Function of Carnitine in the Fatty Acid Oxidase-deficient Insect Flight Muscle

The metabolic function of carnitine in the fatty acid oxidase-deficient flight muscle of the blowfly, Phormia regina, was investigated. Mitochondria from flies did not oxidize palmitate or hexanoate in the presence of adenosine triphosphate, coenzyme A, carnitine, and serum albumin. Palmitoyl carnitine was oxidized at an extremely slow rate, QO2 of 20. The rate of synthesis of the carnitine ester was much slower and is rate limiting in fatty acid oxidation. Hexanoyl carnitine was apparently not metabolized. Flight muscle of flies, however, has a concentration of carnitine of over 4 µmoles per g, wet weight. These findings suggest that the primary function of carnitine in blowfly flight muscle is not related to utilization of fats. Carnitine increased the rate of oxidative decarboxylation of pyruvate-1-14C to acetyl-CoA. The malonate inhibition of the rate of production of 14CO2 from pyruvate-1-14C, but not from pyruvate-3-14C, was relieved by carnitine. The rate of oxidation of pyruvate-3-14C to 14CO2 was inhibited by carnitine. These effects of carnitine may be explained by the formation of acetyl carnitine from carnitine and acetyl-CoA, derived from pyruvate, catalyzed by an active carnitine acetyl transferase. Approximately 90% of the enzyme was found in the mitochondria of the muscle. Acetyl-CoA, in the presence of carnitine, was not oxidized by mitochondria, although acetyl carnitine was oxidized with a QO2 of over 300. This indicates that blowfly mitochondria are not permeable to acetyl-CoA but are permeable to acetyl carnitine. When both pyruvate and acetyl carnitine were available, pyruvate was used preferentially. The concentration of acetyl carnitine in flight muscle of blowflies increased 4-fold on initiation of flight, paralleling the increase in pyruvate. After about 1 min of flight, the level of acetyl carnitine decreased and attained a steady state at a concentration twice that in the muscle at rest. Functions for carnitine in carbohydrate utilization, resulting from the direct action of carnitine on pyruvate metabolism, are proposed. A comparative study of the metabolism of carnitine in blowflies and bees is described.