Isolation and characterization of an olfactory receptor protein for odorant pyrazines

The highly potent bell pepper odorant 2-isobutyl-3-[³H]methoxypyrazine ([³H]IBMP) binds specifically and saturably to bovine and rat nasal epithelium. Specific binding is not detected in 11 other tissues assayed, and in the rat binding is 9 times higher in olfactory than in respiratory epithelium. We have purified to apparent homogeneity a soluble pyrazine odorant binding protein that constitutes ~1% of the total soluble protein in bovine nasal epithelium. Polyacrylamide gel electrophoresis shows a single band of 19,000 Da and gel filtration data suggest that the native protein is a dimer of 38,000 Da. Binding of [³H]IBMP to the purified protein reveals two binding sites (K(d) = 10 x 10^-9 M, B(max) = 135 pmol per mg of protein; K(d) = 3 x 10^-6 M, B(max) = 25 nmol per mg of protein). The binding affinities of a homologous series of pyrazine odorants correlate with the human odor detection thresholds of these compounds. This correlation, together with the regional distribution of the protein, suggests that the protein is a physiologically relevant olfactory receptor.