Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain isobutyl group, making it a non-polar aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Human dietary sources are foods that contain protein, such as meats, dairy products, soy products, and beans and other legumes. It is encoded by the codons UUA, UUG, CUU, CUC, CUA, and CUG.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||131.175 g·mol−1|
|Acidity (pKa)||2.36 (carboxyl), 9.60 (amino)|
|Supplementary data page|
|Refractive index (n),|
Dielectric constant (εr), etc.
|UV, IR, NMR, MS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Like valine and isoleucine, leucine is a branched-chain amino acid. The primary metabolic end products of leucine metabolism are acetyl-CoA and acetoacetate; consequently, it is one of the two exclusively ketogenic amino acids, with lysine being the other. It is the most important ketogenic amino acid in humans.p. 101
Leucine and β-hydroxy β-methylbutyric acid, a minor leucine metabolite, exhibit pharmacological activity in humans and have been demonstrated to promote protein biosynthesis via the phosphorylation of the mechanistic target of rapamycin (mTOR).
The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For leucine, for adults 19 years and older, 42 mg/kg body weight/day.
|Whey protein concentrate, dry powder||10.0-12.0|
|Soy protein concentrate, dry powder||7.5-8.5|
|Pea protein concentrate, dry powder||6.6|
|Soybeans, mature seeds, roasted, salted||2.87|
|Hemp seed, hulled||2.16|
|Beef, round, top round, raw||1.76|
|Fish, salmon, pink, raw||1.62|
|Chicken, broilers or fryers, thigh, raw||1.48|
|Chicken egg, yolk, raw||1.40|
|Edamame (soybeans, green, raw)||0.93|
|Beans, pinto, cooked||0.78|
|Cow milk, whole, 3.25% milk fat||0.27|
|Rice, brown, medium-grain, cooked||0.19|
|Milk, human, mature, fluid||0.10|
As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats. However, results of comparative studies are conflicted. Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men. More studies are needed, preferably ones based on an objective, random sample of society. Factors such as lifestyle choices, age, gender, diet, exercise, etc. must be factored into the analyses to isolate the effects of supplemental leucine as a standalone, or if taken with other branched chain amino acids (BCAAs). Until then, dietary supplemental leucine cannot be associated as the prime reason for muscular growth or optimal maintenance for the entire population.
Both L-leucine and D-leucine protect mice against seizures. D-leucine also terminates seizures in mice after the onset of seizure activity, at least as effectively as diazepam and without sedative effects. Decreased dietary intake of L-leucine promotes adiposity in mice. High blood levels of leucine are associated with insulin resistance in humans, mice, and rodents. This might be due to the effect of leucine to stimulate mTOR signaling. Dietary restriction of leucine and the other BCAAs can reverse diet-induced obesity in wild-type mice by increasing energy expenditure, and can restrict fat mass gain of hyperphagic rats.
Leucine toxicity, as seen in decompensated maple syrup urine disease, causes delirium and neurologic compromise, and can be life-threatening.
A high intake of leucine may cause or exacerbate symptoms of pellagra in people with low niacin status because it interferes with the conversion of L-tryptophan to niacin.
Leucine at a dose exceeding 500 mg/kg/d was observed with hyperammonemia. As such, unofficially, a tolerable upper intake level (UL) for leucine in healthy adult men can be suggested at 500 mg/kg/d or 35 g/d under acute dietary conditions.
Leucine is a dietary amino acid with the capacity to directly stimulate myofibrillar muscle protein synthesis. This effect of leucine arises results from its role as an activator of the mechanistic target of rapamycin (mTOR), a serine-threonine protein kinase that regulates protein biosynthesis and cell growth. The activation of mTOR by leucine is mediated through Rag GTPases, leucine binding to leucyl-tRNA synthetase, leucine binding to sestrin 2, and possibly other mechanisms.
Metabolism in humans
Leucine metabolism in humans
Leucine metabolism occurs in many tissues in the human body; however, most dietary leucine is metabolized within the liver, adipose tissue, and muscle tissue. Adipose and muscle tissue use leucine in the formation of sterols and other compounds. Combined leucine use in these two tissues is seven times greater than in the liver. In healthy individuals, approximately 60% of dietary L-leucine is metabolized after several hours, with roughly 5% (2–10% range) of dietary L-leucine being converted to β-hydroxy β-methylbutyric acid (HMB). Around 40% of dietary L-leucine is converted to acetyl-CoA, which is subsequently used in the synthesis of other compounds.
