A dissertation by Kang (2000) details results of OR-ligand assays for indole, Hedione, and naphthalene. The data show each molecule generating responses in a widespread, discontinuous, random-looking array of odorant receptors. Most of these receptors had not previously been deorphaned, and indeed assays typically suffer from some ORs failing to express on the cell surface (Omura et al, 2021). Most tellingly, Kang’s assay misses two known receptors for both indole and Hedione. To try to gauge the accuracy of the Kang results, we’ve tested a single OR-odorant pair by comparing the odor of indole alone to that of mixtures of indole and two receptor antagonists.
Kang identifies OR2AT4 as a sensitive (log10 EC50 = -6.2) receptor for indole. OR2AT4 is also associated with some sandalwood type aroma compounds, and of the 2AT4 agonists we’ve smelled (sandalore, sandranol, brahmanol), we’ve observed a dermal-fecal-sulfur note to all three, that is lacking from non-agonists of 2AT4 (ebanol, polysantol). We’ve also observed the two known 2AT4 antagonists (raspberry ketone, Phenirat) each mask this unpleasant note, both individually and in combination. Since indole is a fecal smelling compound, its measured activity on the 2AT4 receptor is not at all surprising.
We prepared three test strips with the following aroma compounds added: 1.) indole alone, diluted in carrier oil; 2.) the indole/carrier oil mixture plus raspberry ketone in ethanol; 3.) indole+carrier oil and pure Phenirat. We found that mixture 1 smelled fecal, powdery, and like jasmine flowers. Mixture 2 smelled powdery, floral, and raspberry, with no perceptible fecal note. Mixture 3 smelled fruity and floral, with a diminished fecal undertone that more approaches a woody character.
We conclude that at least some of the Kang data are true measurements of receptor agonism by the compounds tested.
It is still not known why Kang was not able to verify the known receptors OR1G1 and OR52D1 for indole (Sanz et al, 2005) nor the same two receptors for Hedione. But Kang’s results did not detect any activity for these two receptors. Perhaps whatever materials or methods differ between Kang and Sanz et al, may have enabled accurate testing of some ORs for each study and prevented testing those individual ORs for the other.