In vitro and in vivo inhibition of N-acetyl-L-aspartyl-L-glutamate catabolism by N-acylated L-glutamate analogs.

The dipeptide N-acetyl-aspartyl-glutamic acid (NAAG) present in brain and spinal cord tissues may act as a neurotransmitter at excitatory synapses in the central nervous system. However, pharmacological and biochemical studies of NAAG are hampered by its rapid inactivation in vivo and in vitro by an enzyme that cleaves NAAG into N-acetyl-aspartate and glutamate. This enzyme has been previously named N-acetylated alpha-linked acidic dipeptidase (NAALADase). Based upon our earlier studies on the specificity of this enzyme, we have now designed new competitive inhibitors of this peptidase. N-Succinyl-glutamic acid, 4, was almost as potent as N-acetyl-beta-aspartyl-glutamic acid (beta-NAAG), 2, in inhibiting the hydrolysis of [Glu-3H]NAAG by rat brain membranes, with an IC50 value in the micromolar range. The analogous affinities of the substrate NAAG and of N-succinyl-glutamic acid suggest that the N-acetyl moiety is not an absolute requirement for entry into the active site of the enzyme. Therefore, the acronym NAALADase seems to be incorrect, and peptidase activity against NAAG will be used throughout this manuscript when referring to the enzyme that cleaves NAAG and whose activity is inhibited by quisqualate and beta-NAAG. Two N-acylated glutamic acid analogs, 5 and 6, were also found to be effective inhibitors of the in vitro degradation of NAAG, with Ki values in the micromolar range. Compounds 5 and 6 possess two free carboxylic functions on the N-acyl moiety, one of which could interact with the S1 subsite of the enzyme; the other could chelate the Zn++ cation involved in the catalytic hydrolysis of NAAG.(ABSTRACT TRUNCATED AT 250 WORDS)