Metabolism of [U-13C]Leucine in Cultured Astroglial Cells

Abstract

Leucine is rapidly metabolized in astroglial primary cultures. Therefore, it is considered as valuable fuel in brain energy metabolism. Only few of the leucine metabolites generated and released by astroglial cells have been identified. Therefore, a more detailed study was performed analyzing by NMR techniques the 13C-labeled metabolites, which were released by astroglial primary cultures during the degradation of [U-(13)C]leucine. Confirming a former radioactive study this analysis revealed 13C-labeled 2-oxoisocaproate and ketone bodies. Additionally, 13C-labeled alanine, citrate, glutamine, lactate and succinate were identified. Their detailed isotopomer analysis proves that 13C-labeled acetyl-CoA enters the tricarboxylic acid cycle, that intermediates with a characteristic 13C-labeling pattern can be withdrawn at several positions of the cycle, and that in the case of lactate and alanine there appears to be a participation of an active phosphoenolpyruvate carboxykinase and/or malic enzyme pathway. Thus, astroglial cells generate and release into the extracellular fluid not only the leucine catabolites 2-oxoisocaproate and ketone bodies, but also several tricarboxylic acid cycle dependent metabolites.

Glial Metabolism of Isoleucine

“…The astrocytic release of intermediates of BCAAs catabolism ( [9,24,36], present work) is in agreement with the known consumption of amino acids, including BCAAs, by brain tissue for the generation of energy [6,53]. It also is compatible with the hypotheses that one of the functions of these cells is to serve as ''fuel processing plants'' in brain [23] and that BCAAs can serve in brain energy metabolism to an extent of 10% in respect to glucose [6] as additional or alternative fuel molecules.…”

“…Cultured astrocytes remove BCAAs from their incubation medium [9]. Leucine was shown to be metabolized by astrocytes to a certain extent and some of the leucine metabolites released into the extracellular milieu have been identified [9,24]. The isoleucine metabolites, which may be also released by astrocytes, are still unknown.…”

Glial Metabolism of Valine

“…The assignment of metabolites by twodimensional NMR techniques was performed as described previously [32]. As already in the analogous studies of leucine [24] and isoleucine [25] metabolism, the limited number of cultured cells available prevented a corresponding NMR analysis of intracellularly located metabolites derived from 13 C-labelled valine. 15 N NMR studies aiming at the elucidation of the metabolism of the amino nitrogen of valine were not pursued.…”

“…Astrocytes which are considered to serve as ''fuel processing plants'' for neurons and possibly other neural cells [19][20][21] are likely to provide other cells of the brain parenchyma with certain nutrients [22]. The expression of a few enzymes required [11,23] or specific [24] for valine catabolism has been investigated in astrocytes. Cultured astrocytes remove BCAAs from their incubation medium and metabolize them [15].…”

“…Cultured astrocytes remove BCAAs from their incubation medium and metabolize them [15]. Although the catabolites of leucine [15,24] and isoleucine [25] are known, the compounds derived from valine that astrocytes generate and release into their extracellular milieu remain to be determined. Therefore, the aim of the present investigation was to fill this gap in our knowledge and (1) study the ability of astroglial cells to metabolize valine, (2) identify the valinemetabolites released by cultured astroglial cells and consider them as potential fuel molecules for other brain cells, and (3) examine the possibility that the carbon skeleton of valine appears in the TCA cycle as succinyl-CoA, thereby contributing to the anaplerotic function of the astroglial cells.…”