左旋肉毒碱的代谢与营养效应

Metabolic and nutritional effects of L-carnitine

Carnitine is a water solule quaternary ammonium compound,which isa natural constituent of higher organisms,in particular of cells of animal origin.In humans,carnitine is synthesized in liver,brain and kidney starting from protein-bound lysine and methionine.Skeletal and heart muscle cannot synthesize carnitine.Therefore,these tissues are entirely dependent on carnitine uptake from the blood.In tissues and in physiological fluids carnitine is present in a free and an esterified form.The proportion of esterified carnitine may vary considerably with nutritional conditions,exercise and disease states.Tissue carnitine content depends on many factors: dietary carnitine,lysine,methionine and co-factor intake,carnitine synthesis (in uremia carnitine synthesis in the kidney is obviously reduced or absent),carnitine transport inside and outside tissues,and carnitine excretion.The transport of long-chain fatty acid esters to sites of beta-oxidation in the mitochondrial matrix requires L-carnitine.Besides,carnitine acts as a sink and allows a shift of the acyl pressure from the mitochondria to the cytoplasm.It has been suggested that carnitine is also important for the transport of the acyl groups (metabolic energy)from one cell to another cell and into the appropriate cellular compartment.Tissue carnitine content is much higher htan tissue CoA content and so acylcarnitines may also serve as storage for metabolic energy.By modulating the tissue content of acyl-CoA compounds which inhibit many enzyme activities (e.g.pyruvate dehydrogenase activity),carnitine may regulate many metabolic pathways.Carnitine system is located in the crossroads of intermediate metabolism and carnitine deficiency and supplementation may affect lipid,glucose and protein metabolism (and eventually nutrition) not only in primary,but also in secondary carnitine deficiency.Some positive effects of carnitine supplementation have been reported in experimental studies,in newborns,in patients treated with artificial nutrition (e.g. in acutely ill patients,in which carnitine excretion may be elevated),and in several disease states.It may be difficult to identify carnitine depleted patients which could benefit from carnitine Suplementation,because serum carnitine levels may be unrelated to tissue carnitine content.Therefore,a trial of L-carnitine may be considered,when insufficient intake or increased requirements are suspected.