Augmenter of liver regeneration: a flavin-dependent sulfhydryl oxidase with cytochrome c reductase activity.
Augmenter of liver regeneration (ALR; hepatopoietin) is a recently discovered enigmatic flavin-linked sulfhydryl oxidase. An N-terminal His-tagged construct of the short form of the human protein has been overexpressed in Escherichia coli. Several lines of evidence suggest that, contrary to a recent report, human ALR is a disulfide-bridged dimer (linked via C15-C124) with two free cysteine residues (C74 and 85) per monomer. The C15-124 disulfides are not critical for dimer formation and have insignificant impact on the dithiothreitol (DTT) oxidase activity of ALR. Although the crystal structure of rat ALR shows a proximal disulfide (C62-C65) poised to interact with the FAD prosthetic group [Wu, C. K., Dailey, T. A., Dailey, H. A., Wang, B. C., and Rose, J. P. (2003) Protein Sci. 12, 1109-1118], only flavin reduction is evident during redox titrations of the enzyme. ALR forms large amounts of neutral semiquinone during aerobic turnover with DTT. This semiquinone arises, in part, by comproportionation between flavin centers within the dimer. Surprisingly, cytochrome c is about a 100-fold better electron acceptor for ALR than oxygen when DTT is the reducing substrate. These data suggest that this poorly understood flavoenzyme may not function as a sulfhydryl oxidase within the mitochondrial intermembrane space but may communicate with the respiratory chain via the mediation of cytochrome c.
Farrell
Scott R
SR
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA.
Thorpe
Colin
C
D000332:Aerobiosis; D000409:Alanine / Q000235:genetics; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D000693:Anaerobiosis; D000818:Animals; D002384:Catalysis; D003545:Cysteine / Q000235:genetics; D003579:Cytochrome Reductases / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D004227:Dithionite / Q000737:chemistry; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000378:metabolism*; D005420:Flavoproteins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D050862:Oxidoreductases Acting on Sulfur Group Donors; D010777:Photochemistry; D051381:Rats; D040681:Structural Homology, Protein
D000409:Alanine; D003545:Cysteine; D003579:Cytochrome Reductases; D004227:Dithionite; D003576:Electron Transport Complex IV; D005420:Flavoproteins; D010088:Oxidoreductases; D050862:Oxidoreductases Acting on Sulfur Group Donors; C500897:GFER protein, human; C112161:thiol oxidase, 47 kDa