title: Tyrosine residues play an important role in heme detoxification by serum albumin.
abstract: BACKGROUND
  Serum albumin binds avidly to heme to form heme-serum albumin complex, also called methemalbumin, and this binding is thought to protect against the potentially toxic effects of heme. However, the mechanism of detoxification has not been fully elucidated.
  
  
  METHODS
  SDS-PAGE and Western blot were used to determine the efficiency of methemalbumin on catalyzing protein carbonylation and nitration. HPLC was used to test the formation of heme to protein cross-linked methemalbumin.
  
  
  RESULTS
  The peroxidase activity of heme increased upon human serum albumin (HSA) binding. Methemalbumin showed higher efficiency in catalyzing tyrosine oxidation than free heme in the presence of H2O2. Methemalbumin catalyzed self-nitration and significantly promoted the nitration of tyrosine in coexistent protein, but decreased the carbonylation of coexistent protein compared with heme. The heme to protein cross-linked form of methemalbumin suggested that HSA trapped the free radical accompanied by the formation of ferryl heme. When tyrosine residues in HSA were modified by iodination, HSA lost of protection effect on protein carbonylation. The low concentration of glutathione could effectively inhibit tyrosine nitration, but had no effect on protein carbonylation.
  
  
  CONCLUSIONS
  HSA protects against the toxic effect of heme by transferring the free radical to tyrosine residues in HSA, therefore protecting surrounding proteins from irreversible oxidation, rather than by direct inhibiting the peroxidase activity. The increased tyrosine radicals can be reduced by endogenic antioxidants such as GSH.
  
  
  CONCLUSIONS
  This investigation indicated the important role of tyrosine residues in heme detoxification by HSA and suggested a possible novel mechanism.
authors.affiliation: School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Huang
authors.affiliation: School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China.
authors.forename: Youxia
authors.initials: Y
authors.lastname: Shuai
authors.affiliation: School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China.
authors.forename: Hailing
authors.initials: H
authors.lastname: Li
authors.affiliation: School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China. Electronic address: zhgao144@mail.hust.edu.cn.
authors.forename: Zhonghong
authors.initials: Z
authors.lastname: Gao
mesh_terms: D006418:Heme / Q000378:metabolism*; D006801:Humans; D009566:Nitrates / Q000378:metabolism; D010544:Peroxidases / Q000378:metabolism; D050050:Protein Carbonylation; D012709:Serum Albumin / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D009566:Nitrates; D012709:Serum Albumin; D014443:Tyrosine; D006418:Heme; D010544:Peroxidases
keywords: 2,4-Dinitrophenylhydrazine; 3,3′,5,5′-Tetramethylbenzidine; 3-NT; 3-Nitrotyrosine; 4-Aminoantipyrine; 5,5′-Dithiobis-(2-nitrobenzoic acid); AAP; DNP; DNPH; DTNB; Dinitrophenol; HRP; HSA; HSA treated with iodine to modify tyrosine residues; HSA-T; Hb; Heme; Hemoglobin; Horseradish peroxidase; Human serum albumin; Mb; Myoglobin; PBS; Peroxidative activity; Phosphate buffered saline; SA; SDS-PAGE; Serum albumin; Sodium dodecyl sulfate polyacrylamide gel electrophoresis; TMB; Tyrosine