Heme-binding ability of bovine milk proteins
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Heme-binding ability of bovine milk proteins Koichi Orino
Received: 13 May 2020 / Accepted: 17 September 2020 Ó Springer Nature B.V. 2020
Abstract Bovine milk proteins bind calcium and some bind other metal ions or heme. The examination of heme-binding proteins in colostrum and milk using hemin-agarose beads (HA) showed a-casein, j-casein and lactoferrin (Lf) to be heme-binding proteins. aCasein and Lf are iron- and heme-binding proteins, and a- and j-casein bind to HA, as does Lf. j-Casein and Lf have higher affinity to zinc ion than does acasein, and j-casein and Lf interact with a-caseinimmobilized beads (CasB). The addition of a-casein to j-casein bound to CasB decreased the amount of bound j-casein compared with in the absence of acasein, and j-casein likely increases a-casein selfassociation. a-Casein binds Lf bound to neither iron nor heme, as shown by experiments with the apo-form. Beads with immobilized poly-L-lysine bind heme but Lf inhibits this binding. These results indicate that acasein, j-casein and Lf are both heme- and zincbinding proteins, and that a-casein interacts with jcasein and Lf through protein-protein interactions.
Electronic supplementary material The online version of this article (doi:https://doi.org/10.1007/s10534-020-00252-2) contains supplementary material, which is available to authorized users. K. Orino (&) Laboratory of Veterinary Biochemistry, School of Veterinary Medicine, Kitasato University, 23-35-1 Higashi, Towada, Aomori 034-8628, Japan e-mail: [email protected]
Additionally, Lf shows higher affinity to hemin than does poly-L-lysine. Keywords Casein family Heme Lactoferrin Polylysine Zinc ion
Introduction Casein family proteins in bovine milk form micelles containing colloidal calcium and phosphate and aid growth of the neonate, indicating that casein family proteins store and transport bio-available metals (Farrell et al. 2004). The casein family accounts for about 80% of total milk proteins and comprises four major members: aS1-casein, aS2-casein, b-casein and j-casein (Farrell et al. 2004). a-Casein is an iron- and heme-binding protein that also binds various metal ions such as zinc, cadmium and mercury (Pomastowski et al. 2014; Usami et al. 2011). The physiological role of these bound metal ions on the micelle structure remains unknown (Farrell et al. 2004). bLactoglobulin and a-lactalbumin are major proteins in whey and account for about 15% of total milk proteins (Farrell et al. 2004). Milk lactoferrin (Lf) ranges from 0.02 to 0.75 mg mL- 1 and is markedly increased to 1.5 to 5.0 mg mL- 1 in colostrum (McGraph et al. 2016). Lf is known to bind various metal ions, including iron (Adlerova et al. 2008). b-Lactoglobulin
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Biometals
is a member of the lipolalin family and a-lactalbumin is a calcium metalloprotein (Farrell et al. 2004). Beads with immobilized metal ions or proteins are useful to identify and characterize metal-binding proteins and protein-protein interactions (Orino 2016; Naito et
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