SPECIAL TRANSPORT AND NEUROLOGICAL SIGNIFICANCE OF 2 AMINO-ACIDS IN ACONFIGURATION CONVENTIONALLY DESIGNATED AS D

We point out an ability of certain amino acids to be recognized at a b iological receptor site as though their amino group bore, instead of a n at relationship to a carboxylate group, a beta, gamma or delta relat ionship to the same or a second carboxylate group. For aspartate, the unbalanced position of its amino group between a pair of carboxylates allows its occasional biorecognition as a beta- rather than as an alph a-amino acid, whereas for proline and its homologs, their cyclic arran gement may allow the imino group, without its being replicated, to be sensed analogously as falling at either of two distances from the sing le carboxylate group. The greater separation might allow proline to be seen as biologically analogous to gamma-aminobutyric acid. This more remote positioning of the imino group would allow the D-form of both a mino acids to present its amino group in the orientation characteristi c of the natural L-form. The dual modes of recognition should accordin gly be signalled by what appears to be low stereospecificity, actually due to a distinction in the enantiorecognition of the two isomers. Co mpeting recognition for transport between their respective D- and L-fo rms, although it does not prove that phenomenon, has been shown for pr oline and, significantly, even more strongly for its lower homolog, 2- azetidine carboxylate. Such indications have so far revealed themselve s rather inconspicuously for the central nervous system binding of pro line, reviewed here as a possible feature of a role suspected for prol ine in neurotransmission.