Transport of Long-Chain Fatty Acids Into the Mitochondrial Matrix

Summary

The name carnitine is derived from the Latin "carnus" or flesh, as the compound was first isolated from meat. Carnitine is termed a conditionally essential nutrient, as under certain conditions its requirements may exceed the individual's capacity to synthesize it. Carnitine mediates the transport of medium/long-chain fatty acids across mitochondrial membranes, facilitating their oxidation with subsequent energy production; in addition, it facilitates the transport of intermediate toxic compounds out of the mitochondria preventing their accumulation. Because of these key functions, carnitine is concentrated in tissues that utilize fatty acids as their primary dietary fuel, such as skeletal and cardiac (heart) muscle. Dietary sources of carnitine include foods of animal origin, such as meat and dairy products. In general, healthy adults do not require dietary carnitine as carnitine stores are replenished through endogenous synthesis from lysine and methionine in the liver and kidneys. Excess carnitine is excreted via the kidneys. In the US, carnitine is an approved prescription drug for the treatment of primary systemic carnitine deficiency and secondary carnitine deficiency syndromes. Carnitine is also available over-the-counter as a dietary supplement, as an aid to weight loss, to improve exercise performance, and to enhance a sense of well-being.
Carnitine is the generic term for a number of compounds that include L-carnitine, L-acetylcarnitine, acetyl-L-carnitine, and L-propionyl carnitine. The only forms available over-the-counter in the US are L-carnitine and acetyl-L-carnitine. L-carnitine is the biological active form. The D-isomer, which is not biologically active, can compete with the L-isomer potentially increasing the risk of L-carnitine deficiency. Proprionyl-L-carnitine is approved for use in Europe but not in the US.
Carnitine is studied extensively in part because of the important role it plays in fatty acid oxidation and energy production, and because it is a well-tolerated and generally safe therapeutic agent. It is proven treatment in children who have recessive defects in the carnitine transporter system and in individuals treated with pivalate containing antibiotics. Other benefits attributed to carnitine result from the management of secondary carnitine deficiencies. These benefits are supported by preliminary findings and need to be confirmed through well-controlled randomized trials. While there is agreement on carnitine's role as a prescription product for the treatment of primary carnitine deficiencies, its benefits as a dietary supplement in individuals who are carnitine sufficient is debated.

L-CARNITINE

L-Carnitine is a derivative of the amino acid, lysine. Its name is derived from the fact that it was first isolated from meat (carnus) in 1905. Because L-carnitine appeared to act as a vitamin in the mealworm (Tenebrio molitor), it was called vitamin BT. Vitamin BT turned out to be a misnomer when scientists discovered that humans and other higher organisms synthesize L-carnitine. Under certain conditions, the demand for L-carnitine may exceed an individual's capacity to synthesize it, making it a conditionally essential nutrient (1).

FUNCTION

L-Carnitine is synthesized primarily in the liver and also in the kidneys, and must be transported to other tissues. It is most concentrated in tissues that use fatty acids as their primary dietary fuel, such as skeletal and cardiac (heart) muscle. In this regard, L-carnitine plays an important role in energy production by chaperoning activated fatty acids (acyl-CoA) into the mitochondrial matrix for metabolism and chaperoning intermediate compounds out of the mitochondrial matrix to prevent their accumulation.

Transport of long-chain fatty acids into the mitochondrial matrix

The transport of long-chain fatty acids by L-carnitine into the mitochondrial matrix where they can be metabolized to generate energy requires three enzymes located on the mitochondrial outer and inner membranes (diagram). On the outer mitochondrial membrane of skeletal and cardiac muscle cells, carnitine-palmitoyl transferase I (CPTI) catalyzes the formation of acylcarnitine (a fatty acid + L-carnitine) from acyl-CoA (a fatty acid + coenzyme A). A transporter protein called carnitine:acylcarnitine translocase (CT) transports acylcarnitine across the inner mitochondrial membrane. Carnitine-palmitoyl transferase II (CPTII) is associated with the inner mitochondrial membrane and catalyzes the formation of acyl-CoA within the mitochondrial matrix where it can be metabolized through a process called beta-oxidation, ultimately yielding propionyl-CoA and acetyl-CoA (2, 3).

