Lactoperoxidase enzyme system

The lactoperoxidase enzyme is present in raw milk. This enzyme forms part of the lactoperoxidase system, which also includes thiocyanate and hydrogen peroxide produced by aerobic microorganisms in milk. The lactoperoxidase system produces an antimicrobial substance that exerts a preservative effect in raw milk (Blom and Mprtvedt, 1991). 

Ascorbic acid is also known to interact with activated radical species formed by enzyme systems in milk e.g. xanthine oxidase and lactoperoxidase (33 27). 

Control experiments should be included in order to determine whether the cells under investigation contain endogenous systems capable of catalyzing iodination or iodide oxidation. These controls should include a sample in which the enzyme lactoperoxidase is omitted and one to which peroxide is not added. 

Many hypotheses have been advanced for these differences in oxidative susceptibility, including the oxidation potential of milks, and the action of xanthine oxidase and lactoperoxidase, which is controversial. However, there are no substrates for these enzymes in milk. The action of various metallo proteins in milk may be confused as enzymes. These metallo proteins act as powerful lipid oxidation catalysts in the presence of oxygen and redox systems involving ascorbic acid. 

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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