The vast majority of L-leucine metabolism is initially catalyzed by the branched-chain amino acid aminotransferase enzyme, producing α-ketoisocaproate (α-KIC). α-KIC is mostly metabolized by the mitochondrial enzyme branched-chain α-ketoacid dehydrogenase, which converts it to isovaleryl-CoA. Isovaleryl-CoA is subsequently metabolized by isovaleryl-CoA dehydrogenase and converted to MC-CoA, which is used in the synthesis of acetyl-CoA and other compounds. During biotin deficiency, HMB can be synthesized from MC-CoA via enoyl-CoA hydratase and an unknown thioesterase enzyme, which convert MC-CoA into HMB-CoA and HMB-CoA into HMB respectively. A relatively small amount of α-KIC is metabolized in the liver by the cytosolic enzyme 4-hydroxyphenylpyruvate dioxygenase (KIC dioxygenase), which converts α-KIC to HMB. In healthy individuals, this minor pathway – which involves the conversion of L-leucine to α-KIC and then HMB – is the predominant route of HMB synthesis.
A small fraction of L-leucine metabolism – less than 5% in all tissues except the testes where it accounts for about 33% – is initially catalyzed by leucine aminomutase, producing β-leucine, which is subsequently metabolized into β-ketoisocaproate (β-KIC), β-ketoisocaproyl-CoA, and then acetyl-CoA by a series of uncharacterized enzymes.
The metabolism of HMB is catalyzed by an uncharacterized enzyme which converts it to β-hydroxy β-methylbutyryl-CoA (HMB-CoA). HMB-CoA is metabolized by either enoyl-CoA hydratase or another uncharacterized enzyme, producing β-methylcrotonyl-CoA (MC-CoA) or hydroxymethylglutaryl-CoA (HMG-CoA) respectively. MC-CoA is then converted by the enzyme methylcrotonyl-CoA carboxylase to methylglutaconyl-CoA (MG-CoA), which is subsequently converted to HMG-CoA by methylglutaconyl-CoA hydratase. HMG-CoA is then cleaved into acetyl-CoA and acetoacetate by HMG-CoA lyase or used in the production of cholesterol via the mevalonate pathway.
Synthesis in non-human organisms
Leucine is an essential amino acid in the diet of animals because they lack the complete enzyme pathway to synthesize it de novo from potential precursor compounds. Consequently, they must ingest it, usually as a component of proteins. Plants and microorganisms synthesize leucine from pyruvic acid with a series of enzymes:
- Acetolactate synthase
- Acetohydroxy acid isomeroreductase
- Dihydroxyacid dehydratase
- α-Isopropylmalate synthase
- α-Isopropylmalate isomerase
- Leucine aminotransferase
Synthesis of the small, hydrophobic amino acid valine also includes the initial part of this pathway.
Leucine is a branched-chain amino acid (BCAA) since it possesses an aliphatic side-chain that is not linear.
Racemic leucine had been subjected to circularly polarized synchrotron radiation to better understand the origin of biomolecular asymmetry. An enantiomeric enhancement of 2.6% had been induced, indicating a possible photochemical origin of biomolecules' homochirality.
- This reaction is catalyzed by an unknown thioesterase enzyme.
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HMB's mechanisms of action are generally considered to relate to its effect on both muscle protein synthesis and muscle protein breakdown (Figure 1) [2, 3]. HMB appears to stimulate muscle protein synthesis through an up-regulation of the mammalian/mechanistic target of rapamycin complex 1 (mTORC1), a signaling cascade involved in coordination of translation initiation of muscle protein synthesis [2, 4]. Additionally, HMB may have antagonistic effects on the ubiquitin–proteasome pathway, a system that degrades intracellular proteins [5, 6]. Evidence also suggests that HMB promotes myogenic proliferation, differentiation, and cell fusion . ... Exogenous HMB-FA administration has shown to increase intramuscular anabolic signaling, stimulate muscle protein synthesis, and attenuate muscle protein breakdown in humans .
