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Holeček M. Why Are Branched-Chain Amino Acids Increased in Starvation and Diabetes? Nutrients 2020; 12:nu12103087. [PMID: 33050579 PMCID: PMC7600358 DOI: 10.3390/nu12103087] [Citation(s) in RCA: 63] [Impact Index Per Article: 15.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2020] [Revised: 10/04/2020] [Accepted: 10/07/2020] [Indexed: 12/12/2022] Open
Abstract
Branched-chain amino acids (BCAAs; valine, leucine, and isoleucine) are increased in starvation and diabetes mellitus. However, the pathogenesis has not been explained. It has been shown that BCAA catabolism occurs mostly in muscles due to high activity of BCAA aminotransferase, which converts BCAA and α-ketoglutarate (α-KG) to branched-chain keto acids (BCKAs) and glutamate. The loss of α-KG from the citric cycle (cataplerosis) is attenuated by glutamate conversion to α-KG in alanine aminotransferase and aspartate aminotransferase reactions, in which glycolysis is the main source of amino group acceptors, pyruvate and oxaloacetate. Irreversible oxidation of BCKA by BCKA dehydrogenase is sensitive to BCKA supply, and ratios of NADH to NAD+ and acyl-CoA to CoA-SH. It is hypothesized that decreased glycolysis and increased fatty acid oxidation, characteristic features of starvation and diabetes, cause in muscles alterations resulting in increased BCAA levels. The main alterations include (i) impaired BCAA transamination due to decreased supply of amino groups acceptors (α-KG, pyruvate, and oxaloacetate) and (ii) inhibitory influence of NADH and acyl-CoAs produced in fatty acid oxidation on citric cycle and BCKA dehydrogenase. The studies supporting the hypothesis and pros and cons of elevated BCAA concentrations are discussed in the article.
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Affiliation(s)
- Milan Holeček
- Department of Physiology, Faculty of Medicine in Hradec Králové, Charles University, Šimkova 870, 50003 Hradec Králové, Czech Republic
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Thomas M, Davis T, Loos B, Sishi B, Huisamen B, Strijdom H, Engelbrecht AM. Autophagy is essential for the maintenance of amino acids and ATP levels during acute amino acid starvation in MDAMB231 cells. Cell Biochem Funct 2018; 36:65-79. [PMID: 29399832 DOI: 10.1002/cbf.3318] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2017] [Revised: 11/20/2017] [Accepted: 12/26/2017] [Indexed: 12/17/2022]
Abstract
Autophagy plays a major role in the adaptive metabolic response of cancer cells during adverse conditions such as nutrient deprivation. However, specific data that assess metabolite profiles in context with adenosine triphosphate (ATP) availability and cell death susceptibility remain limited. Human breast cancer cells, MDAMB231, and normal breast epithelial cells, MCF12A, were subjected to short-term amino acid starvation and the cellular apoptotic and autophagic responses assessed. The role of autophagy in the control of cellular amino acid, ATP, free fatty acid, and glucose levels during amino acid starvation were compared. We demonstrate that breast cancer cells have an increased metabolic demand contributing to significant amino acid and ATP depletion in a nutrient-poor environment. Upregulation of autophagy was important for the generation of amino acids and free fatty acids and maintenance of cellular ATP levels. In contrast to normal cells, breast cancer cells were unable to maintain the response after 12 hours of amino acid starvation. Regulation of autophagic activity in these environments had indirect consequences on cell death susceptibility. Overall, our data provide support for autophagy as an important survival mechanism capable of providing metabolic substrates when cancer cells are faced with nutrient-deprived environments. SIGNIFICANCE OF STUDY The results obtained in this study helps to expand our current knowledge on how cells respond to environmental changes; the biochemical and metabolic consequences and the physiological processes activated in response. The environmental stress applied in this study is relevant to tumour physiology, and results can be translated to cancer therapeutic and clinical research areas, ultimately assisting in the specific targeting of cancer cells while avoiding harm to normal cells.
