Sulphydryl oxidase EC

Milk contains an enzyme, sulphydryl oxidase (SO), capable of oxidizing sulphydryl groups of cysteine, glutathione and proteins to the corresponding disulphide (reviewed by Farkye, 1992). The enzyme is an aerobic oxidase which catalyses the following reaction  

It undergoes marked self-association and can be purified readily by chromatography on porous glass. The enzyme has a molecular weight of about 89 kDa, a pH optimum of 6.8-7.0, and a temperature optimum of 35°C. Its amino acid composition, its requirement for iron but not for molybdenum and FAD, and the catalytic properties of the enzyme, indicate that sulphydryl oxidase is a distinct enzyme from xanthine oxidase and thiol oxidase (EC 1.8.3.2). 

SO is capable of oxidizing reduced ribonuclease and restoring enzymic activity, suggesting that its physiological role may be the non-random formation of protein disulphide bonds, e.g, during protein biosynthesis. 

SO immobilized on glass beads has the potential to ameliorate the cooked flavour arising from sulphydryl groups exposed upon protein denaturation, but the commercial viability of this system is not known. 

The production of sulphur compounds is believed to be very important in the development of Cheddar cheese flavour. Residual sulphydryl oxidase activity may play a role in initially reoxidizing sulphydryl groups exposed upon heating cheesemilk the sulphydryl groups thus protected may be reformed during the ripening process. 

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