Martin-Requero A, Ayuso M S, Parrilla R
Arch Biochem Biophys. 1986 Apr;246(1):114-27. doi: 10.1016/0003-9861(86)90455-8.
Oxamate, a structural analog of pyruvate, known as a potent inhibitor of lactic dehydrogenase, lactic dehydrogenase, produces an inhibition of gluconeogenic flux in isolated perfused rat liver or hepatocyte suspensions from low concentrations of pyruvate (less than 0.5 mM) or substrates yielding pyruvate. The following observations indicate that oxamate inhibits flux through pyruvate carboxylase: accumulation of substrates and decreased concentration of all metabolic intermediates beyond pyruvate; decreased levels of aspartate, glutamate, and alanine; and enhanced ketone body production, which is a sensitive indicator of decreased mitochondrial free oxaloacetate levels. The decreased pyruvate carboxylase flux does not seem to be the result of a direct inhibitory action of oxamate on this enzyme but is secondary to a decreased rate of pyruvate entry into the mitochondria. This assumption is based on the following observations: Above 0.4 mM pyruvate, no significant inhibitory effect of oxamate on gluconeogenesis was observed. The competitive nature of oxamate inhibition is in conflict with its effect on isolated pyruvate carboxylase which is noncompetitive for pyruvate. Fatty acid oxidation was effective in stimulating gluconeogenesis in the presence of oxamate only at concentrations of pyruvate above 0.4 mM. Since only at low pyruvate concentrations its entry into the mitochondria occurs via the monocarboxylate translocator, from these observations it follows that pyruvate transport across the mitochondrial membrane, and not its carboxylation, is the first nonequilibrium step in the gluconeogenic pathway. In the presence of oxamate, fatty acid oxidation inhibited gluconeogenesis from lactate, alanine, and low pyruvate concentrations (less than 0.5 mM), and the rate of transfer of reducing equivalents to the cytosol was significantly decreased. Whether fatty acids stimulate or inhibit gluconeogenesis appears to correlate with the rate of flux through pyruvate carboxylase which ultimately seems to rely on pyruvate availability. Unless adequate rates of oxaloacetate formation are maintained, the shift of the mitochondrial NAD couple to a more reduced state during fatty acid oxidation seems to decrease mitochondrial oxaloacetate resulting in a decreased rate of transfer of carbon and reducing power to the cytosol.
草氨酸盐是丙酮酸的结构类似物，是一种已知的乳酸脱氢酶强效抑制剂，在分离的灌注大鼠肝脏或低浓度丙酮酸（低于0.5 mM）或产生丙酮酸的底物的肝细胞悬浮液中，乳酸脱氢酶会抑制糖异生通量。以下观察结果表明草氨酸盐抑制丙酮酸羧化酶的通量：底物积累以及丙酮酸之后所有代谢中间产物的浓度降低；天冬氨酸、谷氨酸和丙氨酸水平降低；酮体生成增加，这是线粒体游离草酰乙酸水平降低的敏感指标。丙酮酸羧化酶通量降低似乎并非草氨酸盐对该酶的直接抑制作用所致，而是丙酮酸进入线粒体的速率降低的继发结果。这一假设基于以下观察结果：丙酮酸浓度高于0.4 mM时，未观察到草氨酸盐对糖异生有显著抑制作用。草氨酸盐抑制的竞争性与它对分离的丙酮酸羧化酶的作用相矛盾，后者对丙酮酸是非竞争性的。仅在丙酮酸浓度高于0.4 mM时，脂肪酸氧化才在草氨酸盐存在的情况下有效刺激糖异生。由于仅在低丙酮酸浓度下其进入线粒体是通过单羧酸转运体，从这些观察结果可以得出，丙酮酸跨线粒体膜的转运而非其羧化作用是糖异生途径中的第一个非平衡步骤。在草氨酸盐存在的情况下，脂肪酸氧化抑制了来自乳酸、丙氨酸和低丙酮酸浓度（低于0.5 mM）的糖异生，并且还原当量向细胞质的转移速率显著降低。脂肪酸是刺激还是抑制糖异生似乎与通过丙酮酸羧化酶的通量速率相关，而这最终似乎依赖于丙酮酸的可用性。除非维持足够的草酰乙酸生成速率，脂肪酸氧化过程中线粒体NAD对向更还原状态的转变似乎会降低线粒体草酰乙酸，导致碳和还原能力向细胞质的转移速率降低。