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The stimulation of MPS through mTORc1-signalling following HMB exposure is in agreement with pre-clinical studies (Eley et al. 2008). ... Furthermore, there was clear divergence in the amplitude of phosphorylation for 4EBP1 (at Thr37/46 and Ser65/Thr70) and p70S6K (Thr389) in response to both Leu and HMB, with the latter showing more pronounced and sustained phosphorylation. ... Nonetheless, as the overall MPS response was similar, this cellular signalling distinction did not translate into statistically distinguishable anabolic effects in our primary outcome measure of MPS. ... Interestingly, although orally supplied HMB produced no increase in plasma insulin, it caused a depression in MPB (−57%). Normally, postprandial decreases in MPB (of ~50%) are attributed to the nitrogen-sparing effects of insulin since clamping insulin at post-absorptive concentrations (5 μU ml−1) while continuously infusing AAs (18 g h−1) did not suppress MPB (Greenhaff et al. 2008), which is why we chose not to measure MPB in the Leu group, due to an anticipated hyperinsulinaemia (Fig. 3C). Thus, HMB reduces MPB in a fashion similar to, but independent of, insulin. These findings are in-line with reports of the anti-catabolic effects of HMB suppressing MPB in pre-clinical models, via attenuating proteasomal-mediated proteolysis in response to LPS (Eley et al. 2008).
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A significant increase in blood ammonia concentrations above normal values, plasma leucine concentrations, and urinary leucine excretion were observed with leucine intakes >500 mg · kg⁻¹ · d⁻¹. The oxidation of l-[1-¹³C]-leucine expressed as label tracer oxidation in breath (F¹³CO₂), leucine oxidation, and α-ketoisocaproic acid (KIC) oxidation led to different results: a plateau in F¹³CO₂ observed after 500 mg · kg⁻¹ · d⁻¹, no clear plateau observed in leucine oxidation, and KIC oxidation appearing to plateau after 750 mg · kg⁻¹ · d⁻¹. On the basis of plasma and urinary variables, the UL for leucine in healthy adult men can be suggested at 500 mg · kg⁻¹ · d⁻¹ or ~35 g/d as a cautious estimate under acute dietary conditions.
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Reduced activity of MCC impairs catalysis of an essential step in the mitochondrial catabolism of the BCAA leucine. Metabolic impairment diverts methylcrotonyl CoA to 3-hydroxyisovaleryl CoA in a reaction catalyzed by enoyl-CoA hydratase (22, 23). 3-Hydroxyisovaleryl CoA accumulation can inhibit cellular respiration either directly or via effects on the ratios of acyl CoA:free CoA if further metabolism and detoxification of 3-hydroxyisovaleryl CoA does not occur (22). The transfer to carnitine by 4 carnitine acyl-CoA transferases distributed in subcellular compartments likely serves as an important reservoir for acyl moieties (39–41). 3-Hydroxyisovaleryl CoA is likely detoxified by carnitine acetyltransferase producing 3HIA-carnitine, which is transported across the inner mitochondrial membrane (and hence effectively out of the mitochondria) via carnitine-acylcarnitine translocase (39). 3HIA-carnitine is thought to be either directly deacylated by a hydrolase to 3HIA or to undergo a second CoA exchange to again form 3-hydroxyisovaleryl CoA followed by release of 3HIA and free CoA by a thioesterase.
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Energy fuel: Eventually, most Leu is broken down, providing about 6.0kcal/g. About 60% of ingested Leu is oxidized within a few hours ... Ketogenesis: A significant proportion (40% of an ingested dose) is converted into acetyl-CoA and thereby contributes to the synthesis of ketones, steroids, fatty acids, and other compounds
Figure 8.57: Metabolism of L-leucine Archived 22 March 2018 at the Wayback Machine
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In conclusion, HMB treatment clearly appears to be a safe potent strategy against sarcopenia, and more generally against muscle wasting, because HMB improves muscle mass, muscle strength, and physical performance. It seems that HMB is able to act on three of the four major mechanisms involved in muscle deconditioning (protein turnover, apoptosis, and the regenerative process), whereas it is hypothesized to strongly affect the fourth (mitochondrial dynamics and functions). Moreover, HMB is inexpensive (~30– 50 US dollars per month at 3 g per day) and may prevent osteopenia (Bruckbauer and Zemel, 2013; Tatara, 2009; Tatara et al., 2007, 2008, 2012) and decrease cardiovascular risks (Nissen et al., 2000). For all these reasons, HMB should be routinely used in muscle-wasting conditions especially in aged people. ... 3 g of CaHMB taken three times a day (1 g each time) is the optimal posology, which allows for continual bioavailability of HMB in the body (Wilson et al., 2013)
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