title: Validation of flow cytometric competitive binding protocols and characterization of fluorescently labeled ligands.
abstract: BACKGROUND
  Fluorescently labeled ligands and flow cytometric methods allow quantification of receptor-ligand binding. Such methods require calibration of the fluorescence of bound ligands. Moreover, binding of unlabeled ligands can be calculated based on their abilities to compete with a labeled ligand. In this study, calibration parameters were determined for six fluorescently labeled N-formyl peptides that bind to receptors on neutrophils. Two of these ligands were then used to develop and validate competitive binding protocols for determining binding constants of unlabeled ligands.
  
  
  METHODS
  Spectrofluorometric and flow cytometric methods for converting relative flow cytometric intensities to number of bound ligand/cell were extended to include peptides labeled with fluorescein, Bodipy, and tetramethylrhodamine. The validity of flow cytometric competitive binding protocols was tested using two ligands with different fluorescent properties that allowed determination of rate constants both directly and competitively for one ligand, CHO-NLFNYK-tetramethylrhodamine.
  
  
  RESULTS
  Calibration parameters were determined for six fluorescently-labeled N-formyl peptides. Equilibrium dissociation constants for these ligands varied over two orders of magnitude and depended upon the peptide sequence and the molecular structure of the fluorescent tag. Kinetic rate constants for CHO-NLFNYK-tetramethylrhodamine determined directly or in competition with CHO-NLFNYK-fluorescein were statistically identical.
  
  
  CONCLUSIONS
  Combination of spectrofluorometric and flow cytometric methods allows convenient calculation of calibration parameters for a series of fluorescent ligands that bind to the same receptor site. Competitive binding protocols have been independently validated.
authors.affiliation: Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: A
authors.initials: A
authors.lastname: Waller
authors.forename: D
authors.initials: D
authors.lastname: Pipkorn
authors.forename: K L
authors.initials: KL
authors.lastname: Sutton
authors.forename: J J
authors.initials: JJ
authors.lastname: Linderman
authors.forename: G M
authors.initials: GM
authors.lastname: Omann
mesh_terms: D001667:Binding, Competitive; D002138:Calibration; D005434:Flow Cytometry / Q000379:methods*; D005456:Fluorescent Dyes / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D009504:Neutrophils / Q000378:metabolism; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D015203:Reproducibility of Results; D013050:Spectrometry, Fluorescence / Q000379:methods
chemical_list: D005456:Fluorescent Dyes; D008024:Ligands; D009842:Oligopeptides