title: Measuring myeloperoxidase activity in biological samples.
abstract: BACKGROUND
  Enzymatic activity measurements of the highly oxidative enzyme myeloperoxidase (MPO), which is implicated in many diseases, are widely used in the literature, but often suffer from nonspecificity and lack of uniformity. Thus, validation and standardization are needed to establish a robust method that is highly specific, sensitive, and reproducible for assaying MPO activity in biological samples.
  
  
  RESULTS
  We found conflicting results between in vivo molecular MR imaging of MPO, which measures extracellular activity, and commonly used in vitro MPO activity assays. Thus, we established and validated a protocol to obtain extra- and intracellular MPO from murine organs. To validate the MPO activity assays, three different classes of MPO activity assays were used in spike and recovery experiments. However, these assay methods yielded inconsistent results, likely because of interfering substances and other peroxidases present in tissue extracts. To circumvent this, we first captured MPO with an antibody. The MPO activity of the resultant samples was assessed by ADHP and validated against samples from MPO-knockout mice in murine disease models of multiple sclerosis, steatohepatitis, and myocardial infarction. We found the measurements performed using this protocol to be highly specific and reproducible, and when performed using ADHP, to be highly sensitive over a broad range. In addition, we found that intracellular MPO activity correlated well with tissue neutrophil content, and can be used as a marker to assess neutrophil infiltration in the tissue.
  
  
  CONCLUSIONS
  We validated a highly specific and sensitive assay protocol that should be used as the standard method for all MPO activity assays in biological samples. We also established a method to obtain extra- and intracellular MPO from murine organs. Extracellular MPO activity gives an estimate of the oxidative stress in inflammatory diseases, while intracellular MPO activity correlates well with tissue neutrophil content. A detailed step-by-step protocol is provided.
authors.affiliation: Center for Systems Biology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Pulli
authors.forename: Muhammad
authors.initials: M
authors.lastname: Ali
authors.forename: Reza
authors.initials: R
authors.lastname: Forghani
authors.forename: Stefan
authors.initials: S
authors.lastname: Schob
authors.forename: Kevin L C
authors.initials: KL
authors.lastname: Hsieh
authors.forename: Gregory
authors.initials: G
authors.lastname: Wojtkiewicz
authors.forename: Jenny J
authors.initials: JJ
authors.lastname: Linnoila
authors.forename: John W
authors.initials: JW
authors.lastname: Chen
mesh_terms: D000818:Animals; D004789:Enzyme Activation; D057075:Enzyme Assays / Q000379:methods* / Q000592:standards; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D042541:Intracellular Space / Q000378:metabolism; D007958:Leukocyte Count; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D057054:Molecular Imaging; D009504:Neutrophils / Q000201:enzymology; D009195:Peroxidase / Q000235:genetics / Q000378:metabolism*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D009195:Peroxidase