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Lactoperoxidase is a protein that is common in milk. It is also found in mucus and saliva. This is a type of peroxidase enzyme that reacts with certain organic and inorganic chemicals, and hydrogen peroxide, to generate antibacterial compounds. These compounds are part of the body’s immune system and help protect sensitive areas of the body from colonization by bacteria. Raw milk is often treated with chemicals to activate lactoperoxidase, to extend the shelf life of the milk.
Peroxidases are a widespread group of enzymes found in plants and animals. They contain a heme group at their center. This is an organic compound with a cluster of nitrogen atoms that bind one atom of iron. This iron reacts with the oxygen in hydrogen peroxide (H2O2). One of the oxygen molecules is transferred to an acceptor molecule to generate a temporary, unstable oxidized product that contains oxygen.
Lactoperoxidase can react with bromine and iodine, but the natural acceptor molecule in the body is thiocyanate (SCN-). This compound is a combination of cyanide and sulfur. Its oxidation product, hypothiocyanate (OSCN-), is a potent antibacterial compound, inhibiting the growth of an array of bacteria. In high concentrations, it is able to kill certain types of bacteria, such as Escherichia coli. This combination of lactoperoxidase with thiocyanate and hydrogen peroxide is known as the lactoperoxidase system (LP-s).
The LP-s is part of the body’s natural defense system, also known as the innate immune system. This is background immunity that is always present, in contrast to defenses that are activated upon infection. One of the functions of this is as a respiratory tract antimicrobial defense system. In cystic fibrosis patients, for example, the thiocyanate secretion is impaired. This results in a lowered production of the antimicrobial compound hypothiocyanate, and a subsequent greater risk of infection of the airways.
Another function of the LP-s system is in the digestion of breast milk. Infants have an under-developed immune system and are prone to infection. Having antimicrobial properties in the milk they drink helps to protect their digestive tract from attack by bacteria.
Lactoperoxidase activation is used commercially in raw milk to extend its shelf life, with refrigeration, and to keep it stable longer in ambient Third World temperatures. Since milk is such an excellent source of nutrients, it can quickly become contaminated by microorganisms. While raw milk contains abundant lactoperoxidase, thiocyanate and hydrogen peroxide are only present at low concentrations. These compounds are generally added to activate the LP-s system. This extends the shelf life of refrigerated raw milk by several days, and allows the milk to be kept for seven to eight hours at 111°F (30°C).
The antimicrobial properties of lactoperoxidase also make it desirable for inclusion in toothpaste and mouth rinses, to eliminate bacteria in the mouth. It has also been used to preserve cosmetics against an array of microorganisms, along with a number of other compounds. The LP-s system has an excellent safety record and has been used for a number of years.