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Affiliation(s)
- Mark Thomas
- Department of Physiological Sciences, Faculty of Science, Stellenbosch University, Stellenbosch, South Africa
| | - Tanja Davis
- Department of Physiological Sciences, Faculty of Science, Stellenbosch University, Stellenbosch, South Africa
| | - Ben Loos
- Department of Physiological Sciences, Faculty of Science, Stellenbosch University, Stellenbosch, South Africa
| | - Balindiwe Sishi
- Department of Physiological Sciences, Faculty of Science, Stellenbosch University, Stellenbosch, South Africa
| | - Barbara Huisamen
- Division of Medical Physiology, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa.,Diabetes Discovery Platform, Medical Research Council, Cape Town, South Africa
| | - Hans Strijdom
- Division of Medical Physiology, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa
| | - Anna-Mart Engelbrecht
- Department of Physiological Sciences, Faculty of Science, Stellenbosch University, Stellenbosch, South Africa
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Piccolo BD, Graham JL, Stanhope KL, Fiehn O, Havel PJ, Adams SH. Plasma amino acid and metabolite signatures tracking diabetes progression in the UCD-T2DM rat model. Am J Physiol Endocrinol Metab 2016; 310:E958-69. [PMID: 27094034 PMCID: PMC4935135 DOI: 10.1152/ajpendo.00052.2016] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/09/2016] [Accepted: 04/15/2016] [Indexed: 12/16/2022]
Abstract
Elevations of plasma concentrations of branched-chain amino acids (BCAAs) are observed in human insulin resistance and type 2 diabetes mellitus (T2DM); however, there has been some controversy with respect to the passive or causative nature of the BCAA phenotype. Using untargeted metabolomics, plasma BCAA and other metabolites were assessed in lean control Sprague-Dawley rats (LC) and temporally during diabetes development in the UCD-T2DM rat model, i.e., prediabetic (PD) and 2 wk (D2W), 3 mo (D3M), and 6 mo (D6M) post-onset of diabetes. Plasma leucine, isoleucine, and valine concentrations were elevated only in D6M rats compared with D2W rats (by 28, 29, and 30%, respectively). This was in contrast to decreased plasma concentrations of several other amino acids in D3M and/or D6M relative to LC rats (Ala, Arg, Glu, Gln, Met, Ser, Thr, and Trp). BCAAs were positively correlated with fasting glucose and negatively correlated with plasma insulin, total body weight, total adipose tissue weight, and gastrocnemius muscle weight in the D3M and D6M groups. Multivariate analysis revealed that D3M and D6M UCD-T2DM rats had lower concentrations of amino acids, amino acid derivatives, 1,5-anhydroglucitol, and conduritol-β-opoxide and higher concentrations of uronic acids, pantothenic acids, aconitate, benzoic acid, lactate, and monopalmitin-2-glyceride relative to PD and D2W UCD-T2DM rats. The UCD-T2DM rat does not display elevated plasma BCAA concentrations until 6 mo post-onset of diabetes. With the acknowledgement that this is a rodent model of T2DM, the results indicate that elevated plasma BCAA concentrations are not necessary or sufficient to elicit an insulin resistance or T2DM onset.
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Affiliation(s)
- Brian D Piccolo
- Arkansas Children's Nutrition Center, Little Rock, Arkansas; Department of Pediatrics, University of Arkansas for Medical Science, Little Rock, Arkansas
| | - James L Graham
- Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California; Department of Nutrition, University of California, Davis, California
| | - Kimber L Stanhope
- Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California; Department of Nutrition, University of California, Davis, California
| | - Oliver Fiehn
- West Coast Metabolomics Center, Genome Center, University of California, Davis, California; and King Abdulaziz University, Biochemistry Department, Jeddah, Saudi Arabia
| | - Peter J Havel
- Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California; Department of Nutrition, University of California, Davis, California
| | - Sean H Adams
- Arkansas Children's Nutrition Center, Little Rock, Arkansas; Department of Pediatrics, University of Arkansas for Medical Science, Little Rock, Arkansas; Department of Nutrition, University of California, Davis, California;
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Knapik-Czajka M. Simvastatin increases liver branched-chain α-ketoacid dehydrogenase activity in rats fed with low protein diet. Toxicology 2014; 325:107-14. [DOI: 10.1016/j.tox.2014.09.001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2014] [Revised: 08/24/2014] [Accepted: 09/01/2014] [Indexed: 12/31/2022]
