Summary:The authors investigated concomitant lactate and glucose metabolism in primary neuronal cultures using 13 Cand 1 H-NMR spectroscopy. Neurons were incubated in a medium containing either [1- 13C]glucose and different unlabeled lactate concentrations, or unlabeled glucose and different [3-13 C]lactate concentrations. Overall, 13 C-NMR spectra of cellular extracts showed that more 13 C was incorporated into glutamate when lactate was the enriched substrate. Glutamate 13 C-enrichment was also found to be much higher in lactatelabeled than in glucose-labeled conditions. When glucose and lactate concentrations were identical (5.5 mmol/L), relative contributions of glucose and lactate to neuronal oxidative metabolism amounted to 21% and 79%, respectively. Results clearly indicate that when neurons are in the presence of both glucose and lactate, they preferentially use lactate as their main oxidative substrate. Key Words: Energy metabolismBrain-NMR spectroscopy-TCA cycle-MonocarboxylateGlutamate.

In all trypanosomatids, including Trypanosoma brucei, glycolysis takes place in peroxisome-like organelles called glycosomes. These are closed compartments wherein the energy and redox (NAD(+)/NADH) balances need to be maintained. We have characterized a T. brucei gene called FRDg encoding a protein 35% identical to Saccharomyces cerevisiae fumarate reductases. Microsequencing of FRDg purified from glycosome preparations, immunofluorescence, and Western blot analyses clearly identified this enzyme as a glycosomal protein that is only expressed in the procyclic form of T. brucei but is present in all the other trypanosomatids studied, i.e. Trypanosoma congolense, Crithidia fasciculata and Leishmania amazonensis. The specific inactivation of FRDg gene expression by RNA interference showed that FRDg is responsible for the NADH-dependent fumarate reductase activity detected in glycosomal fractions and that at least 60% of the succinate secreted by the T. brucei procyclic form (in the presence of d-glucose as the sole carbon source) is produced in the glycosome by FRDg. We conclude that FRDg plays a key role in the energy metabolism by participating in the maintenance of the glycosomal NAD(+)/NADH balance. We have also detected a significant pyruvate kinase activity in the cytosol of the T. brucei procyclic cells that was not observed previously. Consequently, we propose a revised model of glucose metabolism in procyclic trypanosomes that may also be valid for all other trypanosomatids except the T. brucei bloodstream form. Interestingly, H. Gest has hypothesized previously (Gest, H. (1980) FEMS Microbiol. Lett. 7, 73-77) that a soluble NADH-dependent fumarate reductase has been present in primitive organisms and evolved into the present day fumarate reductases, which are quinol-dependent. FRDg may have the characteristics of such an ancestral enzyme and is the only NADH-dependent fumarate reductase characterized to date.

Metabolic pathways of the intermediate metabolism of maize root tips were identified and quantified after labeling to isotopic and metabolic steady state using glucose labeled on carbon-1, -2, or -6 with 14C or 13C. The specific radioactivity of amino acids and the 13C-specific enrichment of specific carbons of free glucose, sucrose, alanine and glutamate were measured and used to calculate metabolic fluxes. The non-triose pathways, including synthesis of polysaccharides, accumulation of free hexoses, and to a lesser extent starch synthesis, were found to consume 75% of the glucose entering the root tips. The cycle of synthesis and hydrolysis of sucrose was found to consume about 70% of the ATP produced by respiration. The comparison of the specific radioactivities of amino acids and phospholipid glycerol phosphate after labeling with [1-(14)C] or [6-(14)C]glucose revealed the operation of the pentose phosphate pathway. The transfer of label from [2-(14)C]glucose to carbon-1 of starch glucosyl units confirmed the operation of this pathway and indicated that it is located in plastids. It was found to consume 32% of the hexose phosphates entering the triose pathways. The remaining 68% were consumed by glycolysis. The determination of the specific enrichment of carbohydrate carbons -1 and -6 after labeling with [1-(13)C]glucose indicated that both the conversion of triose phosphates back to hexose phosphates and the transaldolase exchange contributed to this randomization. Of the triose phosphates produced by glycolysis and the pentose phosphate pathway, about 60% were found to be recycled to hexose phosphates, and 28% were directed to the tricarboxylic acid cycle. Of this 28%, two-thirds were found to be directed through the pyruvate kinase branch and one-third through the phosphoenolpyruvate branch. The latter essentially has an anaplerotic function since little malate was found to be converted to pyruvate (malic enzyme reaction).