Lactoperoxidase EC

The occurrence of a peroxidase, lactoperoxidase (LPO), in milk was recognized as early as 1881. It is one of the most heat-stable enzymes in milk its destruction was used as an index of flash pasteurization (now very rarely used) and is now used as an index of super-HTST pasteurization. 

LPO was first isolated in 1943 several isolation procedures have since been published (reviewed by Bjdrck, 1993). 

Significance. Apart from its exploitation as an index of flash or super-HTST pasteurization, LPO is also technologically significant for a number of other reasons  

It is a possible index of mastitic infection although the level of LPO in milk increases on mastitic infection, it is not well correlated with somatic cell count. 

LPO causes non-enzymic oxidation of unsaturated lipids, probably acting through its haem group the heat-denatured enzyme is more active than the native enzyme. 

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Radicals may also be produced enzymatically by the lactoperoxidase system, an antimicrobial system naturally present in milk. Lactoperoxidase (EC 1.11.1.7) catalyzes formation of the antimicrobial agent hypothiocyanite (OSCN ) by oxidation of thiocyanate (SCN ) by HjOj (12). One source of HjOj is the enzyme superoxide dimutase (SOD, EC 1.1S. 1.1), which catalyzes dismutation of to O2... 

The system consists of LP (EC 1.11.1.7) and the substrates thiocyanate and H2O2. The enzyme, a glycoprotein (carbohydrate content 10%), consists of 612 amino acid residues (Mr 78,000, IP 9.6) and Fe-protoporphyrin IX, which as the prosthetic group carries out the catalysis, as described in 2.3.2.2. Thiocyanate takes part in this reaction process as the electron donor AH. Lactoperoxidase is one of the heat stable enzymes of milk, especially when the structure is fixed by Ca ions. It is still active after 30 min at 63 °C (neutral pH) and after 15 s at 72 °C, but it is inactive after only 2.5 s at 80 °C. Acidification (pH 5.3) accelerates the inactivation by liberating the Ca ions. After... 

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