Seven D-amino acids, including D-valine, D-phenylalanine, D-leucine, D-isoleucine, D-tryptophan, D-methionine, and D-alpha-aminobutyric acid, are markedly less stimulative than the corresponding L-isomers that can stimulate the labellar sugar receptor of the fleshfly. A distinct effect of len;th of the amino acid side chain is clearly observed. Esterification and amidation of the alpha-carboxyl group, as well as substitution by hydroxyl and methyl groups, result in extremely decreased responses. Amino acids whose amino groups are located at a position other than the alpha are almost ineffective. With all these rigid stereospecificities of the sugar receptor for amino acids, certain replacement of the alpha-amino group with the hydroxyl or carbonyl group shows a slight increase of the response at neutral pH. Furthermore, certain fatty acids can stimulate the sugar receptor once the solutions are buffered at neutral pH. This observation was further supported by the presence of a remarkable similarity of stimulating effectiveness between amino acids that can stimulate the sugar receptor and those fatty acids. The similarity was shown by testing the response concentration relationships, the stimulating effect of fatty acid derivatives, the effect of treatment with p-chloromercuribenzoate, the behavioral response, and so on.

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