Leucine Amino Acids: Properties, Function, Benefits, and Sources

What is leucine?

In animal nutrition, leucine (Leu) is a crucial branched-chain amino acid (BCAA). In meals that are rich in protein, it is often one of the most abundant amino acids. Leu enhances protein synthesis in skeletal muscle, adipose tissue, and placental cells via activating the mammalian target of rapamycin (mTOR) signaling pathway. Leu inhibits protein degradation and improves energy metabolism, which includes glucose uptake, mitochondrial biogenesis, and fatty acid oxidation, to power protein synthesis. In skeletal muscle, about 80% of Leu is converted to α-ketoisocaproate (α-KIC) and Η-hydroxy-β-methylbutyrate (HMB), while the rest is typically utilized for protein synthesis. Hence, it has been suggested that Leu's metabolites regulate some of its actions. In numerous in vitro and in vivo studies, both α-KIC and HMB have lately attracted a lot of interest as dietary supplements that regulate energy balance, limit protein breakdown, and enhance protein synthesis. There is encouraging evidence that leu and its metabolites can improve the development and well-being of various animal species.

Leucine metabolism

The metabolism of leucine in the body can be categorized into two stages. The initial step involves the reversible transamination of leucine to produce α-ketoisocaproate, alongside the simultaneous generation of glutamate from α-ketoglutarate. This reaction is facilitated by branched-chain amino acid aminotransferase (BCAT), which is abundantly expressed in muscle tissue. There exist two mammalian BCAT isoenzymes: a cytosolic form (BCATc) and a mitochondrial form (BCATm). BCATc is found in the placenta, ovary, brain, mammary tissue, and small intestine, while BCATm is expressed in most peripheral tissues, excluding the liver. Glutamate, a product of branched-chain amino acid (BCAA) transamination, is either amidated with ammonia to produce glutamine or transaminated with pyruvate to yield alanine in many tissues, including skeletal muscle, placenta, and mammary tissue. Notably, the liver lacks the BCAT isoenzyme. The availability of leucine is contingent upon either external (dietary) sources or endogenous (protein breakdown) mechanisms. Leucine from dietary protein can circumvent initial metabolism in the liver, leading to a significant increase in plasma leucine levels and the activation of leucine signaling in peripheral tissues following a meal. The subsequently generated α-KIC is released into the bloodstream and absorbed by various tissues, where it may be oxidized or utilized for the re-synthesis of Leu. The leucine released from the liver into the bloodstream may be utilized by skeletal muscle for protein synthesis or the production of alanine and glutamine. In extrahepatic tissues, Leucine is transformed to α-Ketoisocaproate, which can then be re-synthesized into Leucine or further oxidized.

The subsequent phase of Leu catabolism involves the irreversible oxidative decarboxylation of α-KIC, catalyzed by the BCKD complex situated within the mitochondrion. The BCKD complex exhibits low activity in skeletal muscle, high activity in the liver, and intermediate activity in the heart and kidney. Consequently, most α-KIC oxidation transpires in the liver. In the liver mitochondria, almost 90% of all synthesized α-KIC is oxidized to isovaleryl CoA (IVA-CoA), resulting in the production of β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) and the ultimate metabolites acetoacetate and acetyl-CoA. In the liver cytosol, the residual α-KIC is converted to HMB by the enzyme α-KIC dioxygenase (KICD). Subsequently, HMB, produced only from Leu, is either excreted by the kidneys or converted to HMG-CoA, the precursor for cholesterol production. Consequently, the second catabolic step dedicates the Leu carbon skeleton to the breakdown route.

Pathways of leucine metabolism in animals. (Duan Y., et al., 2016)

Leucine structure

Like other amino acid, leucine has an amino group and a carboxyl group. The side chain of leucine is a branched aliphatic chain with the structure –CH₂–CH(CH₃)₂, consisting of a two-carbon unit (–CH₂) attached to a branching carbon (–CH) that has two methyl groups (–CH₃) attached.

