Sites and regulation of carnitine biosynthesis in mammals.

Although the pathway of carnitine biosynthesis in mammals is known, the location of active synthesis of carnitine and regulation of the pathway have not been clearly defined. Studies in several laboratories have shown that the enzymes that collectively convert epsilon-N-trimethyllysine (epsilon-N-TML) to gamma-butyrobetaine are found in all tissues studied in rats and humans, but distribution of the final enzyme of the pathway, gamma-butyrobetaine, 2-oxoglutarate dioxygenase (gamma-butyrobetaine hydroxylase) is variable from one species to another. Evidence from studies in rats and humans indicates that uptake and metabolism of epsilon-N-TML by the kidney is necessary for carnitine biosynthesis from circulating epsilon-N-TML. Limited data now available suggest that some of the intracellularly derived epsilon-N-TML is metabolized to gamma-butyrobetaine and carnitine in the tissue of origin, and some is released into the circulation. epsilon-N-TML in mammals is apparently derived from lysine residues in proteins, which are methylated and later released by protein hydrolysis. This source probably provides sufficient substrate for carnitine biosynthesis. Carnitine biosynthesis from epsilon-N-TML is not regulated by end-product feedback mechanisms. Hepatic gamma-butyrobetaine hydroxylase activity in rats and humans is developmentally regulated, and is increased by dietary L-thyroxine in adult rats. No other mechanisms for regulation of carnitine biosynthesis have been identified.