Short Communication Compartmentation of Liver Phosphoenolpyruvate Carboxykinase in the Aquatic Turtle Pseudemys Scripta Elegans: a Reassessment

Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the conversion of oxaloacetate to pyruvate in the first step of gluconeogenesis. Since oxaloacetate is impermeable to the inner mitochondrial membrane, the localisation of PEPCK within the cell plays a major role in defining substrate preferences for gluconeogenesis. As a result, the immunochemically distinct isoenzymes of PEPCK found within the mitochondrial and the cytosolic cell fractions vary in their proportion to one another between organs and species and with prandial state. For animals that routinely undergo periods of prolonged anoxia supported by anaerobic glycolysis (with the accumulation of tissue lactate), the localisation and activity of PEPCK may have an influence on the rate at which lactate is used to replete glycogen on recovery. The red-eared turtle (Pseudemys scripta elegans) is such an animal, with anoxic dives characteristic of this species lasting up to 4 weeks at 3 ̊C (Ultsch, 1985). During 24h of forced anoxic submergence at 22 ̊C, liver lactate accumulates to 45mmol l21 and liver glycogen is depleted by 83% (Penney, 1974). In this species, a previous study reported that 72% of hepatic PEPCK activity was localised in the cytosol (Penney and Kornecki, 1973). This would tend to suggest that gluconeogenesis is not specific in its source of carbon for pyruvate formation. Since previous studies from our laboratory have noted a predominant proportion of liver mitochondrial PEPCK activity in the western painted turtle, a related aquatic turtle species (Chrysemys picta bellii, Buck et al. 1993), we undertook to re-examine the intracellular localisation of PEPCK in the red-eared turtle. Adult red-eared turtles were obtained from a local supplier (Delta Aquatics, BC) and kept in an outdoor tank equipped with a flow-through water supply at 20 ̊C and with a basking platform. Animals were fed chopped beef heart and liver ad libitum and were sampled over a 4 week period in mid summer. After the turtle had been killed by decapitation and the plastron had been removed, a 1 g sample of liver tissue was taken from the right hepatic lobe and placed in 5 volumes of