Branched-chain amino acid
A branched-chain amino acid (BCAA) is an amino acid having an aliphatic side-chain with a branch (a central carbon atom bound to three or more carbon atoms). Among the proteinogenic amino acids, there are three BCAAs: leucine, isoleucine, and valine. Non-proteinogenic BCAAs include 2-aminoisobutyric acid.
The three proteinogenic BCAAs are among the nine essential amino acids for humans, accounting for 35% of the essential amino acids in muscle proteins and 40% of the preformed amino acids required by mammals. Synthesis for BCAAs occurs in all locations of plants, within the plastids of the cell, as determined by presence of mRNAs which encode for enzymes in the metabolic pathway.
BCAAs fill several metabolic and physiologic roles. Metabolically, BCAAs promote protein synthesis and turnover, signaling pathways, and metabolism of glucose. Oxidation of BCAAs may increase fatty acid oxidation and play a role in obesity. Physiologically, BCAAs take on roles in the immune system and in brain function. BCAAs are broken down effectively by dehydrogenase and decarboxylase enzymes expressed by immune cells, and are required for lymphocyte growth and proliferation and cytotoxic T lymphocyte activity. Lastly, BCAAs share the same transport protein into the brain with aromatic amino acids (Trp, Tyr, and Phe). Once in the brain BCAAs may have a role in protein synthesis, synthesis of neurotransmitters, and production of energy.
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; for isoleucine 19 mg/kg body weight/day; for valine 24 mg/kg body weight/day. For a 70 kg (154 lb) person this equates to 2.9, 1.3 and 1.7 g/day. Diets that meet or exceed the RDA for total protein (0.8 g/kg/day; 56 grams for a 70 kg person), meet or exceed the RDAs for branched-chain amino acids.
Dietary BCAAs have been used in an attempt to treat some cases of hepatic encephalopathy. They can have the effect of alleviating symptoms of hepatic encephalophathy, but there is no evidence they benefit mortality rates, nutrition, or overall quality of life as further research is necessary.
Certain studies suggested a possible link between a high incidence of amyotrophic lateral sclerosis (ALS) among professional American football players and Italian soccer players, and certain sports supplements including BCAAs. In mouse studies, BCAAs were shown to cause cell hyper-excitability resembling that usually observed in ALS patients. The proposed underlying mechanism is that cell hyper-excitability results in increased calcium absorption by the cell and thus brings about cell death, specifically of neuronal cells which have particularly low calcium buffering capabilities. Yet any link between BCAAs and ALS remains to be fully established. While BCAAs can induce a hyperexcitability similar to the one observed in mice with ALS, current work does not show if a BCAA-enriched diet, given over a prolonged period, actually induces ALS-like symptoms.
Blood levels of the BCAAs are elevated in obese, insulin resistant humans and in mouse and rat models of diet-induced diabetes, suggesting the possibility that BCAAs contribute to the pathogenesis of obesity and diabetes. BCAA-restricted diets improve glucose tolerance and promote leanness in normal weight mice, restores insulin sensitivity and normal body weight to obese mice and promotes insulin sensitivity in obese rats.
Five enzymes play a major role in the parallel synthesis pathways for isoleucine, valine, and leucine: threonine dehydrogenase, acetohydroxyacid synthase, ketoacid reductoisomerase, dihydroxyacid dehygrogenase and aminotransferase. Threonine dehydrogenase catalyzes the deamination and dehydration of threonine to 2-ketobutyrate and ammonia. Isoleucine forms a negative feedback loop with threonine dehydrogenase. Acetohydroxyacid synthase is the first enzyme for the parallel pathway performing condensation reaction in both steps – condensation of pyruvate to acetoacetate in the valine pathway and condensation of pyruvate and 2-ketobutyrate to form acetohydroxybtylrate in the isoleucine pathway. Next ketoacid reductisomerase reduces the acetohydroxy acids from the previous step to yield dihydroxyacids in both the valine and isoleucine pathways. Dihydroxyacid dehygrogenase converts the dihyroxyacids in the next step. The final step in the parallel pathway is conducted by amino transferase, which yields the final products of valine and isoleucine. A series of four more enzymes – isopropylmalate synthase, isopropylmalate isomerase, isopropylmalate dehydrogenase, and aminotransferase – are necessary for the formation of leucine from 2-oxolsovalerate.
Degradation of branched-chain amino acids involves the branched-chain alpha-keto acid dehydrogenase complex (BCKDH). A deficiency of this complex leads to a buildup of the branched-chain amino acids (leucine, isoleucine, and valine) and their toxic by-products in the blood and urine, giving the condition the name maple syrup urine disease.
The BCKDH complex converts branched-chain amino acids into acyl-CoA derivatives, which after subsequent reactions are converted either into acetyl-CoA or succinyl-CoA that enter the citric acid cycle.
While most amino acids are oxidized in the liver, BCAAs are primarily oxidized in the skeletal muscle and other peripheral tissues. The effects of BCAA administration on muscle growth in rat diaphragm was tested, and concluded that not only does a mixture of BCAAs alone have the same effect on growth as a complete mixture of amino acids, but an amino acid mixture with all but BCAAs has no effect on rat diaphragm muscle growth. Administration of either isoleucine or valine alone had no effect on muscle growth, although administration of leucine alone appears to be nearly as effective as the complete mixture of BCAAs. Leucine indirectly activates p70 S6 kinase as well as stimulates assembly of the eIF4F complex, which are essential for mRNA binding in translational initiation. P70 S6 kinase is part of the mammalian target of rapamycin complex (mTOR) signaling pathway, and has been shown to allow adaptive hypertrophy and recovery of rat muscle. At rest protein infusion stimulates protein synthesis 30 minutes after start of infusion, and protein synthesis stays elevated for another 90 minutes. Infusion of leucine at rest produces a six-hour stimulatory effect and increased protein synthesis by phosphorylation of p70 S6 kinase in skeletal muscles. Following resistance exercise, without BCAA administration, a resistance exercise session does not affect mTOR phosphorylation and even produces a decrease in Akt phosphorylation. Some phosphorylation of p70 S6 kinase was discovered. When BCAAs were administered following a training session, sufficient phosphorylation of p70 S6 kinase and S6 indicated activation of the signaling cascade.
Role in diabetes mellitus type 2
In addition to cell signaling, the mTOR pathway also plays a role in beta cell growth leading to insulin secretion. High glucose in the blood begins the process of the mTOR signaling pathway, in which leucine plays an indirect role. The combination of glucose, leucine, and other activators cause mTOR to start signaling for the proliferation of beta cells and the secretion of insulin. Higher concentrations of leucine cause hyperactivity in the mTOR pathway, and S6 kinase is activated leading to inhibition of insulin receptor substrate through serine phosphorylation. In the cell the increased activity of mTOR complex causes eventual inability of beta cells to release insulin and the inhibitory effect of S6 kinase leads to insulin resistance in the cells, contributing to development of type 2 diabetes.
Metformin is able to activate AMP kinase which phosphorylates proteins involved in the mTOR pathway, as well as leads to the progression of mTOR complex from its inactive state to its active state. It is suggested that metformin acts as a competitive inhibitor to the amino acid leucine in the mTOR pathway.
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