Glial Metabolism of Isoleucine

Abstract

Isoleucine, together with leucine and valine, constitutes the group of branched-chain amino acids (BCAAs). BCAAs are transported from the blood into the brain parenchyma, where they can serve several distinct functions. Since brain tissue is known to oxidatively metabolize BCAAs to CO(2), they are considered as fuel material in brain energy metabolism. Also, in the case of leucine, cultured astrocytes have been reported to be able to completely oxidize BCAA. While the metabolism of leucine by astroglia-rich primary culture (APC) has already been studied in detail, the metabolic fates of isoleucine and valine in these cells remained to be identified. Therefore, in the present study an NMR analysis was performed of (13)C-labelled metabolites generated in the catabolism of [U-(13)C]Ile by astrocytes and released by them into the incubation medium. APC potently removed isoleucine from the medium and metabolized it. The major isoleucine metabolites released from APC are 2-oxo-3-methylvalerate, 2-methylbutyrate, 3-hydroxy-2-methylbutyrate and propionate. To a lesser extent, APC generate and release also [2,3-(13)C]glutamine, [4,5-(13)C]glutamine and (13)C-labelled isotopomers of lactate and citrate. These results show that APC can release into the extracellular milieu catabolites and several TCA cycle dependent metabolites resulting from the degradation of isoleucine.

Glial Metabolism of Valine

“…The concentration of labelled citrate in the incubation medium was too low for quantification respect to other released compounds. This is analogous to the release of 2-methylbutyrate, 3-hydroxy-2-methylbutyrate and propionate from astroglial cells when they are incubated with isoleucine [25]. The occurrence of isobutyrate in the incubation medium may not only be due to the hydrolysis of the thioester bond of isobutyryl-CoA and the subsequent release of the acylate from the cells, but may also be due to non-enzymic decarboxylation of 2-oxoisovalerate by reaction with hydrogen peroxide.…”

“…Furthermore, together with their cognate branchedchain 2-oxo acids, they can participate in maintaining the glutamate-glutamine cycle between astrocytes and neurons [9-11, 20, 21]. Indeed, cultured astrocytes were shown to release also 2-oxoisocaproate [24] and 2-oxo-3-methylbutyrate [25]. It is very likely that-similar to 2-oxoisocaproate-2-oxoisovalerate crosses the plasma membrane of astroglial cells by facilitated transport mediated by monocarboxylate transporter 1 [33].…”

“…Cell viability was assessed by determining the appearance of lactate dehydrogenase in the medium using the modification [25] of a described method [27]. By definition, 0% viability corresponds to 100% activity of lactate dehydrogenase in the medium [28].…”

“…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.…”

“…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.…”