Carnitine is an amino acid derivative and nutrient involved in lipid (fat) metabolism in mammals and other eukaryotes. It is in the chemical compound classes of β-hydroxyacids and quaternary ammonium compounds, and because of the hydroxyl-substituent, it exists in two stereoisomers, the biologically active enantiomer L-carnitine, and the essentially biologically inactive D-carnitine.[1][better source needed] Both are available through chemical synthesis,[not verified in body] and the L-form is continuously biosynthesized in eukaryotic organisms from the proteinogenic amino acids lysine and methionine.[2] In such eukaryotic cells, it is specifically required for the transport of fatty acids from the intermembraneous space in the mitochondria into the mitochondrial matrix during the catabolism of lipids, in the generation of metabolic energy.[1][better source needed] Carnitine was originally found as a growth factor for mealworms and labeled vitamin BT, although carnitine is not by biochemical definition a true vitamin.[3][4] It is used efficaciously, clinically, in the treatment of some conditions, e.g. systemic primary carnitine deficiency,[5] and it is available over the counter as a nutritional supplement, though its efficacy for most conditions for which it is advertised is controversial or not yet established.