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Loos B, Engelbrecht AM, Lockshin RA, Klionsky DJ, Zakeri Z. The variability of autophagy and cell death susceptibility: Unanswered questions. Autophagy 2013; 9:1270-85. [PMID: 23846383 PMCID: PMC4026026 DOI: 10.4161/auto.25560] [Citation(s) in RCA: 105] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Impaired autophagic machinery is implicated in a number of diseases such as heart disease, neurodegeneration and cancer. A common denominator in these pathologies is a dysregulation of autophagy that has been linked to a change in susceptibility to cell death. Although we have progressed in understanding the molecular machinery and regulation of the autophagic pathway, many unanswered questions remain. How does the metabolic contribution of autophagy connect with the cell’s history and how does its current autophagic flux affect metabolic status and susceptibility to undergo cell death? How does autophagic flux operate to switch metabolic direction and what are the underlying mechanisms in metabolite and energetic sensing, metabolite substrate provision and metabolic integration during the cellular stress response? In this article we focus on unresolved questions that address issues around the role of autophagy in sensing the energetic environment and its role in actively generating metabolite substrates. We attempt to provide answers by explaining how and when a change in autophagic pathway activity such as primary stress response is able to affect cell viability and when not. By addressing the dynamic metabolic relationship between autophagy, apoptosis and necrosis we provide a new perspective on the parameters that connect autophagic activity, severity of injury and cellular history in a logical manner. Last, by evaluating the cell’s condition and autophagic activity in a clear context of regulatory parameters in the intra- and extracellular environment, this review provides new concepts that set autophagy into an energetic feedback loop, that may assist in our understanding of autophagy in maintaining healthy cells or when it controls the threshold between cell death and cell survival.
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Affiliation(s)
- Ben Loos
- Department of Physiological Sciences; Faculty of Natural Sciences; University of Stellenbosch; Stellenbosch, South Africa
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Stimulation of rat liver branched-chain alpha-keto acid dehydrogenase activity by low doses of bezafibrate. Toxicology 2013; 306:101-7. [PMID: 23485652 DOI: 10.1016/j.tox.2013.02.011] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2012] [Revised: 01/25/2013] [Accepted: 02/12/2013] [Indexed: 11/22/2022]
Abstract
Multienzyme branched-chain alpha-ketoacid dehydrogenase complex (BCKDH) catalyzes the regulatory step of oxidative catabolism of indispensable branched-chain amino acids (BCAA). The activity of the BCKDH complex is regulated by a reversible phosphorylation, end-product inhibition and by changes in the gene expression of BCKDH component enzymes. It has been shown previously that a high dose of bezafibrate (an agent added to rat chow at final concentration of 0.5%) changes mRNA levels of BCKDH-related enzymes and increases dephosphorylation of the complex leading to stimulation of liver BCKDH activity and the enhanced BCAA catabolism. The aim of the present study was to determine an in vivo effect of low, clinically relevant doses of bezafibrate on BCKDH activity in rat liver. Bezafibrate was administrated for 14 days by gastric gavage to Wistar male rats (fed low-protein chow; 8% protein) at one of the following daily doses of 5, 10 and 20mg/kgb.wt. The control group was given the vehicle (0.3% methylcellulose) only. The actual BCKDH and total BCKDH activities were assayed spectrophotometrically before and after incubation with a broad-specificity phosphatase, respectively. The mRNA levels of the selected genes (BCKDH catalytic subunits and regulatory enzymes) were quantified by means of semi-quantitative RT-PCR. Current catalytic activity of BCKDH (described as BCKDH activity state - the proportion of the BCKDH complex in its active dephosphorylated form) increased by 2.1 ± 0.2, 2.3 ± 0.2 and 2.7 ± 0.2 fold (p<0.01). Changes in BCKDH activity did not correspond with changes in mRNA levels of the complex catalytic subunits. Moreover, mRNA levels of regulatory enzymes remained unaltered. Initially bezafibrate caused a transient insignificant reduction in body weight, but it had no effect on the final body weight. The highest dose of bezafibrate induced hepatomegaly. In conclusion, these data indicate that under conditions of dietary protein restriction low, clinically relevant doses of bezafibrate have a similar adverse effect on rat liver BCKDH activity and BCAA degradation rate as the high experimental dose. Up-regulation of liver BCKDH activity by low doses of bezafibrate appears to result mainly from changes in phosphorylation status of the complex (increased dephosphorylation) and is not associated with elevations in mRNA levels of BCKDH enzymatic components.
