Carnitine palmitoyltransferase activity is present and high in the muscle and liver of lampreys (Agnatha)

The unusual energy metabolism of elasmobranch fishes

The unusual energy metabolism of elasmobranchs is characterized by limited or absent fatty acid oxidation in cardiac and skeletal muscle and a great reliance on ketone bodies and amino acids as oxidative fuels in these tissues. Other extrahepatic tissues in elasmobranchs rely on ketone bodies and amino acids for aerobic energy production but, unlike muscle, also appear to possess a significant capacity to oxidize fatty acids. This organization of energy metabolism is reflected by relatively low plasma levels of non-esterified fatty acids (NEFA) and by plasma levels of the ketone body ss-hydroxybutyrate that are as high as those seen in fasted mammals. The preference for ketone body oxidation rather than fatty acid oxidation in muscle of elasmobranchs under routine conditions is opposite to the situation in teleosts and mammals. Carbohydrates appear to be utilized as a fuel source in elasmobranchs, similar to other vertebrates. Amino acid- and lipid-fueled ketogenesis in the liver, the lipid storage site in elasmobranchs, sustains the demand for ketone bodies as oxidative fuels. The liver also appears to export NEFA and serves a buoyancy role. The regulation of energy metabolism in elasmobranchs and the effects of environmental factors remain poorly understood. The metabolic organization of elasmobranchs was likely present in the common ancestor of the Chondrichthyes ca. 400million years ago and, speculatively, it may reflect the ancestral metabolism of jawed vertebrates. We assess hypotheses for the evolution of the unusual energy metabolism of elasmobranchs and propose that the need to synthesize urea has influenced the utilization of ketone bodies and amino acids as oxidative fuels.

Metabolic organization of the spotted ratfish, Hydrolagus colliei (Holocephali: Chimaeriformes): insight into the evolution of energy metabolism in the chondrichthyan fishes

The metabolic organization of a holocephalan, the spotted ratfish (Hydrolagus colliei), was assessed using measurements of key enzymes of several metabolic pathways in four tissues and plasma concentrations of free amino acids (FAA) and non-esterified fatty acids (NEFA) to ascertain if the Holocephali differ metabolically from the Elasmobranchii since these groups diverged ca. 400 Mya. Activities of carnitine palmitoyl transferase indicate that fatty acid oxidation occurs in liver and kidney but not in heart or white muscle. This result mirrors the well-established absence of lipid oxidation in elasmobranch muscle, and more recent studies showing that elasmobranch kidney possesses a capacity for lipid oxidation. High activities in oxidative tissues of enzymes of ketone body metabolism, including D-beta-hydroxybutyrate dehydrogenase, indicate that, like elasmobranchs, ketone bodies are of central importance in spotted ratfish. Like many carnivorous fishes, enzyme activities demonstrate that amino acids are metabolically important, although the concentration of plasma FAA was relatively low. NEFA concentrations are lower than in teleosts, but higher than in most elasmobranchs and similar to that in some "primitive" ray-finned fishes. NEFA composition is comparable to other marine temperate fishes, including high levels of n-6 and especially n-3 polyunsaturated fatty acids. The metabolic organization of the spotted ratfish is similar to that of elasmobranchs: a reduced capacity for lipid oxidation in muscle, lower plasma NEFA levels, and an emphasis on ketone bodies as oxidative fuel. This metabolic strategy was likely present in the common chondrichthyan ancestor, and may be similar to the ancestral metabolic state of fishes.

The metabolic organization of a primitive air-breathing fish, the Florida gar (lepisosteus platyrhincus)

The metabolic organization of the air-breathing Florida gar, Lepisosteus platyrhincus, was assessed by measuring the maximal activities of key enzymes in several metabolic pathways in selected tissues, concentrations of plasma metabolites including nonesterified fatty acids (NEFA), free amino acids (FAA) and glucose as well as tissue FAA levels. In general, L. platyrhincus has an enhanced capacity for carbohydrate metabolism as indicated by elevated plasma glucose levels and high activities of gluconeogenic and glycolytic enzymes. Based upon these properties, glucose appears to function as the major fuel source in the Florida gar. The capacity for lipid metabolism in L. platyrhincus appears limited as plasma NEFA levels and the activities of enzymes involved in lipid oxidation are low relative to many other fish species. L. platyrhincus is capable of oxidizing both D- and L-beta-hydroxybutyrate, with tissue-specific preferences for each stereoisomer, yet the capacity for ketone body metabolism is low compared with other primitive fishes. Based on enzyme activities, the metabolism of the air-breathing organ more closely resembles that of the mammalian lung than a fish swim bladder. The Florida gar sits phylogenetically and metabolically in an intermediate position between the "primitive" elasmobranchs and the "advanced" teleosts. The apparently unique metabolic organization of the gar may have evolved in the context of a bimodal air-breathing environmental adaptation.

Acetyl-CoA carboxylase 2 mutant mice are protected against obesity and diabetes induced by high-fat/high-carbohydrate diets

Malonyl-CoA, generated by acetyl-CoA carboxylases ACC1 and ACC2, is a key metabolite in the control of fatty acid synthesis and oxidation in response to dietary changes. ACC2 is associated to the mitochondria, and Acc2-/- mice have a normal lifespan and higher fatty acid oxidation rate and accumulate less fat. Mutant mice fed high-fat/high-carbohydrate diets weighed less than their WT cohorts, accumulated less fat, and maintained normal levels of insulin and glucose, whereas the WT mice became type-2 diabetic with hyperglycemic and hyperinsulinemic status. Fatty acid oxidation rates in the soleus muscle and in hepatocytes of Acc2-/- mice were significantly higher than those of WT cohorts and were not affected by the addition of insulin. mRNA levels of uncoupling proteins (UCPs) were significantly higher in adipose, heart (UCP2), and muscle (UCP3) tissues of mutant mice compared with those of the WT. The increase in the UCP levels along with increased fatty acid oxidation may play an essential role in the regulation of energy expenditure. Lowering intracellular fatty acid accumulation in the mutant relative to that of the WT mice may thus impact glucose transport by higher GLUT4 activity and insulin sensitivity. These results suggest that ACC2 plays an essential role in controlling fatty acid oxidation and is a potential target in therapy against obesity and related diseases.

Metabolic organization of liver and somatic muscle of landlocked sea lamprey,Petromyzon marinus, during the spawning migration

The metabolic organization of liver and muscle of the landlocked sea lamprey, Petromyzon marinus, during the spring spawning migration was assessed by measuring activities of key enzymes for several metabolic pathways, the oxidative properties of mitochondria, and plasma concentrations of nonesterified fatty acids (NEFAs) and free amino acids. These determinations indicate that several metabolic sources are used to fuel the energy requirements of muscle. Lamprey muscle has a high capacity to oxidize lipids but the plasma NEFAs are lower than those reported for other species. Of the NEFAs measured in plasma, 18:0 was prominent, accounting for 23% of the total NEFA content of the plasma. High plasma concentrations of ketogenic amino acids and high levels of tissue ketogenic/ketolytic enzyme activities indicate that ketone bodies may also be a major fuel source for migrating sea lampreys. Based on mitochondrial oxidation and enzyme measurements, glutamine catabolism in somatic muscle of lampreys is less important than in other fish red muscle.