The results of a study of the conformational stability in poly-α-amino acids and circular dichroism and potentiometric titration investigations on leucine random copolymers in water suggest that leucine might be the amino acid most prone to producing α-helical structures. Leucine may be most abundant in the helical sections of proteins, according to this. Insulin, lysozyme, myogen, myoglobin, papain, ribonuclease A, staphylococcal nuclease, subtilisin BPN′, and fifteen other proteins with 2473 residues and known sequence and conformation as determined by X-ray crystallography were examined. Among these proteins, 888 are located in helical areas, with 422 of those residues occupying the internal turns of helical domains. The helical portions of proteins contained the largest percentages of Glu, Ala, Leu, and His, but the inner helical cores of proteins obviously contained the most prevalent residue, Leu. Glu and Asp are located at the N-terminal helix-coil border regions, while His, Lys, and Arg are at the C-terminal helical ends, indicating a preference for polar residues. These results are in agreement with those of Ptitsyn's (1969) study of seven proteins having a total of 1,132 residues. Ile, Met, and Val are most commonly found in the β-regions of proteins, according to a more thorough examination in this survey. In proteins, namely in the whole helical and β-regions, leu was discovered to be the most powerful structure-forming residue. The presence of leucine as the most abundant residue surrounding the heme in five of the heme proteins proved the functional-structural role of the amino acid. Hydrophobic residues help the polar residues with substrate binding and enzymic catalysis by creating a non-aqueous environment; this is supported by the fact that leucine is more abundant as a neighbor to active-site residues in enzymes. To demonstrate the structure-function relationship of proteins and to clarify why the majority of leucine residues in the insulin, hemoglobin, and cytochrome c homologs remain unchanged, we provide examples of conservative and non-conservative leucine mutations in heme proteins. Lastly, leucine's high frequency in proteins' inner helical cores and its great helix-forming capacity in synthetic copolypeptides imply that it may play a significant role as nucleation centers during the folding and evolution of big protein molecules.

Leucine pKa

At physiological pH, leucine shows electrical neutrality for both its α-Carboxyl and α-Amine groups, with pKa values of 2.3 and 9.6, respectively. Leucine usually has its carboxyl group deprotonated (-COO⁺) and its amine group protonated (-NH₃⁺) at physiological pH.

Physicochemical properties of the common protein amino acids. (Amaya-Farfan J., et al., 2003)

The polarity of amino acids. (Amaya-Farfan J., et al., 2003)

Charge of leucine

The net charge is zero at physiological pH (~7.4) because the positive charge of the amine group (+1) and the negative charge of the carboxyl group (-1) balance each other out. Because the amino and carboxyl groups' positive and negative charges cancel each other out, leucine is electrically neutral at physiological pH.

Leucine sources

Since leucine is an amino acid that is found in all proteins (albeit it is more plentiful in proteins derived from animals), it is very difficult to determine how much leucine one needs in relation to the overall amount of protein consumed. The goal, then, is to avoid or regain lean mass loss in the elderly by consuming 25-30 g of protein and 3 g of leucine at each of the three main meals. It is challenging to construct a diet according to these standards since food composition databases seldom display the quantities of leucine included in foods.

Content in leucine, protein and energy per 100 g of food (list 1). (Rondanelli M., et al., 2021)

Content in leucine, protein and energy per 100 g of food (list 2). (Rondanelli M., et al., 2021)

Leucine amino acids at Creative Peptides

Leucine function

There is a strong relationship between protein and energy metabolism; when energy substrates are scarce, protein synthesis is halted. Myokines, AMPK, and mTORC1 all have a role in Leu control and the communication between protein and energy metabolism. In addition to its critical involvement in protein turnover, leu is involved in energy metabolism. It can promote energy transfer from fat cells to muscle cells, which means fat cells store less fat and muscle cells use more fat. Leu plays a pivotal role in mediating fatty acid oxidation and mitochondrial biogenesis, which in turn enhance energy expenditure in skeletal muscle cells, via activating AMPK and SIRT1. Whether Leu itself mediates energy metabolism or if the effects are contributed to by its metabolites α-KIC and/or HMB is still not established. One such way that Leu could control energy metabolism is via influencing nitric oxide production. How Leu and its metabolites regulate insulin secretion, glucose homeostasis, fatty acid oxidation, and mitochondrial biosynthesis will be discussed in the section that follows.

