Lactoferrin (LF) is a human protein that has many functions. Also known as lactotransferrin, it is found in breast milk and in mucosal secretions, such as saliva, tears, and gastric secretions. This protein has strong antibacterial activity, due to both iron-binding properties and the intrinsic structure of the protein. It also modulates inflammatory events. In combination with hypothiocyanate, lactoferrin is used to treat cystic fibrosis patients.
The antimicrobial defense system of the respiratory tract involves layers of defense mechanisms that protect it, and the lungs, against microorganisms that have been inhaled. Lactoferrin is an essential part of this system. It can be broken down, by protease activity, into smaller fragments to produce two small peptides known as lactoferricin and kaliocin-1, both of which have antimicrobial activity.
Lactoferrin is an iron-binding protein, but unlike most proteins that bind iron, it does not contain a heme group. For this reason, the protein is in the category of non-heme iron-binding proteins. The ability to bind iron causes it to have inhibitory properties toward a range of gram-positive and gram-negative bacteria, and some fungi. These organisms need iron for growth, but cannot utilize it if has been sequestered by lactoferrin. The ability to stop the growth of the microorganisms without killing them is known as being bacteriostatic.
In some cases, gram-negative bacteria produce molecules that sequester iron, and secrete them into the environment to bind up all the iron. This can thwart the bacteriostatic properties of lactoferrin. It appears that part of the protein structure itself is antibacterial, however, even in the absence of iron. This combination of factors accounts for the potent antibacterial activities of this molecule.
Lactoferrin is the second most common protein in human breast milk. It is most abundant in milk produced in the days immediately following birth, and appears to supply newborn infants with the iron they need to thrive. This protein is also present in cow’s milk, but at much lower levels than in human milk.
This molecule is also involved in fighting inflammation. It is stored in the granules of neutrophils — the most common variety of white blood cells in humans. These cells respond quickly to sites of inflammation or bacterial infection, and secrete lactoferrin. This molecule interacts with cells at inflammation sites to inhibit the production of cytokines that would cause inflammation. There is also some evidence that it inhibits the binding of viruses to cells, including HIV.
LF is used in the treatment of cystic fibrosis, a disease that affects many parts of the body, including the lungs. It causes mucus to build up in them, and susceptibility to disease is increased. Part of the reason for this increased vunerability is that components of the immune system, such as LF and hypothiocyanate, are produced at low levels or are lacking entirely. Hypothiocyanate is normally generated as an indirect result of lactoperoxidase activity, another protein that is involved in the respiratory tract antimicrobial defense system. A current treatment for cystic fibrosis involves treatment with LF and hypothiocyanate.