Enzymes indigenous to milk: Lactoperoxidase.

Abstract

Lactoperoxidase (LPO), the peroxidase isolated from milk, is widely distributed in nature. LPO is the second most abundant enzyme in milk and its primary role is to protect the mammary gland and the gut of infants against bacterial infections. LPO is isolated from milk by cation-exchange chromatography. It consists of a single polypeptide chain of 612 amino acid residues and is a 78 kDa glycoprotein with a heme group at its active site. LPO has high thermal stability in milk and has been used as an index of the pasteurization efficiency of milk; the enzyme shows its maximum activity in milk at pH 6.0. Variations in enzyme level are influenced by the sexual cycle of the cow, season of the year, feeding practices, and breed type. LPO catalyzes the oxidation of thiocyanate by hydrogen peroxide to yield thiocyanogen, which is then hydrolyzed to hypothiocyanate. The LPO system can inhibit the growth and metabolism of different species of microorganisms, can be applied at ambient temperatures, and is recommended as an alternative to chilling for the preservation of raw milk. The antimicrobial effect of the LPO system stems from the reaction of unstable hypothiocyanite with sulfhydryl groups in bacterial cell membrane proteins. The LPO system has been applied in combination with various other processes for the preservation of a selection of dairy products.