Sulphydryl oxidase: Properties and applications

Sulphydryl oxidase has been isolated and purified from bovine milk and partially characterized. The enzyme is obtained from the whey fraction produced by chymosin (rennin) treatment of skim milk. Several procedures have been developed for its isolation, all of which use to advantage its concentration-dependent aggregate size. This enzyme exhibits a much broader substrate specificity than that shown by other ‘aerobic oxidases’ presently characterized which catalyse the formation of disulphides from thiols. Using molecular oxygen as an electron acceptor in a twoelectron reduction to hydrogen peroxide, the enzyme catalyses oxidation of cysteine and its analogues, some volatile thiols, peptides, and also thiols in proteins.Vis-a-vis protein-disulphide oxidoreductase and protein-disulphide isomerase, this enzyme does not catalyse thiol-disulphide interchange and thus functions only in de novo formation of disulphides. Enzyme-bound iron and most likely a free sulphydryl group are essential for catalytic activity; however, the carbohydrate moiety of this glycoprotein is probably not required. The broad substrate specificity suggests several potential industrial uses for the enzyme, including flavour modification in the food industry or synthesis of disulphides in pharmaceuticals. Consequently, immobilized forms were developed and characterized. Reactors containing sulphydryl oxidase covalently immobilized on porous glass and silica have been examined for their efficacy in eliminating the cooked flavour in ultra-high temperature sterilized milk. Both the degree of oxidation and cooked flavour were correlated with a normalized residence time in the reactor.