Hertz L, Yu A C, Schousboe A
Department of Pharmacology, University of Saskatchewan, Saskatoon, Canada.
J Neurosci Res. 1992 Oct;33(2):289-96. doi: 10.1002/jnr.490330212.
Uptake and oxidative metabolism of [14C]malate as well as its incorporation into aspartate, glutamate, glutamine, and GABA were studied in cultured cerebral cortical neurons (GABAergic), cerebellar granule neurons (glutamatergic), and cerebral cortical astrocytes. All cell types exhibited high affinity uptake of malate (Km 10-85 microM) with slightly higher Vmax values in neurons (0.1-0.2 nmol x min-1 x mg-1) than in astrocytes (0.06 nmol x min-1 x mg-1). Malate was oxidatively metabolized in all three cell types with nominal rates of 14CO2 production of 2-15 pmol x min-1 x mg-1. The oxidation of malate was only slightly inhibited by 5 mM aminooxyacetic acid (AOAA). In granule cell preparations [14C]malate was incorporated into aspartate and glutamate and, to a much less extent, into glutamine. This incorporation was blocked by 5 mM AOAA. Astrocytes exhibited slightly higher incorporation rates into aspartate and glutamate, but in these cells glutamine was labelled to a considerable extent. AOAA (5 mM) inhibited the incorporation by 60-70%. In cultures of cerebral cortical neurons, very low levels of radioactivity derived from [14C]malate were found in aspartate and glutamate, and GABA was not labelled at all. Glutamine had the same specific activity as glutamate, indicating that the low rates of incorporation of radioactivity into amino acids in this preparation is likely to exclusively represent metabolism of malate in the small population of astrocytes (5% of total cell number), contaminating the neuronal cultures. The findings suggest that exogenous malate to a quantitatively limited extent may serve as a precursor for transmitter glutamate in glutamatergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)
在培养的大脑皮质神经元（γ-氨基丁酸能）、小脑颗粒神经元（谷氨酸能）和大脑皮质星形胶质细胞中，研究了[¹⁴C]苹果酸的摄取、氧化代谢及其掺入天冬氨酸、谷氨酸、谷氨酰胺和γ-氨基丁酸的情况。所有细胞类型均表现出对苹果酸的高亲和力摄取（Km为10 - 85微摩尔），神经元中的最大摄取速率（Vmax）（0.1 - 0.2纳摩尔·分钟⁻¹·毫克⁻¹）略高于星形胶质细胞（0.06纳摩尔·分钟⁻¹·毫克⁻¹）。苹果酸在所有三种细胞类型中均发生氧化代谢，¹⁴CO₂生成的标称速率为2 - 15皮摩尔·分钟⁻¹·毫克⁻¹。5毫摩尔的氨基氧乙酸（AOAA）对苹果酸氧化的抑制作用很轻微。在颗粒细胞制剂中，[¹⁴C]苹果酸掺入天冬氨酸和谷氨酸，掺入谷氨酰胺的程度则小得多。这种掺入被5毫摩尔的AOAA阻断。星形胶质细胞中天冬氨酸和谷氨酸的掺入速率略高，但在这些细胞中谷氨酰胺有相当程度的标记。5毫摩尔的AOAA使掺入率降低60 - 70%。在大脑皮质神经元培养物中，天冬氨酸和谷氨酸中源自[¹⁴C]苹果酸的放射性水平极低，γ-氨基丁酸根本没有被标记。谷氨酰胺与谷氨酸具有相同的比活性，表明该制剂中放射性掺入氨基酸的低速率可能仅代表污染神经元培养物的少量星形胶质细胞（占细胞总数的5%）中苹果酸的代谢。这些发现表明，在数量有限的情况下，外源性苹果酸可能作为谷氨酸能神经元中递质谷氨酸的前体。（摘要截断于250字）
