Tissue Carnitine Homeostasis in Very-Long-Chain Acyl-CoA Dehydrogenase–Deficient Mice

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Abstract
Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is the most common long-chain fatty acid oxidation defect and presents with heterogeneous clinical manifestations. Accumulation of long-chain acylcarnitines and deficiency of free carnitine have often been proposed to play an important role in disease pathogenesis. The VLCAD-deficient mouse exhibits similar clinical and biochemical phenotypes to those observed in humans and, therefore, represents an excellent model to study VLCAD deficiency. We measured carnitine and acylcarnitine profiles in liver, skeletal muscle (SkM), bile, and blood from VLCAD knock-out mice and controls under nonstressed and various stress conditions. Carnitine and acylcarnitines were extracted from body fluids with methanol and from tissues with acetonitrile, respectively, and were analyzed as their butyl esters using electrospray ionization tandem mass spectrometry. Fasting combined with a cold challenge for 8 h significantly induced liver long-chain acylcarnitine and free carnitine production. Acylcarnitines in SkM predominantly accumulated during exercise with a concomitant decrease of free carnitine. Changes in blood free carnitine did not correlate with carnitine homeostasis in liver and SkM. Our results demonstrate different tissue-specific long-chain acylcarnitine profiles in response to various stressors, which may be of importance with respect to the heterogeneous clinical manifestations of VLCAD deficiency in humans. Furthermore, we conclude that carnitine biosynthesis in the liver seems sufficiently active to maintain liver carnitine levels during increased demand. Our data suggest that carnitine supplementation in long-chain β-oxidation defects may not be required, and blood carnitine concentrations do not reflect tissue carnitine homeostasis.