title: A flavin-dependent sulfhydryl oxidase in bovine milk.
abstract: Both metal and flavin-dependent sulfhydryl oxidases catalyze the net generation of disulfide bonds with the reduction of oxygen to hydrogen peroxide. The first mammalian sulfhydryl oxidase to be described was an iron-dependent enzyme isolated from bovine milk whey (Janolino, V.G., and Swaisgood, H.E. (1975) J. Biol. Chem. 250, 2532-2537). This protein was reported to contain 0.5 atoms of iron per 89 kDa subunit and to be completely inhibited by ethylenediaminetetraacetate (EDTA). However the present work shows that a soluble 62 kDa FAD-linked and EDTA-insensitive sulfhydryl oxidase apparently constitutes the dominant disulfide bond-generating activity in skim milk. Unlike the metalloenzyme, the flavoprotein is not associated tightly with skim milk membranes. Sequencing of the purified bovine enzyme (>70% coverage) showed it to be a member of the Quiescin-sulfhydryl oxidase (QSOX) family. Consistent with its solubility, this bovine QSOX1 paralogue lacks the C-terminal transmembrane span of the long form of these proteins. Bovine milk QSOX1 is highly active toward reduced RNase and with the model substrate dithiothreitol. The significance of these new findings is discussed in relation to the earlier reports of metal-dependent sulfhydryl oxidases.
authors.affiliation: Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Jaje
authors.forename: Holly N
authors.initials: HN
authors.lastname: Wolcott
authors.forename: Olajumoke
authors.initials: O
authors.lastname: Fadugba
authors.forename: Diane
authors.initials: D
authors.lastname: Cripps
authors.forename: Austin J
authors.initials: AJ
authors.lastname: Yang
authors.forename: Ian H
authors.initials: IH
authors.lastname: Mather
authors.forename: Colin
authors.initials: C
authors.lastname: Thorpe
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D002645:Chickens; D002850:Chromatography, Gel; D004529:Egg White / Q000032:analysis; D005260:Female; D008892:Milk / Q000201:enzymology*; D008969:Molecular Sequence Data; D010088:Oxidoreductases / Q000302:isolation & purification / Q000378:metabolism*; D016415:Sequence Alignment; D050862:Oxidoreductases Acting on Sulfur Group Donors
chemical_list: D010088:Oxidoreductases; D050862:Oxidoreductases Acting on Sulfur Group Donors