Lactate dehydrogenase (LDH; E.C. 220.127.116.11) is a crucial enzyme involved in energy metabolism in muscle, facilitating the production of ATP via glycolysis during oxygen deprivation by recycling NAD+. The present study investigated purified LDH from the muscle of 20？h anoxic and normoxic T. s. elegans, and LDH from anoxic muscle showed a significantly lower (47%) for L-lactate and a higher value than the normoxic form. Several lines of evidence indicated that LDH was converted to a low phosphate form under anoxia: (a) stimulation of endogenously present protein phosphatases decreased the of L-lactate of control LDH to anoxic levels, whereas (b) stimulation of kinases increased the of L-lactate of anoxic LDH to normoxic levels, and (c) dot blot analysis shows significantly less serine (78%) and threonine (58%) phosphorylation in anoxic muscle LDH as compared to normoxic LDH. The physiological consequence of anoxia-induced LDH dephosphorylation appears to be an increase in LDH activity to promote the reduction of pyruvate in muscle tissue, converting the glycolytic end product to lactate to maintain a prolonged glycolytic flux under energy-stressed anoxic conditions. 1. Introduction Lactate dehydrogenase (LDH; E.C. 18.104.22.168) is a critical enzyme involved in anaerobic metabolism. LDH catalyzes the following reversible reaction: In this capacity, LDH favors the pyruvate reducing direction in skeletal muscle tissue, converting the glycolytic end product to lactate and regenerating the NAD+ pools to maintain a prolonged glycolytic flux . This process is especially critical to those organisms that enter periodically into hypoxic/anoxic environments, where maintaining NAD+/NADH balance is essential for ATP production. Under low oxygen insult, organisms often rely solely on the glycolytic pathway to produce ATP. The greatly reduced production of ATP via glycolysis, as compared to that of oxidative phosphorylation, results in difficult challenges for anoxia-tolerant organisms to overcome. Several of these organisms employ alternate anaerobic pathways to increase ATP yield and/or depress their metabolic rate to survive the low oxygen stress . Furthermore, these organisms typically need to safeguard against the accumulation of acidic glycolytic end products such as lactate, which disrupts cellular homeostasis throughout prolonged exposure to anoxia . Freshwater turtles, Trachemys scripta elegans, have demonstrated a remarkable ability to survive submerged in cold water for 4-5 months during the winter to escape freezing air temperatures. While submerged,
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