Structure/activity relationships in the arginine taste pathway of the channel catfish

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Abstract
To characterize the molecular/structural requirements for activation or antagonism of the arginine taste pathways in catfish, Ictalurus punctatus, structure/activity studies were performed using integrated multiunit responses and cross-adaptation. Of all the guanidinium-containing compounds tested, only L-arginine, L-α-amino-β-guanidino propionic acid (L-AGPA) and L-arginine methyl and ethyl esters were strong stimuli. Results of functional group substitutions and modification of the L-arginine parent molecule indicated that: (i) stereospecificity was observed with D-arginine being a much less effective stimulus than L-arginine; (ii) an L-amino group must be present and unblocked (α-chloro-guanidino-N-valeric acid and N-acetyl L-arginine were weak or inactive stimuli); (iii) a free carboxylic acid group was not necessary for activity; (iv) the distance between the anomeric carbon and the guanidinium group was not critical (L-AGPA, having two methylene groups less than L-arginine was a moderately strong stimulus as was L-canavanine) and (v) modification or substitution of the guanidinum group by other basic groups including amine, methyl or dimethylamine or by an isosterc (ureido) resulted in loss of stimulatory ability. In general, those stimuli and analogs that were good cross-adapters of L-arginine stimulation were also good competitors for L-[3H]arginine binding to a partial membrane fraction (P2) from catfish taste epithelium. On the other hand, compounds that were poor cross-adapting stimuli were also poor binding competitors. While D-arginine was a poor stimulus, it did cross-adapt L-arginine and competed well with L-[3H]arginine for binding to fraction P2.

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