Lactoferrin and Other Iron Binding Proteins

Lactoferrin is an iron-binding protein that occurs in milk and colostrum and has potential for use as a food antimicrobial (78). Lactoferrin is a glycoprotein with a molecular weight of around 76,500 Da. Lactoferrin has two iron-binding sites per molecule and, for each Fe3+ bound, requires one bicarbonate (hco3 ). Lactoferrin is potentially active in milk due to the low iron concentration and presence of bicarbonate (79). The exact biological role of lactoferrin is unknown; however, it may act as a barrier to infection of the nonlactating mammary gland and protect the gastrointestinal tract of the newborn against infection (78).

Lactoferrin is inhibitory to a number of microorganisms including Bacillus subtilis, B. stearothermophilus, Listeria monocytogenes, Micrococcus, E. coli, and Klebsiella (18). There is evidence that the most effective form of lactoferrin is the iron-unsaturated, or apolactoferrin, form (80). Some Gram-negative bacteria may be resistant because they adapt to low-iron environments by producing siderophores such as phenolates and hydroxamates. Microorganisms with a low iron requirement, such as lactic acid bacteria, would not be inhibited by lactoferrin (78).

Lactoferricin B is a small peptide (25 amino acids) produced by acid-pepsin hydrolysis of bovine lactoferrin (81). The compound is inhibitory to Shigella, Salmonella, Yersinia enterocolitica, E. coli 0157:H7, Staphylococcus aureus, L. monocytogenes, and Candida species at concentrations ranging from 1.9 to 125 mg/mL (82). Another iron-binding molecule, ovotransferrin or conalbumin, occurs in egg albumen. Each ovotransferrin molecule has two iron-binding sites and, like lactoferrin, it binds an anions, such as bicarbonate or carbonate with each ferric iron bound. Ovotransferrin is inhibitory against both Grampositive and Gram-negative bacteria, but the former are generally more sensitive, with Bacillus and Micrococcus species being particularly sensitive (75). Some yeasts are also sensitive.

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