Most of the characteristics of sulfhydryl oxidase (EC 1.8.3.) reported in this chapter are those of the enzyme isolated from the skim milk membrane vesicles obtained from bovine milk (1-3). The enzyme is an integral membrane iron-containing glycoprotein found in mammary tissue, kidney, and pancreas (4). Immunofluorescent staining also indicated its location in the endothelial cells lining the capillaries of the kidney, heart, and small intestine (4). The enzyme catalyzes oxidation of sulfhydryl groups of cysteine, cysteine-containing peptides, and proteins to disulfides with molecular oxygen as the electron acceptor according to the stoichiometry given below (1, 5):

This enzyme differs from thiol oxidase (EC not only by its substrate specificity and protein characteristics but also in the fact that water is a product of the thiol oxidase-catalyzed oxidation (6), whereas, hydrogen peroxide is the product of sulfhydryl oxi-dase-catalyzed reactions. Also, sulfhydryl oxidase differs from glutathione oxidase (EC because of its broader substrate specificity and it does not contain FAD (6). Hence, the enzyme is a sulfhydryl:oxygen oxidoreductase (EC 1.8.3. ), but it has not been assigned a serial number as yet.

The mammalian enzyme is also distinct from micro-bial sulfhydryl oxidase. The microbial enzyme from Aspergillus niger is a soluble flavoprotein that oxidizes small thiol compounds such as dithiothreitol (DTT)

but does not exhibit much activity with proteins such as reduced RNase, whereas the mammalian enzyme exhibits no activity with DTT, contains iron, and rapidly oxidizes reduced proteins (7). A mammalian flavoprotein sulfhydryl oxidase that oxidizes DTT has been isolated from the male reproductive tract (8-10).

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