Department of Biology, Institute of Biochemistry, Carleton University, Ottawa, Canada.
Department of Biology and Chemistry, Institute of Biochemistry, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada.
J Comp Physiol B. 2019 Apr;189(2):271-281. doi: 10.1007/s00360-018-1200-3. Epub 2019 Jan 10.
The African clawed frog, Xenopus laevis endures whole body dehydration which can increase its reliance on anaerobic glycolysis for energy production. This makes the regulation of the terminal enzyme of glycolysis, lactate dehydrogenase (LDH), crucial to stress survival. We investigated the enzymatic properties and posttranslational modification state of purified LDH from the skeletal muscle of control and dehydrated (30% total body water loss) X. laevis. LDH from the muscle of dehydrated frogs showed a 93% reduction in phosphorylation on threonine residues and an 80% reduction of protein nitrosylation. LDH from dehydrated muscle also showed a 74% lower V in the pyruvate oxidizing direction and a 78% decrease in V in the lactate reducing direction along with a 33% lower K for pyruvate and a 40% higher K for lactate. In the presence of higher levels of urea and molecular crowding by polyethylene glycol, used to mimic conditions in the cells of dehydrated animals, the K values of control and dehydrated LDH demonstrated opposite responses. In the pyruvate oxidizing direction, control muscle LDH was unaffected by these additives, whereas the affinity for pyruvate dropped further for LDH from dehydrated muscle. The opposite effect was more pronounced in the lactate reducing direction as control LDH showed an increased affinity for lactate, whereas LDH from dehydrated animals showed a further reduction in affinity. The physiological consequences of dehydration-induced LDH regulation appear to poise the enzyme towards lactate production when urea levels are high and lactate catabolism when urea levels are low, perhaps helping to maintain glycolysis under dehydrating conditions whilst providing for the ability to recycle lactate upon rehydration.
非洲爪蟾（Xenopus laevis）可以忍受全身脱水，这会增加其对无氧糖酵解产生能量的依赖。这使得糖酵解的终末酶，乳酸脱氢酶（LDH）的调节对压力生存至关重要。我们研究了来自对照和脱水（总身体水分损失 30%）的非洲爪蟾骨骼肌中的纯化 LDH 的酶学特性和翻译后修饰状态。脱水青蛙肌肉中的 LDH 上的苏氨酸残基磷酸化减少了 93%，蛋白质硝化减少了 80%。脱水肌肉中的 LDH 还表现出丙酮酸氧化方向的 V 降低了 74%，乳酸还原方向的 V 降低了 78%，同时丙酮酸的 K 值降低了 33%，乳酸的 K 值升高了 40%。在存在更高水平的尿素和由聚乙二醇引起的分子拥挤的情况下，这些添加剂用于模拟脱水动物细胞中的条件，对照和脱水 LDH 的 K 值表现出相反的反应。在丙酮酸氧化方向上，对照肌肉 LDH 不受这些添加剂的影响，而脱水肌肉 LDH 对丙酮酸的亲和力进一步降低。在乳酸还原方向上，这种相反的效果更为明显，因为对照 LDH 对乳酸的亲和力增加，而脱水动物的 LDH 对乳酸的亲和力进一步降低。脱水诱导的 LDH 调节的生理后果似乎使酶在尿素水平高时偏向于乳酸生成，而在尿素水平低时偏向于乳酸分解代谢，这有助于在脱水条件下维持糖酵解，同时为再水合时回收乳酸提供能力。
