Interdependence of N-acetyl aspartate and high-energy phosphates in healthy human brain.

Because cellular and extract data have suggested that N-acetylaspartate (NAA) reflects neuronal mitochondrial function, we evaluated the quantitative relationship between NAA, high-energy phosphates, and ADP levels in the hippocampus and occipital lobe of 15 healthy volunteers. The ADP levels are calculated using the creatine kinase equilibrium and quantified (31)P and total creatine measurements. Using high-field quantitative MR spectroscopic imaging, we find that NAA and ADP concentrations in the hippocampal body are 9.7 +/- 1.5mM and 35 +/- 8microM, respectively. In the occipital lobe, NAA and ADP are 11.9 +/- 1.9mM and 32 +/- 12microM, respectively. There is a statistically significant positive correlation between NAA and ADP, with R = +0.80, p < 2 x 10(-7)in the hippocampal body. In an adjacent hippocampal NAA voxel, the correlation between NAA and ADP had a R = +0.62, p < 3 x 10(-4), whereas, in the occipital lobe, R = +0.67, p < 5 x 10(-5). There was no significant relationship NAA and either ATP or phosphocreatine. This positive relationship of NAA with ADP suggests a directional process wherein energetics may modulate mitochondrial function.