Feeding versus infusion: a novel approach to study the NAA metabolism in rat brain

Using in vivo (13)C-NMR spectroscopy, the energy metabolism in rat brain has commonly been studied via infusion of (13)C-labeled substrates on a minute to hour time scale. In the present study, as a novel approach, (13)C-enriched animal chow was administered over several days and compared with a 2 h infusion of [U-(13)C(6)]-glucose. Rats received chow containing either [U-(13)C(6)]-glucose or [U-(13)C]-biomass (a mixture of proteins, lipids, DNA, and carbohydrates) during 3 to 5 days. During feeding with (13)C-labeled glucose and biomass, in vivo (13)C-NMR spectroscopy was carried out daily and revealed slow but successive label incorporation into a large number of metabolites. Lipids and proteins were not significantly (13)C-enriched during a 2 h infusion of (13)C-labeled glucose, but became the most prominent resonances in the (13)C feeding experiment. Likewise, feeding with (13)C-enriched biomass led to additional (13)C-label incorporation into creatine, urea carbons and glycogen. Finally, only the acetyl moiety of N-acetyl-aspartate (NAA) became significantly enriched during the 2 h infusion experiment, whereas the aspartyl moiety remained at natural abundance levels. In the feeding experiments, however, label incorporation into all carbons of NAA could be observed. Moreover, isotopomer analysis of brain extracts revealed that the acetyl moiety of NAA in feeding experiments was always more strongly (13)C-enriched than its aspartyl moiety, suggesting that the turnover of the acetyl moiety is faster than that of the aspartyl moiety. The different enrichment kinetics of acetyl and aspartyl moiety could be explained by the existence of two different metabolic pathways reflecting the compartmentalised synthesis of NAA.