title: A sensitive proximity ligation assay for active PSA.
abstract: Prostate-specific antigen (PSA) is a widely used marker for prostate cancer. The utility of PSA tests is limited by their inability to differentiate prostate cancer from non-malignant conditions such as benign prostatic hyperplasia and prostatitis. In circulation, PSA occurs in various complexed and free forms, and specific determination of some of these can be used to improve the diagnostic accuracy of PSA tests. We have previously identified peptides that specifically bind to enzymatically active PSA and using such a peptide we have developed an immunopeptidometric assay for this form of PSA. However, the sensitivity of that assay is too low to measure active PSA at clinically important levels. Recently a novel sensitive immunoassay for analysis of proteins, termed the proximity ligation assay, has been established. Here we describe a sensitive implementation of the proximity ligation assay, which utilizes a PSA-binding peptide and antibody as probes to detect active PSA. The assay has a sensitivity of 0.07 microg/l, which is approximately ten-fold lower than that of our previous assay. It does not cross-react with inactive proPSA or the highly similar kallikrein hK2. Our results show that a highly sensitive immunopeptidometric assay can be developed using proximity ligation. This principle should facilitate establishment of specific assays for active forms of other proteases.
authors.affiliation: Department of Clinical Chemistry, Biomedicum/Helsinki University Central Hospital, FI-00029 Helsinki, Finland.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhu
authors.forename: Hannu
authors.initials: H
authors.lastname: Koistinen
authors.forename: Ping
authors.initials: P
authors.lastname: Wu
authors.forename: Ale
authors.initials: A
authors.lastname: Närvänen
authors.forename: Edith
authors.initials: E
authors.lastname: Schallmeiner
authors.forename: Simon
authors.initials: S
authors.lastname: Fredriksson
authors.forename: Ulf
authors.initials: U
authors.lastname: Landegren
authors.forename: Ulf-Håkan
authors.initials: UH
authors.lastname: Stenman
mesh_terms: D000918:Antibody Specificity; D014408:Biomarkers, Tumor / Q000032:analysis; D006801:Humans; D007118:Immunoassay / Q000379:methods* / Q000592:standards; D008297:Male; D017430:Prostate-Specific Antigen / Q000032:analysis*; D011471:Prostatic Neoplasms / Q000175:diagnosis*; D012680:Sensitivity and Specificity
chemical_list: D014408:Biomarkers, Tumor; D017430:Prostate-Specific Antigen