Methylmercury has a selective effect on mitochondria in cultured astrocytes in the presence of [U- 13C]glutamate

Abstract

The effect of methylmercury on glutamate metabolism was studied by 13C magnetic resonance spectroscopy. Cerebral cortical astrocytes were pretreated with methylmercury, either 1 μM for 24 h, or 10 μM for 30 min, and subsequently with 0.5 mM [U- 13C]glutamate for 2 h. Labeled glutamate, glutamine, aspartate and glutathione were present in cell extracts, and glutamine, aspartate and lactate in the medium of all groups. HPLC analysis of these amino acids showed no changes in concentrations between groups. Surprisingly, the amounts of [U- 13C]glutamate and unlabeled glucose taken up by the astrocytes were unchanged. Furthermore, the amounts of most metabolites synthesized from [U- 13C]glutamate were also unchanged in all groups. However, formation of [U- 13C]lactate was decreased in the 10 μM methylmercury group. This was not observed for labeled aspartate. It is noteworthy that both [U- 13C]lactate and [U- 13C]aspartate can only be derived from [U- 13C]glutamate via mitochondrial metabolism. [U- 13C]glutamate enters the tricarboxylic acid cycle (located in mitochondria) after conversion to 2-[U- 13C]oxoglutarate and [U- 13C]aspartate is formed from [U- 13C]oxaloacetate, as is [U- 13C]lactate. [U- 13C]lactate can also be formed from [U- 13C]malate. This differential effect on labeled aspartate and lactate indicates cellular compartmentation and thus selective vulnerability of mitochondria within the astrocytes to the effects of methylmercury. The decreased lactate production from glutamate might be detrimental to surrounding cells since lactate has been shown to be an important substrate for neurons.