Regulation of energy metabolism through the modulation of acetyl CoA:CoA ratios

The enzyme, pyruvate dehydrogenase (PDH), catalyzes the conversion of pyruvate to acetyl CoA, a pivotal reaction in glucose metabolism. In the mitochondrial matrix, decreased free CoA, relative to acetyl CoA, inhibits the activity of PDH. Carnitine acetyl-transferase (CAT) catalyzes the transfer of the acetyl group from acetyl CoA to L-carnitine, freeing CoA to participate in the PDH reaction (diagram). Acetyl-L-carnitine can be exported from the mitochondria through the activity of CT (2, 3).

Removal of short- and medium-chain fatty acids from the mitochondria

Within the mitochondrial matrix, short- and medium-chain fatty acids can be transferred from CoA to L-carnitine, allowing short and medium-chain acyl-carnitines to be exported from the mitochondria. This process provides free CoA needed for energy metabolism, as well as a mechanism to export excess acetyl and acyl groups from the mitochondria. This mechanism may also play a role in the depletion of L-carnitine during the metabolism of certain drugs (see Drug Interactions) (2).

DEFICIENCY

Primary carnitine deficiencies

The primary carnitine deficiencies, systemic carnitine deficiency and myopathic carnitine deficiency, are relatively rare hereditary disorders.

Systemic carnitine deficiency: Primary systemic carnitine deficiency is a genetic disorder that is usually detected in infancy or early childhood. It is characterized by low serum L-carnitine levels, and if untreated may result in life-threatening damage to the liver, heart, or brain. Also known as carnitine carrier deficiency, the underlying cause is a mutation in the gene coding for the protein that transports L-carnitine into cells. As a result of this defect, intestinal absorption of dietary L-carnitine is poor and reabsorption by the kidney is impaired, resulting in increased urinary loss of L-carnitine (1).

Myopathic carnitine deficiency: Primary myopathic carnitine deficiency is also a genetic disorder in which carnitine deficiency is limited to skeletal and cardiac muscle. Serum L-carnitine levels are generally normal. The symptoms of myopathic carnitine deficiency include muscle pain and progressive muscle weakness. Symptoms may begin in childhood or adulthood. The myopathic form of primary carnitine deficiency is generally less severe than the systemic form (1).

Secondary carnitine deficiencies

Secondary carnitine deficiencies may be hereditary or acquired. In all cases, they are characterized by decreased availability of free L-carnitine. In such cases, total L-carnitine levels may be normal, but free L-carnitine levels are decreased.

Hereditary causes: Hereditary causes of secondary carnitine deficiency include genetic defects in amino acid degradation (e.g., propionic aciduria) and lipid metabolism (e.g., medium chain acyl-CoA dehydrogenase deficiency).

Increased L-carnitine loss: Hemodialysis (see Disease Treatment), Fanconi syndrome, and the metabolism of some medications (see Drug Interactions) may result in substantial L-carnitine loss, resulting in L-carnitine deficiency (1).

Insufficient L-carnitine synthesis: Malabsorption syndromes and diets that chronically lack L-carnitine and its precursors (see Nutrient interactions) may increase the risk of secondary carnitine deficiency. Premature infants may be at risk of secondary L-carnitine deficiency when fed soy-based formulas without added L-carnitine. Therefore, it is recommended that non-milk based infant formulas be fortified with the amount of L-carnitine normally found in human milk (11 mg/liter). Although dietary L-carnitine comes mainly from animal sources, even strict vegetarians can generally synthesize enough L-carnitine to prevent deficiency (3).

Nutrient interactions

The synthesis of L-carnitine is catalyzed by the concerted action of five different enzymes. This process requires two essential amino acids (lysine and methionine), iron (Fe2+), vitamin C, vitamin B6, and niacin in the form of nicotinamide adenine dinucleotide (NAD) (1). One of the earliest symptoms of vitamin C deficiency is fatigue, thought to be related to decreased synthesis of L-carnitine (4).

SOURCES

Biosynthesis

The normal rate of L-carnitine biosynthesis in humans ranges from 0.16 to 0.48 mg/kg of body weight/day (1). Thus, a 70 kg (154 1b) person would synthesize from 11 to 34 mg/day. This rate of synthesis combined with the reabsorption of about 95% of the L-carnitine filtered by the kidneys is enough to prevent deficiency in generally healthy people, including strict vegetarians (3).

