Heat-induced covalent complex between casein micelles and beta-lactoglobulin from goat's milk: identification of an involved disulfide bond.
Goat milk is characterized by a very low heat stability that could be attributed, in part, to the covalent interaction between whey proteins and casein micelles. However, the formation of such a complex in goat milk has never been evidenced. This study was designed to assess whether heat-induced covalent interaction occurs between purified casein micelles and beta-lactoglobulin. We used a multiple approach of ultracentrifugation of heated mixture, chromatographic fractionation of resuspended pellets, sequential enzyme digestion of disulfide-linked oligomers, and identification of disulfide-linked peptides by on-line liquid chromatography-electrospray ionization mass spectrometry (LC-ESI/MS), and tandem MS. We identified three different types of disulfide links: (1) expected intermolecular bridges between beta-Lg molecules; (2) disulfide bond involving two kappa-casein molecules; and (3) a disulfide bond between two peptides, one from beta-Lg and the other from kappa-casein. The involved sites in this last bond were Cys(160) of beta-Lg and Cys(88) of kappa-casein. Although the identified heterolinkage is possibly only one of several different types, the results of this study constitute the first direct evidence of the formation of a covalent complex between casein micelles and beta-lactoglobulin derived from goat milk.
Henry
Gwénaële
G
INRA, Laboratoire de Recherche de Technologie Laitière, 65 rue de Saint-Brieuc, 35042 Rennes Cedex France.
Mollé
Daniel
D
Morgan
François
F
Fauquant
Jacques
J
Bouhallab
Saïd
S
D000818:Animals; D002364:Caseins / Q000737:chemistry*; D002853:Chromatography, Liquid; D004220:Disulfides / Q000737:chemistry*; D005260:Female; D006041:Goats; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry*; D008823:Micelles; D008892:Milk / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods
D002364:Caseins; D004220:Disulfides; D007782:Lactoglobulins; D008823:Micelles