Enzyme Kinetics of N-Acetyl-Aspartylglutamate in the Cingulated Cortices in ASD: A 1H-MRS Model

Background:  The L-glutamate (Glu) and L-acetyl-aspartate (NAA) are products of N-acetyl-aspartyl-glutamate (NAAG) which require the participation of neurons, oligodendrocytes and atrocytes. On the one hand, NAAG is synthesized from NAA and Glu by an NAAG synthase, forming a dedicated pool of Glu that also cannot be further metabolized, and on the other hand, NAAG is then hydrolyzed by NAAG peptidase releasing Glu which activates the mGluR3 receptor. Enzymes stabilize transition states for reactions, and thus lower the activation energy required. A common measure for how much a reaction is sped up is called the rate enhancement, equal to the ratio of the catalyzed rate to the uncatalyzed rate. This ratio varies widely, ranging from one (which is technically no longer an enzyme - merely a protein) to 1.4 × 10¹⁷ for oritidine-monophosphate decarboxylase (an enzyme involved in DNA synthesis). At high concentrations, some substrates also inhibit the enzyme activity. Substrate inhibition occurs with about 20% of all known enzymes. It happens when two molecules of substrate can bind to the enzyme, and thus block activity. Altered NAAG metabolism has been described, in some neurological conditions but not in autism spectrum disorders (ASD). Our previous studies using proton-Magnetic Resonance Spectroscopy (¹H-MRS) in bilateral anterior (ACC) and posterior cingulate cortex (PCC) have described the altered neurometabolic patterns in adults with ASD.

Objectives: To study the enzyme kinetics of NAAG in vivo, using ¹H-MRS.

Methods:  Single-voxel ( ¹H-MRS) in bilateral ACC and PCC, in 19 adults with a clinical diagnosis of ASD and 41 controls, matched for age, gender. Autism quotients (AQ) score were assessed. The affinity between enzymes and substrates associated with NAAG was measured. The Michaelis-Menten constant is calculated. One-way ANOVA and Bonferroni correction were applied.

Results: The ASD group had a significant increase of Km (NAA) = [5,79x10⁶ (mM)]; R² = -27.05 compared with controls (TD) in ACC.

Conclusions:  Altered enzyme kinetics N-acetylaspartylglutamate levels were found in cingulated cortices by ¹H-MRS in individuals with ASD, suggesting new therapeutic avenues.