Potential ligands of OBPs. Twenty nine potential ligands were tested in a competitive binding assay for their binding to OBP16, OBP18 and OBP21. Each purified recombinant protein was mixed with the fluorescent probe N-phenyl-1-naphthylamine (1-NPN), both at the concentration of 2 μM in Tris buffer. Each mixture was titrated with a ligand and the displacement of the fluorescent probe from the complex was used to evaluate the relative dissociation constants, as described in the Methods section. Compounds with higher affinities are shown in the top panel, while compound with lower affinity are shown in the lower panel. OBP16 and OBP18 showed higher affinities for most compounds than the previously characterized OBP21. Of particular interest is the high binding affinity of OBP18 to oleic acid.