SYNTHETIC ANALOGS OF CONANTOKIN-G - NMDA ANTAGONISTS ACTING THROUGH ANOVEL POLYAMINE-COUPLED SITE

Conantokin-G (con-G) is a 17-amino-acid polypeptide that acts as an N- methyl-D-aspartate (NMDA) antagonist. This action has been attributed to a specific but noncompetitive inhibition of the positive modulatory effects of polyamines at NMDA receptors. Con-G possesses several unus ual structural features, including five gamma-carboxyglutamate (Cia) r esidues and a high degree of helicity in aqueous media. Previous struc ture-activity studies indicated that one or more Gla residues are nece ssary for NMDA antagonist activity, Con-G analogues were synthesized w ith alanine (Ala), serine (Ser), and phosphoserine substituted for Gla to assess the contribution of individual Gla residues to biological a ctivity and secondary structure. Replacement of Gla in positions 3 and 4 resulted in polypeptides with markedly reduced and no NMDA antagoni st actions, respectively. In contrast, Gla residues in positions 7, 10 , and 14 are not required for NMDA antagonist actions because the pote ncies of con-G analogues containing Ser(7), Ser(10) Ala(14) and Ser(14 ) to inhibit spermine-stimulated [H-3]MK-801 binding are similar to th e parent peptide. Moreover, the Ala(7) derivative of con-G was about f ourfold more potent than the parent peptide both as an inhibitor of sp ermine-stimulated increases in [H-3]MK-801 binding (IC50 of similar to 45 nM) and in reducing NMDA-stimulated increases in cyclic GMP levels (IC50 of similar to 77 nM) in cerebellar granule cell cultures. Altho ugh con-G and its analogues assumed mixtures of 3(10) and alpha-helice s, no clearcut relationship was evinced between the NMDA antagonist pr operties of these peptides and the degree of helicity they assumed in aqueous solutions. Together with the inability of con-G to affect 5,7- dichloro[H-3] kynurenic acid, [H-3]CGP-39653, and [H-3]ifenprodil bind ing, these data are consistent with the hypothesis that this polypepti de acts at a unique, polyamine-associated site on NMDA receptors.