Mechanisms for leucine to stimulate protein synthesis in skeletal muscle. (Duan Y., et al., 2016)

Leucine benefits

Among the branch-chain amino acids (BCAAs), leucine is known to activate mTOR complex 1 and its downstream targets, promoting protein synthesis. Conversely, leucine-derived KIC prevents the breakdown of proteins. These characteristics have led to the clinical application of BCAA supplementation in the treatment of burns, sepsis, and liver disease. Supplementing the diets of rats with leucine or BCAAs decreases their body fat mass and weight while increasing their glucose oxidation. These results provide more evidence that leucine supplementation could help with weight management and preventing type 2 diabetes. A knockout model for mitochondrial BCAT (BCATm) showed the importance of leucine signaling. The BCATm−/− mice exhibited a substantial decrease in body mass and fat, as well as an increase in plasma BCAA levels. In the animal model, improving glucose and insulin tolerance leads to a significant rise in energy expenditure, even when food intake is increased. The cycle is initiated by leucine-induced enhanced protein synthesis and countered by accelerated proteolysis due to the removal of KIC-mediated inhibition. These results provide compelling evidence that leucine signaling is involved in increasing energy expenditure and insulin sensitivity, two mechanisms that contribute to weight loss. Leucine is recognized as an insulin secretagog in pancreatic islet β-cells, where it promotes insulin secretion in an acute manner. Patients with diabetes who take leucine regularly and successfully improve their glycemic control also have greater insulin release from their islet cells. The islet leucine pool has a dual purpose: it provides energy and increases glutaminolysis by acting as an allosteric activator of glutamate dehydrogenase (GDH). During periods of low blood glucose, leucine-mediated glutaminolysis is demonstrated to play a pivotal role in the release of insulin between meals.

An energy-wasting futile cycle in mitochondrial BCAT (BCATm) knockout mice. (Chuang D T., 2013)

What is leucine used for?

(1) Appetite control

The function of leucine in regulating hormones and controlling hunger has been the subject of some intriguing scientific studies. The body's supply of leucine is unlike any other. Some interesting words and phrases borrowed from molecular biology are making their way into medical jargon as the field expands its influence in the medical field. "Mammalian target of rapamycin" (mTOR) is one such example. The Streptomyces hygroscopicus bacterium produces the antibiotic rapamycin. Because it dampens some parts of the immune response, it helps keep transplant recipients' bodies from rejecting their new organs. The mTOR pathway is an important target of rapamycin. Human cell chemistry incorporates the mTOR site. It appears to play a role in regulating the production of proteins and the expansion of cells. Additionally, it acts as a go-between for the insulin-induced cellular actions. For a some now, people have known that leucine activates mTOR in several bodily regions. The hypothalamus is responsible for controlling food intake in rats, and a recent study found that injecting rats with leucine caused them to eat much less than usual 151. Leucine, it seems, suppresses hunger by activating the mTOR site in the brain. Rats given the mTOR inhibitor rapamycin ate more and had a greater hunger than rats given a placebo. Another way to suppress hunger is to inject tiny doses of leptin into the brain, which likewise activates mTOR. Rapamycin blocks mTOR activity, which reduces leptin's effect on food intake. It suggests that leucine is involved in the synthesis and secretion of leptin from adipose tissue. Once again, it seems that leucine's impact is mediated by mTOR activation. Researchers observed that adding leucine to rat adipose tissue increased their leptin secretion in vitro. The mTOR antagonist rapamycin152 inhibited this leucine effect. The elimination of leucine from rats' diets resulted in a nearly 40% reduction in blood leptin levels after meals. Based on these findings, leucine, leptin, and mTOR are highly related to hunger regulation.

