Stereoselective detection of amino acids by lobster olfactory receptor neurons

1. Biochemical and electrophysiological assays were used to test the hypothesis that the olfactory system of the Caribbean spiny lobster, Panulirus argus, contains populations of chemosensory receptors that are differentially sensitive to the L- and D-stereoisomers of the amino acid alanine. 2. Independent binding sites for L-alanine (dissociation constant (KD) of 6.6 microM and maximum binding (Bmax) of 16.8 fmole/microgram protein) and for D-alanine (KD of 21.6 microM and Bmax of 17.8 fmole/microgram protein) were characterized biochemically. The interaction of ligand with each binding site is rapid, reversible and saturable with respect to both time and concentration. 3. Based on a difference of at least 20% in the relative sensitivity of an olfactory receptor cell to alanine enantiomers, 44% and 34% of the 77 neurons tested were classified as L-alanine and D-alanine sensitive, respectively. The relative sensitivity to alanine enantiomers was independent of the concentration tested. Stereoselective receptors are likely for 17 of 20 other amino acids tested. 4. The congruence of biochemical and electrophysiological results leads to the conclusion that the lobster's responses to D- and L-alanine are mediated by receptors specific for each stereoisomer and that the receptors are differentially distributed among receptor cells.