Using APCI-MS to Study the Dynamics of Odor Binding under Simulated Peri-Receptor Conditions

Odor binding protein (OBP) has been found in the olfactory systems of many insect and mammalian species. It binds a broad range of volatile organic compounds (VOCs) and one of its proposed functions in odor perception is to solubilize hydrophobic VOCs in the mucosal layer of the olfactory cleft and assist their transfer from the air phase to the olfactory receptors. The relative binding power of OBP for different volatile compounds has been measured experimentally, with values expressed as the dissociation constant, but dynamic studies are scarce. To address the lack of information, in vitro systems were built that could measure the release kinetics from OBP using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) as well as determine odor uptake and release by OBP under simulated in vivo conditions using direct mass spectrometry of gas-phase odors. The in vitro system was capable of introducing VOCs at a constant rate, or as pulses (to mimic the odor delivery on a breath-by-breath basis). Competition between VOCs for the binding sites on OBP was also studied experimentally.