Oxygen activation by sulfhydryl oxidase and the enzyme's interaction with peroxidase

Sulfhydryl oxidase is a metalloglycoprotein in milk which catalyzes oxidation of thiols to their corresponding disulfides using molecular oxygen as an electron acceptor. Cysteine, peptides, and proteins all serve as substrates for this oxidative activity. Investigation of the various possible active oxygen species suggests that the enzyme-bound forms of singlet oxygen and a hydroperoxy group may be produced during catalysis. However, the possible intermediate superoxide anions or hydroxyl radicals did not appear to be formed. Evidence was obtained for a direct interaction between sulfhydryl oxidase and horseradish peroxidase which results in an enhancement of the thiol oxidative activity. This interaction also induced a change in the peroxidase absorption spectra consistent with formation of the horseradish peroxidase-II form of the enzyme. Stimulation of oxidase activity also was observed in the presence of oxytocin and certain concentrations of oxidized glutathione.