Role of hypothalamic mTOR signaling in the central anorectic action of leptin. (Cota D., et al., 2006)

(2) Management of Type 2 diabetes

Impaired glucose absorption and hyperglycemic blood glucose excursions are symptoms of type 2 diabetes, which is defined by peripheral tissues' diminished sensitivity to circulating insulin levels. In order to enhance postprandial glycemic control in individuals with type 2 diabetes, the insulinotropic characteristics of amino acids—and leucine in particular—may be relevant. Patients with type 2 diabetes who have had the disease for a long time often have a reduced insulinotropic response to carbs, but this response can be greatly enhanced when protein and amino acids are consumed together. The pancreatic ß-cell may no longer respond correctly to glucose, but it is still very functional when it comes to other stimuli, such as amino acids. In people with long-term type 2 diabetes, the insulin response to carbohydrate consumption can be enhanced two-to fourfold when leucine is consumed with protein. When compared to healthy, normoglycemic controls who consumed the same amount of carbohydrates, the insulin response actually increases to the level seen in these individuals. Research has shown that patients with type 2 diabetes benefit from a higher postprandial insulin response, which lowers postprandial hyperglycemia and increases blood glucose uptake. Research showing that a little protein hydrolysate and leucine taken with each main meal improves 24-hour glycemic control by lowering the incidence of postprandial hyperglycemia has been reported in subsequent investigations. Since it has been shown that protein and/or leucine can effectively promote endogenous insulin release in type 2 diabetes, research into the therapeutic advantages of long-term protein and/or leucine supplementation in this population is necessary.

(3) Mitigate Sarcopenia

Sarcopenia, the age-related weakening and atrophy of muscles, lowers functional capacity and raises the danger of developing chronic metabolic disease. It seems that a disturbance in the control of muscle protein turnover is one of the primary causes of sarcopenia. New research shows that dietary and exercise-related anabolic stimuli have less of an effect on the elderly. The reduced muscle protein synthesis response to food intake in the elderly may be adequately compensated for by increasing the leucine content of a meal. An amino acid mixture with an increased leucine content (from 26 to 41%; or 1.7 to 2.8 g leucine) allows the elderly to maintain a postprandial muscle protein synthesis response similar to that of young people, according to a recent paper by Katsanos' team. The results were corroborated by Rieu's group, who found that older men whose diets were rich in leucine had increased rates of muscle protein synthesis. Therefore, it has been suggested that a good way for the elderly to increase their muscle protein synthesis response to food is to increase the amount of leucine in their meals (>2.8 g leucine). Although leucine's effect on muscle protein breakdown has been extensively demonstrated in rat models, it is yet unclear whether extra leucine injections may likewise prevent muscular protein breakdown in living organisms. Research on the effects of leucine co-ingestion on postprandial muscle protein breakdown is limited, despite the fact that numerous studies have shown lower rates of muscle protein breakdown after intravenous infusion of leucine in humans. Keep in mind that the insulin response that follows eating a meal usually lowers the rates of muscle protein breakdown, so there's less need for (extra) leucine to further limit postprandial muscle protein breakdown. On the other hand, a subset of the elderly who are already at a higher risk of experiencing accelerated muscle protein breakdown owing to comorbidities and cachexia may benefit greatly from learning more about the effects of leucine co-ingestion on this process. In a nutshell, the supposed anabolic effects of amino acids, and leucine in particular, should be useful for enhancing postprandial protein balance and making up for the diminished synthetic response of muscle proteins to meal consumption in the aged. In light of this, taking a leucine supplement has been suggested as a viable nutritional approach to slow down the aging process and, by extension, sarcopenia.