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Sridhar S, Botbol Y, Macian F, Cuervo AM. Autophagy and disease: always two sides to a problem. J Pathol 2011; 226:255-73. [PMID: 21990109 DOI: 10.1002/path.3025] [Citation(s) in RCA: 229] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2011] [Revised: 09/25/2011] [Accepted: 10/03/2011] [Indexed: 12/18/2022]
Abstract
Autophagy is a process traditionally known to contribute to cellular cleaning through the removal of intracellular components in lysosomes. In recent years, intensive scrutiny at the molecular level to which autophagy has been subjected has also contributed to expanding our understanding of the physiological role of this pathway. Added to the well-characterized role in quality control, autophagy has proved to be important in the maintenance of cellular homeostasis and of the energetic balance, in cellular and tissue remodelling, and cellular defence against extracellular insults and pathogens. It is not a surprise that, in light of this growing number of physiological functions, connections between autophagic malfunction and human pathologies have also been strengthened. In this review, we focus on several pathological conditions associated with primary or secondary defects in autophagy and comment on a recurring theme for many of them, ie the fact that autophagy can often exert both beneficial and aggravating effects on the progression of disease. Elucidating the factors that determine the switch between these dual functions of autophagy in disease has become a priority when considering the potential therapeutic implications of the pharmacological modulation of autophagy in many of these pathological conditions.
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Affiliation(s)
- Sunandini Sridhar
- Department of Developmental and Molecular Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
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8
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Loos B, Lochner A, Engelbrecht AM. Autophagy in heart disease: a strong hypothesis for an untouched metabolic reserve. Med Hypotheses 2011; 77:52-7. [PMID: 21482032 DOI: 10.1016/j.mehy.2011.03.021] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2010] [Revised: 03/05/2011] [Accepted: 03/09/2011] [Indexed: 12/20/2022]
Abstract
Autophagy is a conserved catabolic process for long-lived proteins and organelles and is primarily responsible for nonspecific degradation of redundant or faulty cell components. Although autophagy has been described as the cell's major adaptive strategy in response to metabolic challenges, its influence on the cell's energy profile is poorly understood. In the myocardium, autophagy is active at basal levels and is crucial for maintaining its contractile function. Defects in the autophagic machinery cause cardiac dysfunction and heart failure. In this paper we propose that (1) autophagy contributes significantly to the metabolic balance sheet of the heart. (2) Increased autophagy contributes to an improved myocardial energy profile through changing the cardiac substrate preference. (3) Substrates generated through autophagy give rise to an alternative for ATP production with an oxygen-sparing effect. These elements identify autophagy in a new context of myocardial metabolic interregulation, which we discuss in the settings of myocardial infarction, heart failure and the diabetic heart. It is hoped that the hypothesis presented can lead to new insights aimed at exploiting autophagy to improve existing metabolic-based therapy in heart disease.
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Affiliation(s)
- B Loos
- Department of Physiological Sciences, Faculty of Natural Sciences, University of Stellenbosch, Stellenbosch 7600, South Africa.
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Bajotto G, Murakami T, Nagasaki M, Sato Y, Shimomura Y. Decreased enzyme activity and contents of hepatic branched-chain alpha-keto acid dehydrogenase complex subunits in a rat model for type 2 diabetes mellitus. Metabolism 2009; 58:1489-95. [PMID: 19586643 DOI: 10.1016/j.metabol.2009.04.029] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/05/2008] [Accepted: 04/02/2009] [Indexed: 01/18/2023]
Abstract
The mitochondrial branched-chain alpha-keto acid dehydrogenase complex (BCKDC) is responsible for the committed step in branched-chain amino acid catabolism. In the present study, we examined BCKDC regulation in Otsuka Long-Evans Tokushima Fatty (OLETF) rats both before (8 weeks of age) and after (25 weeks of age) the onset of type 2 diabetes mellitus. Long-Evans Tokushima Otsuka (LETO) rats were used as controls. Plasma branched-chain amino acid and branched-chain alpha-keto acid concentrations were significantly increased in young and middle-aged OLETF rats. Although the hepatic complex was nearly 100% active in all animals, total BCKDC activity and protein abundance of E1alpha, E1beta, and E2 subunits were markedly lower in OLETF than in LETO rats at 8 and 25 weeks of age. In addition, hepatic BCKDC activity and protein amounts were significantly decreased in LETO rats aged 25 weeks than in LETO rats aged 8 weeks. In skeletal muscle, E1beta and E2 proteins were significantly reduced, whereas E1alpha tended to increase in OLETF rats. Taken together, these results suggest that (1) whole-body branched-chain alpha-keto acid oxidation capacity is extremely reduced in OLETF rats independently of diabetes development, (2) the aging process decreases BCKDC activity and protein abundance in the liver of normal rats, and (3) differential posttranscriptional regulation for the subunits of BCKDC may exist in skeletal muscle.
