Liposomes having high sensitivity to odorants

The conditions to increase the sensitivities of liposomes to odorants were examined. The results obtained are as follows. (1) The minimum concentration of amyl acetate to induce the membrane potential changes (threshold) in phosphatidylcholine (PC) liposomes was about 10^-4 M and addition of 10 or 20% phospatidylserine (PS) lowered the threshold to about 10^-9 M, which was lower than the thresholds for amyl acetate in the turtle and frog olfactory systems. (2) Similar to amyl acetate, addition of PS to PC greatly lowered the threshold for β-ionone. On the other hand, addition of PS to PC in certain ratio increased the threshold for citral, suggesting that addition of PS to PC does not always increase the responses to all odorants. (3) The membrane fluidity change of the liposomes in response to odorants occurred at similar concentration region where the membrane potential changes occurred. The presence of CaCl₂ in external solution much greatly increased both the magnitude of the membrane potential changes and the membrane fluidity changes of the PC-PS liposomes in response to amyl acetate than the presence of NaCl and MgCl₂. These results suggest that the membrane fluidity change is related to generation of the membrane potential change. (4) It was estimated that adsorption of less than a few molecules of amyl acetate on single liposome elicits detectable changes in the membrane potential and the membrane fluidity.