Department of Medical Biochemistry, Semmelweis University, Budapest, 1094, Hungary; MTA-SE Lendület Neurobiochemistry Research Group, Hungary.
Department of Medical Biochemistry, Semmelweis University, Budapest, 1094, Hungary; MTA-SE Laboratory for Neurobiochemistry, Hungary.
Neurochem Int. 2017 Oct;109:41-53. doi: 10.1016/j.neuint.2017.03.008. Epub 2017 Mar 11.
GABA is catabolized in the mitochondrial matrix through the GABA shunt, encompassing transamination to succinic semialdehyde followed by oxidation to succinate by the concerted actions of GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH), respectively. Gamma-hydroxybutyrate (GHB) is a neurotransmitter and a psychoactive drug that could enter the citric acid cycle through transhydrogenation with α-ketoglutarate to succinic semialdehyde and d-hydroxyglutarate, a reaction catalyzed by hydroxyacid-oxoacid transhydrogenase (HOT). Here, we tested the hypothesis that the elevation in matrix succinate concentration caused by exogenous addition of GABA, succinic semialdehyde or GHB shifts the equilibrium of the reversible reaction catalyzed by succinate-CoA ligase towards ATP (or GTP) hydrolysis, effectively negating substrate-level phosphorylation (SLP). Mitochondrial SLP was addressed by interrogating the directionality of the adenine nucleotide translocase during anoxia in isolated mouse brain and liver mitochondria. GABA eliminated SLP, and this was rescued by the GABA-T inhibitors vigabatrin and aminooxyacetic acid. Succinic semialdehyde was an extremely efficient substrate energizing mitochondria during normoxia but mimicked GABA in abolishing SLP in anoxia, in a manner refractory to vigabatrin and aminooxyacetic acid. GHB could moderately energize liver but not brain mitochondria consistent with the scarcity of HOT expression in the latter. In line with these results, GHB abolished SLP in liver but not brain mitochondria during anoxia and this was unaffected by either vigabatrin or aminooxyacetic acid. It is concluded that when mitochondria catabolize GABA or succinic semialdehyde or GHB through the GABA shunt, their ability to perform SLP is impaired.
GABA 在线粒体基质中通过 GABA 分流途径进行分解代谢，包括转氨基作用生成琥珀酸半醛，然后分别由 GABA 转氨酶 (GABA-T) 和琥珀酸半醛脱氢酶 (SSADH) 协同氧化为琥珀酸。γ-羟基丁酸 (GHB) 是一种神经递质和精神活性药物，可通过与 α-酮戊二酸的转氢作用进入柠檬酸循环，生成琥珀酸半醛和 d-羟基戊二酸，该反应由羟酸-氧酸转氢酶 (HOT) 催化。在这里，我们检验了这样一个假设，即外源性添加 GABA、琥珀酸半醛或 GHB 会导致基质琥珀酸浓度升高，从而使琥珀酸-CoA 连接酶催化的可逆反应平衡向 ATP（或 GTP）水解方向移动，有效地否定了底物水平磷酸化 (SLP)。通过在分离的小鼠脑和肝线粒体中缺氧时询问腺嘌呤核苷酸转运蛋白的方向性来解决线粒体 SLP 问题。GABA 消除了 SLP，而 GABA-T 抑制剂维加特林和氨基氧乙酸挽救了这一现象。琥珀酸半醛在正常氧条件下是一种非常有效的底物，可使线粒体产能量，但在缺氧条件下，它与 GABA 一样可消除 SLP，并且对维加特林和氨基氧乙酸有抗性。GHB 可以适度地使肝线粒体，但不能使脑线粒体产能量，这与后者中 HOT 表达的稀缺性一致。与这些结果一致，GHB 在缺氧时消除了肝线粒体但没有脑线粒体中的 SLP，并且这不受维加特林或氨基氧乙酸的影响。综上所述，当线粒体通过 GABA 分流途径分解代谢 GABA 或琥珀酸半醛或 GHB 时，它们进行 SLP 的能力受损。
