Secondary carnitine deficiency.
For any given tissue the normal carnitine content is that which is necessary for an optimal rate of long-chain fatty acid oxidation. Tissues especially rich in carnitine are liver, muscle and heart. The endogenous rate of carnitine biosynthesis from lysine and methionine is not known to be influenced by fluctuations in the levels of the parent amino acids, as exemplified by hypermethioninaemic patients. Inadequate dietary supply of carnitine, leading to a deficiency, may occur in vegetarians and especially in subjects on total parenteral nutrition. Premature babies are especially at risk in this respect, and this has led to the addition of carnitine to solutions for intravenous alimentation. It has been suggested that carnitine plays an important role in the intramitochondrial regulations of coenzyme A homeostasis by expelling short-chain and medium-chain acyl groups from the mitochondrion in the form of acylcarnitines. These esters are preferentially excreted into the urine and thus result in a depletion of the body's carnitine stores. Important conditions in this respect are the inherited organic acidurias and disorders of fatty acid oxidation. Urinary acylcarnitines can be identified by indirect gas chromatographic or direct mass spectrometric methods. Patients on haemodialysis treatment will lose carnitine in the dialysis fluid, whereas excessive urinary losses of free and acetylated carnitine occur in the Fanconi syndrome. Secondary carnitine deficiency may be accompanied by a moderate degree of muscular dysfunction. Reassuringly, however, no signs of hepatic or cardiac involvement, as often seen in primary carnitine deficiency, have been observed.