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Leucine to Stimulate Protein Synthesis

By: Robert Baird

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Leucine is one of the three branched-chain amino acids, with valine and isoleucine being the other two. To maintain a positive nitrogen balance in adults is essential. However, leucine can be synthesized from the keto acids a-ketoisovalera to, a-isopropylmalate, - isopropylmala to, or a-ketoisocaproate (KIC). Catabolism of the amino acid results in the production of carbon dioxide along the following pathway: a-ketoisocaproate --> isovaleryl-CoA, --> -methylcrotonyl-CoA, --> -methylglutoconyl-CoA, --> -hydroxy--methylglutaryl-CoA, --> acetoacetate or acetyl-CoA. Catabolism of leucine can take place in the liver, kidney, muscle, heart, and adipose tissue. Because of the increase in leucine catabolism that occurs in skeletal muscle during exercise, studies have linked the effects of leucine supplementation on performance. This will review the effects of leucine supplementation on protein synthesis and protein degradation.

Animal Studies

Leucine has been shown to affect amino acid and protein metabolism. With in vitro studies indicating a potential for leucine to stimulate protein synthesis via a variety of mechanisms, scientists have examined the effects of different feeding strategies on leucine metabolism, and the effects of leucine feedings on protein synthesis and nitrogen balance. After a 3-day fast in sheep, leucine arterial concentrations increased, leucine turnover in muscle decreased, hepatic use was unchanged, leucine oxidation increased, and protein synthesis decreased. Fasting promoted skeletal muscle catabolism to provide precursors for hepatic protein synthesis.

As leucine in the diet is progressively increased, the rate of leucine oxidation stays low until the leucine blood concentration exceeds the amount needed for the maximal rate of weight gain. Plasma concentrations of leucine in rats were low when dietary levels of leucine were low and increased with increasing dietary leucine content. When leucine is administered to rats enterally, first-pass effects extract 27%.103 Only 3% of this amount was used by the liver to synthesize new protein, while the rest was believed to represent first-pass use of leucine in intestinal protein synthesis and other metabolic pathways in the splanchnic bed. With leucine enterally administered to dogs, 31.4% was used for protein synthesis, 27.9% was deaminated, 6.0% was oxidized by the splanchnic region, and 4.8% was oxidized by the liver. leucine administered via infusion to lambs raised plasma leucine levels up to 15 times above baseline. Plasma amino acid concentrations were lowered, but protein synthesis rates in skeletal muscles and the whole body did not change.

The addition of glucose or sucrose can change the kinetics of leucine metabolism. Pigs fed glucose had a hindlimb uptake of leucine that was three times greater than pigs fed sucrose. Preprandial protein synthesis was higher in the sucrose group whereas postprandial protein synthesis was higher in the glucosetreated group.

These observations indicate that dietary carbohydrate source can influence both pre- and postprandial aspects of leucine metabolism. Additional work in this area examined the effects of leucine on insulin, using the islet cells from rats with chronic renal failure. In this condition there is impaired insulin secretion. Insulin secretion in response to leucine was significantly decreased in isolated islet cells. Insulin release improved when the rats underwent parathyroidectomy or treatment with verapamil. Verapamil blocks the action of parathyroid hormone (PTH) on the islets. The high levels of PTH induced a secondary state of impaired leucine-induced insulin secretion The defective pathway involves abnormal leucine activation of glutamate dehydrogenase, impaired use of a-ketoglutarate, and a reduction in the maximal reaction rate of glutaminase. The interrelationships of leucine, insulin, and PTH require further study to determine if protein synthesis and insulin production are impacted.

Human Studies

The effects of leucine during fasted conditions have also been studied in humans. Each subject underwent three separate 14-day fasts, 34 mm/day of leucine infused on days 1-7 and a third control fast in which no infusions were given. The daily urinary urea nitrogen (UUN) excretion was similar for the control and leucine treatments, while KIC infusion significantly reduced daily urine urea nitrogen (UUN) excretion. In this study, KIC infusions decreased nitrogen wasting during starvation, whereas leucine, studied under identical conditions, did not. In study on abdominal surgery patients, KIC and leucine were infused at 70 mmol/day. KIC was found to decrease nitrogen wastage, whereas under the same conditions leucine did not.

Mendall et al. administered patients with Duchenne muscular dystrophy (DMD) either a placebo or oral leucine at a dose of 0.2 g/kg/day in a randomized, doubleblind, controlled trial. Although there were some transient improvements over 1 month, after 1 year, leucine failed to produce a therapeutic response. Research on the effects of leucine in cirrhotic patients indicates that it can increase oxidation without affecting protein synthesis in vivo.

In contrast to the above studies, infusions of either leucine or leucine plus glucose were found to decrease muscle proteolysis Both leucine and glucose infusions decreased the aromatic amino acids and the basic amino acids in muscle. Glucose stimulated insulin, which decreased plasma essential amino acid concentrations. This effect was augmented by leucine. lower amino acid levels were also found in track athletes administered 50 mg/kg/day of leucine for 10-weeks in a randomized, double-blind, placebo-controlled, crossover study. leucine concentrations had decreased significantly in the placebo group during the first 5- weeks, but not during the second 5 weeks. Leucine concentrations did not change in the treatment group. The total serum amino acid pool decreased significantly in all the subjects during the 10-week training period. Of all the amino acids, glutamine decreased the most. Net protein synthesis was not measured; therefore, it is difficult to interpret the findings. The fact that leucine and other amino acids were lower during the treatment condition may be indicative of increased protein synthesis or oxidation. Note that these subjects were ingesting 1.26 g/kg/day of protein. The effects of leucine occurred during a metabolic state in which protein intake was greater than the RDA.

Safety and Toxicity

Infusions of large amounts of leucine into rats do not cause an imbalance in plasma amino acids, even under severe catabolic conditions. In addition, oral loading of a 15-month-old girl with propionic acidemia produced no ketoacidosis. Infusions and oral loading of leucine appear to be well tolerated. Toxic cases of leucine are rare in the literature and overload of leucine in young rats produced no significant changes in growth, food consumption, and hematological and immune responsiveness when the basic diet was balanced. Of greater concern is lack of leucine, which can decrease nitrogen balance and decrease antitumor activity.

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