(4) Improve body weight control

A diet rich in BCAAs, such as leucine, which accounts for as much as 20% of protein, can help with weight management. Wistar rats lose weight just as much as those undergoing the HPD when given dietary leucine supplements for three weeks. Evidence of comparable weight loss effects has been found in a number of further investigations including both obese animals and humans. For 10 weeks, a study with C57BL/6 mice found that animals given 1.5% leucine-fortified water had a 32% decrease in weight increase (P =<0.05) and a 25% decrease in adiposity (P = <0.01). After 14 days of HFD plus BCAA, mice gained less weight, had smaller adipocytes, and less epididymal adipose tissue than mice given just high-fat diet (HFD). An additional finding from the study of approximately 4400 middle-aged men and women is that the likelihood of becoming overweight or obese is inversely proportional to the amount of BCAA consumed. Similarly, Brazilian men who consumed the most BCAAs had a lower risk of obesity, according to a different study. There was a notable impact of leucine specifically.

There are two possible explanations for why leucine supplementation reduces the weight gain caused by the HFD. Reduced food intake caused by leucine is one possible explanation. Administering leucine centrally reduces food intake and body weight in obese animals. This is achieved by activating the mTOR pathway in the hypothalamic arcuate nucleus, which in turn controls the release of hormones like ghrelin and leptin, which may impact food intake in the intestines and fat stores. An other potential mechanism could be the elevation in resting energy expenditure caused by leucine. Leucine modulates thermogenic tissue mitochondrial uncoupling of oxidative phosphorylation, which in turn increases energy expenditure and decreases body weight in HFD-induced obese mice via upregulating the expression of uncoupling protein 3 (UCP-3). The same study also found a reduced respiratory exchange ratio. There have been other investigations in male C57/BL6 mice that have shown comparable metabolic and molecular effects. In mice that are caused to be overweight by a HFD, the leucine-supplemented group shows an increase in total energy expenditure along with decreases in body weight, fat mass, and respiratory quotient. Fewer calories consumed and more energy expended could be the fundamental mechanisms.

Mechanisms of the Beneficial Effects of Dietary Leucine Supplementation. (Yao K., et al., 2016)

Summary of the Effects of Dietary Leucine on Lipid and Glucose Metabolism. (Yao K., et al., 2016)

(5) Improve inflammation phenotypes

Additionally, it has been noted that leucine supplementation improves inflammatory characteristics. Adding leucine to a HFD (doubling intake) prevents macrophage infiltrates from showing up and reduces levels of the proinflammatory cytokine TNF-α and the macrophage marker F4/80 in the perigonadal fat by 40-45%. This, in turn, may improve insulin resistance in other tissues and improve glucose metabolism throughout the body. Similarly, adiponectin, an anti-inflammatory cytokine, was found to be increased in HFD-induced obese mice when they were given leucine (24 g/kg food; 200% of normal level) and low-dose resveratrol (12.5 mg/kg diet) for 6 weeks. Leucine, along with other supplements like icariin and metformin, normalizes inflammatory markers like TNF-α, IL-6, IL-1β, MCP-1, and CRP, which is in line with these studies. Obese animal models show a significant reduction in macrophage infiltration and the expression levels of proinflammatory cytokines TNF-α and MCP-1 after being treated with leucine, either alone or in combination with other substances. The specific way leucine-mediated inflammatory phenotypes work is still not known, however it may be reliant on the suppression of NF-κB by mTOR.

FAQ

Is Leucine polar or nonpolar?

Valine, leucine, and isoleucine are branched-chain aliphatic amino acids that engage in hydrophobic interactions due to their large, nonpolar R-groups. Essential amino acids are all three of them. Disease of the maple syrup peeps occurs when their catabolism is flawed. Isoleucine is present in protein in just one of its four stereoisomers, and it has asymmetrical centers at the α- and β-carbons. Within water-soluble globular proteins, the large side chains often bind together. Amino acid residues that are hydrophobic help to maintain the polymer's three-dimensional structure.

References

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