Carnitine (earlier known as vitamin Bt) is a quaternary amine, beta-hydroxy-gamma-trimethylaminobutyrate. It is a very hygroscopic compound and is found in biological samples both as the free carnitine and as the ester of a wide variety of acyl compounds. Of the two types of carnitine, L-and D-carnitine, only L-carnitine is biologically active. This has been shown in a number of species (Grandjean et al., 1993).
Carnitine is synthesized in the liver and kidneys. The synthesis depends on two precursors, L-lysine and methionine, as well as ascorbic acid, nicotinamide, vitamin B6 and iron (Borum, 1991). Deficiency in any cofactor will cause L-carnitine deficiency. In rats, total acid-soluble carnitine and free carnitine in plasma and tissues were reduced in a vitamin B6 deficiency but increased when vitamin B6 was provided in a repletion diet (Cho and LeKlem, 1990; Ha et al., 1994). It has been suggested that early features of scurvy (fatigue and weakness) may be attributed to carnitine deficiency. Vitamin C is a cofactor for two alpha-ketoglutarate-requiring dioxygenase reactions (epsilon-N-trimethyllysine hydroxylase and gamma-butyrobetaine hydroxylase) in the pathway of carnitine biosynthesis. Carnitine concentrations are variably low in some tissues of vitamin C-deficient guinea pigs (Rebouche, 1991). Choline has also been shown to affect carnitine homeostasis in humans and guinea pigs (Daily and Sachan, 1995). Choline supplementation resulted in decreased urinary excretion of carnitine in young adult women; choline resulted in the conservation of carnitine in guinea pigs.
Carnitine appears to be absorbed across the gut by an active process dependent on Na+ as well as by a passive diffusion that may be important for the absorption of large doses of the factor. The uptake of carnitine from the intestinal lumen into the mucosa is rapid, and about one-half of the carnitine taken up is acetylated in that tissue. Tissues such as cardiac muscle and skeletal muscle require carnitine for normal fuel metabolism but cannot synthesize carnitine and are totally dependent on the transport of carnitine from other tissues. Free carnitine is excreted in urine, with the principal excretory product being trimethylamine oxide (Mitchell, 1978). Carnitine is highly conserved by the human kidney, which reabsorbs more than 90% of filtered carnitine, thus, playing an important role in the regulation of carnitine concentration in blood. For the dog, 95% to 98% of the carnitine body pool is in skeletal muscle and the heart (Rebouche and Engel, 1983). With rats, Flores et al. (1996) recently reported that the small intestine is a considerable and previously unrecognized proportion of the carnitine pool of suckling animals.