Food sources

Meat, poultry, fish, and dairy products are the richest sources of L-carnitine. Tempeh (fermented soybeans), wheat, and avocados contain some L-carnitine, while fruits, vegetables, and grains contain relatively little L-carnitine to the diet. Omnivorous diets have been found to provide 20 to 200 mg/day of L-carnitine for a 70 kg person, while strict vegetarian diets may provide as little as 1 mg/day for a 70 kg person. Between 63% and 75% of L-carnitine from food is absorbed, compared to 15%-20% from oral supplements (3, 5). Non-milk based infant formulas (e.g., soy formulas) should be fortified so that they contain 11 mg/liter. Some carnitine-rich foods and their carnitine content in milligrams (mg) are listed in the table below (3).

*A 3-ounce serving of meat is about the size of a deck of cards.

Supplements

Intravenous L-carnitine is available by prescription only for the treatment of primary and secondary L-carnitine deficiencies.

Oral L-carnitine is available by prescription for the treatment of primary and secondary L-carnitine deficiencies. It is also available without a prescription as a nutritional supplement. Supplemental doses usually range from 500 mg to 2,000 mg/day.

Acetyl-L-carnitine is available without a prescription as a nutritional supplement. In addition to providing L-carnitine, it provides acetyl groups, which may be used in the formation of the neurotransmitter, acetylcholine. Supplemental doses usually range from 500 mg to 2,000 mg/day (6).

Propionyl-L-carnitine is available in Europe, but not the U.S. It provides L-carnitine as well as propionate, which may be utilized as an intermediate during energy metabolism (5).

See a diagram of the chemical structures of L-carnitine, acetyl-L-carnitine, and propionyl-L-carnitine.

DISEASE PREVENTION

Aging

Age-related declines in mitochondrial function and increases in mitochondrial oxidant production are thought to be important contributors to the adverse affects of aging. Tissue L-carnitine levels have been found to decline with age in humans and animals (7). Feeding aged rats acetyl-L-carnitine (ALCAR) reversed age-related declines in tissue L-carnitine levels and reversed a number of age-related changes in mitochondrial function, but high doses of ALCAR increased liver mitochondrial oxidant production (8). More recently, a series of studies in aged rats found that supplementation with either ALCAR or alpha-lipoic acid, a mitochondrial cofactor and antioxidant, improved mitochondrial energy metabolism, decreased oxidative stress, and improved memory (9, 10). Interestingly, supplementation with the combination of ALCAR and alpha-lipoic acid resulted in significantly greater improvement than either compound alone. While these findings are very exciting, the researchers involved caution that these studies used relatively high doses of the compounds and only for a short time (one month). It is not yet known whether taking relatively high doses of these two naturally occurring substances will benefit rats in the long-term or will have similar effects in humans. Clinical trials in humans are planned, but it will be several years before the results are available.

For more information about aging and oxidative stress, see the article, Aging with Dr. Tory Hagen, in the Fall/Winter 2000 Linus Pauling Institute Newsletter.

DISEASE TREATMENT

Conditions related to myocardial ischemia (insufficient blood supply to the heart muscle)

In the studies discussed below it is important to note that treatment with L-carnitine or propionyl-L-carnitine was used as an adjunct (in addition) to appropriate medical therapy not in place of it.

Myocardial infarction (heart attack):Myocardial infarction (MI) occurs when atherosclerotic plaque in a coronary artery ruptures. The resultant blood clot can obstruct the blood supply to the heart muscle, causing in injury or damage to the heart. L-Carnitine treatment has been found to reduce injury to heart muscle resulting from ischemia in several animal models (11). In humans, the administration of L-carnitine immediately after the diagnosis of MI has resulted in improved clinical outcomes in several small clinical trials. In one trial, half of 160 men and women diagnosed with a recent MI were randomly assigned to receive 4 grams/day of L-carnitine in addition to standard pharmacological treatment. After one year of treatment, mortality was significantly lower in the L-carnitine supplemented group (1.2% vs. 12.5%), and attacks of angina were less frequent (12). Not all clinical trials have found L-carnitine supplementation to be beneficial after MI. In a randomized, double blind, placebo controlled trial, 60 men and women diagnosed with an acute MI were treated with either intravenous L-carnitine (6 grams/day) for 7 days followed by oral L-carnitine (3 grams/day) for 3 months or placebo (13). After 3 months, mortality did not differ between the 2 groups, nor did echocardiographic measures of cardiac function. In a larger placebo-controlled trial, 472 patients treated in an intensive care unit within 24 hours of having their first MI were randomly assigned to intravenous L-carnitine therapy (9 grams/day) for 5 days followed by oral L-carnitine (6 grams/day) for 12 months or a placebo in addition to standard medical therapy (14, 15). Although there were no significant differences in mortality or the incidence of congestive heart failure (CHF), left ventricular volumes were significantly lower in the L-carnitine treated group at the end of one year, suggesting that carnitine therapy may limit adverse effects of acute MI on the heart muscle. Based on these findings, a randomized placebo-controlled trial in 4,000 patients with acute MI is planned to determine the effect of L-carnitine therapy on the incidence of heart failure 6 months after MI (15).

