate to the interior of the micelles. This was confirmed in a quite independent way with a non-reacting agent, myoglobin (mol. wt = 17000). It is postulated that the casein micelle has a sponge-like structure in which the 3 different subunits are distributed fairly uniformly. Micellar fractions obtained by differential centrifugation from skim-milk and colostrum were examined for their content of α s1 -, β-, κ- and para-κ-caseins by the use of CPA as described by Ribadeau Dumas (1968). The results confirm that there is a real difference in κ-casein content of the micelles, according to their rate of sedimentation.