Isolation and characterization of an olfactory receptor protein for odorant pyrazines.

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Abstract
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. A soluble pyrazine odorant binding protein that constitutes .apprxeq. 1% of the total soluble protein in bovine nasal epithelium was purified to apparent homogeneity. Polyacrylamide gel electrophoresis shows a single band of 19,000 Da [dalton], 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 2 binding sites (Kd = 10 .times. 10-9 M, Bmax [maximum binding] = 135 pmol per mg of protein; Kd = 3 .times. 10-6 M, Bmax = 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.