Heart Failure: Impairment of the heart's ability to pump enough blood for all of the body's needs is known as heart failure. In coronary artery disease, the accumulation of atherosclerotic plaque in the coronary arteries may prevent parts of the heart muscle from getting adequate circulation, ultimately resulting in damage and impaired pumping ability. Myocardial infarction (MI) may also damage the heart muscle, resulting in the development of heart failure. Because physical exercise increases the demand on the weakened heart, measures of exercise tolerance are frequently used to monitor the severity of heart failure. Echocardiography is also used to determine the left ventricular ejection fraction (LVEF), an objective measure of the heart's pumping ability. An LVEF of less than 40% is indicative of systolic heart failure (16).

The addition of L-carnitine to standard medical therapy for heart failure has been evaluated in several clinical trials. In a randomized, single-blind, placebo-controlled trial in 30 heart failure patients, oral administration of 1.5 grams/day of propionyl-L-carnitine for 1 month resulted in significantly improved measures of exercise tolerance and a slight but significant decrease in left ventricular size compared to placebo (17). A larger randomized, double blind, placebo-controlled trial compared the addition of propionyl-L-carnitine (1.5 grams/day) to the treatment regimen of 271 heart failure patients to a placebo in 266 patients for 6 months (18). Overall, exercise tolerance was not different between the two groups. However, in those with higher LVEF values (greater than 30%), exercise tolerance was significantly improved in those taking propionyl-L-carnitine compared to placebo, suggesting that propionyl-L-carnitine may help to improve exercise tolerance in higher functioning heart failure patients..

Angina pectoris:Angina pectoris is chest pain that occurs when the coronary blood supply is insufficient to meet the metabolic needs of the heart muscle (ischemia). The addition of L-carnitine or propionyl-L-carnitine to pharmacologic therapy for chronic stable angina has been found to modestly improve exercise tolerance and decrease electrocardiographic signs of ischemia during exercise testing in a limited number of angina patients. In a randomized, placebo-controlled crossover trial in 44 men with chronic stable angina, 2 grams/day of L-carnitine for 4 weeks significantly increased the exercise workload tolerated prior to the onset of angina and decreased ST segment depression (electrocardiographic evidence of ischemia) during exercise compared to placebo (19). In a more recent randomized placebo-controlled trial in 47 men and women with chronic stable angina, the addition of 2 grams/day of L-carnitine for 3 months significantly improved exercise duration and decreased the time required for exercise-induced ST segment changes to return to baseline compared to placebo (20).

Intermittent claudication in peripheral arterial disease

In peripheral arterial disease, atherosclerosis of the arteries supplying the lower extremities may diminish blood flow to the point that it is insufficient to supply the metabolic needs of exercising muscles, leading to ischemic leg or hip pain known as claudication (21). In a randomized placebo-controlled study of 495 patients with intermittent claudication, 2 grams/day of propionyl-L-carnitine for 12 months significantly increased maximal walking distance and the distance walked prior to the onset of claudication in patients whose initial maximal walking distance was less than 250 meters (22). However, no significant response to propionyl-L-carnitine treatment was observed in more mildly affected patients whose initial maximal walking distance was greater than 250 meters. In a double blind, randomized, placebo-controlled trial of 155 patients with disabling claudication in the U.S. and Russia, 2 grams/day of propionyl-L-carnitine for 6 months significantly improved walking distance and claudication onset time compared to placebo. One study compared the efficacy of L-carnitine and propionyl-L-carnitine administered intravenously for the treatment of intermittent claudication, and concluded that propionyl-L-carnitine was more effective than L-carnitine when the same amount of carnitine was provided (23).