[Stereopharmacology of carnitine].

L-Carnitine (L-beta-hydroxy-gamma-N,N,N-trimethylaminobutyric acid) plays an essential role in fatty acid transport in the mitochondrion. Conditions that appear to benefit from exogenous supplementation of L-carnitine include anorexia, chronic fatigue, cardiovascular disease, hypoglycemia, male infertility, muscular myopathies, renal failure and dialysis. D-Carnitine is not biologically active and might interfere with the proper utilization of the L isomer, and so there are claims that the racemic mixture (DL-carnitine) should be avoided. Despite the fact that it is known about the systemic manifestations of oral intake of this compound, oral supplementation with DL-carnitine for treatment of primary and secondary carnitine deficiency syndromes has been used in Russia for 25 years. The purpose of the present review was to contrast the differences in pharmacokinetics, phannacodynamics, biochemistry, and toxicity between treatments of L- and DL-carnitine. There is some evidence that L-carnitine and D-carnitine compete for uptake in small intestine and tubular re-absorption in kidneys. After intestinal absorption, L- and D-carnitine is transferred to organs whose metabolism is dependent on fatty acid oxidation, such as heart and skeletal muscle, and D-carnitine competitively depletes muscle level of L-carnitine. Whereas L-carnitine is found to be essential for the oxidation of fatty acids, D-carnitine causes a depletion of L-carnitine, and hindered fatty acid oxidation and energy formation. Pharmacological effects of carnitine are stereospecific, since L-carnitine was effective in various animals and clinical studies, while D- and DL-carnitine was found to be ineffective or toxic, for example, to muscle cells and to the myocardium. DL-Carnitine causes symptoms of myasthenia and cardiac arrhythmias, which disappeared after L-carnitine administration. Clinically toxic effect of D-carnitine was described in patients with renal failure on long-term haemodialysis, in adriamycin (doxorubicin) cardiotoxicity and in stable angina pectoris.