Preferential sites of tryptic cleavage on the major bovine caseins within the micelle

Abstract

Caseins, major proteins in bovine milk, are structured in micelles within which the detailed casein organisation remains unclear. We have used limited proteolysis to find the most exposed regions of the caseins within the micelle. Under hydrolysis conditions, only 9% of colloidal calcium and 15.6% of the colloidal inorganic phosphate were removed from the micelle after 4 h. The released peptides during tryptic hydrolysis were separated from the residual micelle by ultracentri- fugation. They were subsequently analysed and characterised by reversed-phase HPLC coupled on- line with electrospray ion source mass spectrometry (ESI-MS). After 4 h of hydrolysis, the p-casein had completely disappeared while about 40% of us,-casein and 37% of x-casein remained undiges- ted. The initial distribution, i.e., 91 % caseins and 9% peptides prior to hydrolysis, was modified after hydrolysis to give 64% peptides remaining within the micelles together with 13% caseins and 22% of the peptides produced were released from the micelles. Among the 61 peptides identified, 34 arose from p-casein, 16 from us1-casein, II from us2-casein and none from x-casein. For the lat- ter casein, the failure ta detect peptides was due to its lower concentration in the large micelles corn- pared with that of the Pand us1-caseins. The peptides derived from p-casein were mainly released from the micelle, while peptides from bath us1- and us2-caseins were preferentially retained within the resi- dual micelles, suggesting that ail the caseins are accessible ta trypsin but are differentially involved in the micellar framework. Our results suggest that us-caseins play a significant role in the micellar cohesion via salt binding with colloidal calcium phosphate and hydrophobie interaction, with itself and with the other caseins. © Inra/Elsevier, Paris.