Enzymes of energy metabolism and gluconeogenesis in the Pacific blue marlin, Makaira nigricans

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Abstract
Enzymes of energy metabolism were measured in marlin red and white muscle and heart to determine the relative capacities for various pathways. Red muscle and heart are highly aerobic tissues with the capacity to oxidize carbohydrate, fat, and amino acids. White muscle possesses an extremely high capacity for anaerobic glycolysis as well as a capacity for carbohydrate oxidation which is unusually high by teleost white muscle standards. The great similarity in metabolic potential between tuna and marlin muscles suggests a similar capacity for metabolic heat production. However, tuna muscles are "hot" while marlin muscles are "cold." The implications of this are discussed in the context of energy metabolism and endothermy in the scombroid fish. Levels of gluconeogenic enzymes in liver and kidney as well as enzymes possibly involved in glycogenesis in white muscle suggest that the major substrate for glycogenesis in this tissue is endogenous lactate and that the major route for conversion of pyruvate to phosphoenolpyruvate is reversal of the pyruvate kinase reaction.
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