Hydrogen turnover and subcellular compartmentation of hepatic [2-13C]glutamate and [3-13C]aspartate as detected by 13C NMR

C-13 NMR monitored the dynamics of exchange from specific hydrogens of hepatic [2-C-13]glutamate and [3-C-13]aspartate with deuterons from intracellular heavy water providing information on alpha-ketoglutarate/glutamate exchange and subcellular compartmentation. Mouse livers were perfused with [3-C-13]alanine in buffer containing or not 50% (H2O)-H-2 for increasing periods of time (I min < t < 30 min). Liver extracts prepared at the end of the perfusions were analyzed by high resolution C-13 NAIR (150.13 MHz) with H-1 decoupling only and with simultaneous H-1 and H-2 decoupling. C-13-H-2 Couplings and H-2-induced isotopic shifts observed in the glutamate C2 resonance, allowed to estimate the apparent rate constants (forward, reverse; min(-1)) for (i) the reversible exchange of [2-C-13]glutamate H2 as catalyzed mainly by aspartate aminotransferase (0.32, 0.56), (ii) the reversible exchange of [2-C-13]glutamate H3(proS) as catalyzed by NAD(P) isocitrate dehydrogenase (0.1, 0.05), and (iii) the irreversible exchanges of glutamate H3(proR) and H3(proS) as catalyzed by the sequential activities of mitochondrial aconitase and NAD isocitrate dehydrogenase of the tricarboxylic acid cycle (0.035), respectively. A similar approach allowed to determine the rates of H-1-H-2 exchange for the H2 (0.4, 0.5) or H3(proR) (0.3, 0.2) or the H2 and H3(proS) hydrogens (0.20, 0.23) of [3-C-13]aspartate isotopomers. The ubiquitous subcellular localization of H-1-H-2 exchange enzymes and the exclusive mitochondrial localization of pyruvate carboxylase and the tricarboxylic acid cycle resulted in distinctive kinetics of deuteration in the H2 and either or both 113 hydrogens of [3-C-13]glutamate and [3-C-13]aspartate, allowing to follow glutamate and aspartate trafficking through cytosol and mitochondria.