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

Abstract

Using in vivo 13C-NMR spectroscopy, the energy metabolism in rat brain has commonly been studied via infusion of 13C-labeled substrates on a minute to hour time scale. In the present study, as a novel approach, 13C-enriched animal chow was administered over several days and compared with a 2 h infusion of [U-13C6]-glucose. Rats received chow containing either [U-13C6]-glucose or [U-13C]-biomass (a mixture of proteins, lipids, DNA, and carbohydrates) during 3 to 5 days. During feeding with 13C-labeled glucose and biomass, in vivo 13C-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 13C-enriched during a 2 h infusion of 13C-labeled glucose, but became the most prominent resonances in the 13C feeding experiment. Likewise, feeding with 13C-enriched biomass led to additional 13C-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 13C-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. Copyright © 2003 John Wiley & Sons, Ltd.