Gluconeogenesis in liver and kidney of common murre (Uria aalge).

Phosphoenolpyruvate carboxykinase (PEPCK) in murre liver occurs in both cytoplasmic and mitochondrial forms. During a 3-day fast, hepatic PEPCK increases from 9.1 U/g with 19% cytosolic to 12.2 U/g with 35% cytosolic. The increase in activity is due almost entirely to increased cytosolic activity. PEPCK in murre kidney was present only in the mitochondrial compartment. Gluconeogenesis in vitro was determined in both hepatocytes and kidney tubules isolated from 3-day-fasted murres. In hepatocytes, lactate was the best substrate, but both pyruvate and alanine were good gluconeogenic substrates. This observation is consistent with the existence of a cytosolic form of PEPCK. In the kidney, glycerol was the best substrate but was only slightly better than lactate. Alanine and pyruvate were not as effective as gluconeogenic precursors, presumably because of the lack of cytosolic PEPCK. We propose that the major site of gluconeogenesis from amino acids in the murre is the liver, since this is a much larger organ than the kidney and has a cytosolic form of PEPCK necessary for gluconeogenesis from oxidized substrates.