Carnitine

Carnitine, also known as L-carnitine (levocarnitine) is a quaternary ammonium compound derived from the amino acid lysine and is responsible for the transport of fatty acids from the cytosol into the mitochondria. It is often sold as a nutritional supplement. Originally found as a growth factor for mealworms and labled Vitamin Bt. Natural carnitine is the L-stereoisomer. It can be synthesised within the body from the amino acids lysine or methionine. Vitamin C (ascorbic acid) is essential to the synthesis of carnitine. It has been speculated that during growth or pregnancy the requirement of carnitine could exceed its natural production.

Role in fatty acid metabolism

Fatty acids must be activated before they can be carried into the mitochondria, where fatty acid oxidation occurs. This process occurs in two steps:

The formula for the above is:
RCOO- + CoA + ATP + H2O → RCO-CoA + AMP + PPi + 2H+
This reaction is reversible and its equilibrium lies near 1. However, pyrophosphate is hydrolized by a pyrophosphatase, which drives the reaction forward, and to completion.

Once activated, the acyl CoA is transported into the mitochondrial matrix. This occurs via a series of similar steps:

  1. Acyl CoA is conjugated to carnitine by carnitine acyltransferase (palmitoyltransferase) I located on the outer mitochondrial membrane
  2. Acyl carnitine is shuttled inside by a translocase
  3. Acyl carnitine is converted to acyl CoA by carnitine acyltransferase (palmitoyltransferase) II located on the inner mitochondrial membrane

It is important to note that carnitine acyltransferase I undergoes allosteric inhibition as a result of malonyl CoA, an intermediate in fatty acid biosynthesis, in order to prevent futile cycling between beta-oxidation and fatty acid synthesis.

Natural sources

The best source of natural carnitine is in red meat and dairy products. Other natural sources of Carnitine include nuts and seeds (e.g pumpkin, sunflower, sesame), legumes or pulses (beans, peas, lentils, peanuts), vegetables (artichokes, asparagus, beet greens, broccoli, brussels sprouts, collard greens, garlic, mustard greens, okra, parsley), fruits (apricots, bananas), cereals (buckwheat, corn, millet, oatmeal, rice bran, rye, whole wheat, wheat bran, wheat germ) and other 'health' foods (bee pollen, brewer's yeast, carob, kale).

PRODUCT