Thiocyanate lactoperoxidase oxidation

The lactoperoxidase system produces various compounds for antimicrobial protection of milk, the principal one of which is hypothiocyanite (OCNS ) as a result of the lactoperoxidase-catalyzed oxidation of thiocyanate (SCN ) by H2O2. No overheating should take place during the pasteurization process, to avoid damaging lactoperoxidase . 

Small amounts of hydrogen peroxide in raw milk can activate the lactoperoxidase-catalyzed oxidation of thiocyanate to produce a bacterial inhibitor (Hogg and Jago 1970). Inhibitory compounds resulting from oxygen metabolism can produce initially slow starter culture growth in industrial dairy fermentations if the milk has been excessively agitated. 

Wojciechowski G, Huang L, Ortiz de Montellano PR (2005) Autocatalytic modification of the prosthetic heme of horseradish but not lactoperoxidase by thiocyanate oxidation products. A role for heme-protein covalent crosslinking. J Am Chem Soc 127 15871-15879... 

Thomas EL (1985) Products of lactoperoxidase-catalyzed oxidation of thiocyanate and halides. In Pruitt KM, Tenovuo JO (eds) The lactoperoxidase system. Chemistry and biological significance. Marcel Deikker, New York, pp 31-53... 

Thus the chloride/chlorine couple has a redox potential of 1.36 V and can be oxidised by myeloperoxidase and eosinophil peroxidase lactoperoxidase can oxidise bromide (E° = 1.07 V) whereas thyroid peroxidase can only oxidise thiocyanate (E° = 0.77 V) and iodide (E° = 0.54V). All these reactions are two-electron oxidations and there is no evidence for a stable one-electron intermediate, i.e., compound II does not build up as a stable intermediate, and in fact can be inhibitory if it does form in significant concentrations [21]. 

Modi S, Deodhar SS, Behere DV, Mitra S (1991) Horseradish peroxidase catalyzed oxidation of thiocyanate by hydrogen peroxide comparison with lactoperoxidase-catalysed oxidation and role of distal histidine. Biochem 30 118-124... 

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... 

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