A Bicyclic Amino Acid to Improve Discriminations among Transport Systems

Abstract

Abstract Introduction of an amino and a carboxyl group onto carbon 2 of the bicycloheptane ring of norcamphor, by way of the corresponding spirohydantoin, produced a model substrate in which rigidly specified positions are taken by carbon atoms corresponding to those of the ordinary branched chain amino acids. This substance is transported by the system serving for amino acids with apolar (usually, branched) side chains in all cells and tissues tested. No measurable transport of it by any other system has yet been observed, the Na+-dependent systems of various erythrocytes and the Ehrlich cell having been specifically excluded. No unsuspected heterogeneity was detected in the Na+-independent transport of amino acids with apolar side chains by its use. We have used another new synthetic amino acid to illustrate how this substance can be used in conjunction with α-(methylamino)-isobutyric acid for determining by what routes a given amino acid is transported. When the bicyclic amino acid was prepared instead by way of the aminonitrile, the product contained only 31% (rather than the 92% obtained by way of the hydantoin) of the generally effective component, as separated by chromatography on sulfonated polystyrene resin. These two components are presumed to be the endo and exo isomers, although conformations have not yet been assigned. Of the two forms, only the one migrating more slowly on resin columns was taken up by Escherichia coli and that by the system serving for branched chain amino acids. The same product was taken up 2.5 times as fast by the human red blood cell, 3.7 times as fast by the pigeon red blood cell, and 5 times as fast in the Ehrlich cell, as the a isomer. The binding of b-2-aminobicyclo[2,2,1]heptane-2-carboxylic acid to the leucine-binding protein of E. coli and its inhibition of the binding of leucine were far weaker than predicted from its Km and Ki values for transport and inhibition of leucine transport. The bicyclic amino acid has permitted new discriminations of transport agencies and interactions, and may be expected to assist in the further discrimination of transport systems and other biological receptor systems. Determination of the absolute configuration and conformation of the active isomer should assist in the description of specific transport receptor sites, both that which accepts it and those that exclude it.