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Affiliation(s)
- Gustavo Bajotto
- Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan
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10
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Abstract
Autophagy is a process of cellular "self-eating" in which portions of cytoplasm are sequestered within double-membrane cytosolic vesicles termed autophagosomes. The autophagosome cargo is delivered to the lysosome, broken down, and the resulting amino acids recycled after release back into the cytosol. Autophagy occurs in all eukaryotes and can be up-regulated in response to various nutrient limitations. Under these conditions, autophagy may become essential for viability. In addition, autophagy plays a role in certain diseases, acting to prevent some types of neurodegeneration and cancer, and in the elimination of invading pathogens. We review the current information on the mechanism of autophagy, with a focus on its role in protein metabolism and intracellular homeostasis.
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Affiliation(s)
- Noboru Mizushima
- Department of Physiology and Cell Biology, Tokyo Medical and Dental University, Tokyo 113-8519, Japan.
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Lynch CJ, Halle B, Fujii H, Vary TC, Wallin R, Damuni Z, Hutson SM. Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR. Am J Physiol Endocrinol Metab 2003; 285:E854-63. [PMID: 12812918 DOI: 10.1152/ajpendo.00153.2003] [Citation(s) in RCA: 88] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Leucine has been shown to stimulate adipose tissue protein synthesis in vivo as well as leptin secretion, protein synthesis, hyper-plastic growth, and tissue morphogenesis in in vitro experiments using freshly isolated adipocytes. Recently, others have proposed that leucine oxidation in the mitochondria may be required to activate the mammalian target of rapamycin (mTOR), the cytosolic Ser/Thr protein kinase that appears to mediate some of these effects. The first irreversible and rate-limiting step in leucine oxidation is catalyzed by the branched-chain alpha-keto acid dehydrogenase (BCKD) complex. The activity of this complex is regulated acutely by phosphorylation of the E1alpha-subunit at Ser293 (S293), which inactivates the complex. Because the alpha-keto acid of leucine regulates the activity of BCKD kinase, it has been suggested as a potential target for leucine regulation of mTOR. To study the regulation of BCKD phosphorylation and its potential link to mTOR activation, a phosphopeptide-specific antibody recognizing this site was developed and characterized. Phospho-S293 (pS293) immunoreactivity in liver corresponded closely to diet-induced changes in BCKD activity state. Immunoreactivity was also increased in TREMK-4 cells after the induction of BCKD kinase by a drug-inducible promoter. BCKD S293 phosphorylations in adipose tissue and gastrocnemius (which is mostly inactive in vivo) were similar. This suggests that BCKD complex in epididymal adipose tissue from food-deprived rats is mostly inactive (unable to oxidize leucine), as is the case in muscle. To begin to test the leucine oxidation hypothesis of mTOR activation, the dose-dependent effects of orally administered leucine on acute activation of S6K1 (an mTOR substrate) and BCKD were compared using the pS293 antibodies. Increasing doses of leucine directly correlated with increases in plasma leucine concentration. Phosphorylation of S6K1 (Thr389, the phosphorylation site leading to activation) in adipose tissue was maximal at a dose of leucine that increased plasma leucine approximately threefold. Changes in BCKD phosphorylation state required higher plasma leucine concentrations. The results seem more consistent with a role for BCKD and BCKD kinase in the activation of leucine metabolism/oxidation than in the activation of the leucine signal to mTOR.
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Affiliation(s)
- Christopher J Lynch
- Department of Cellular & Molecular Physiology (MC H166, Rm C4757), Penn State College of Medicine, 500 University Drive, Hershey, PA 17033, USA.
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12
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Nakai N, Kobayashi R, Popov KM, Harris RA, Shimomura Y. Determination of branched-chain alpha-keto acid dehydrogenase activity state and branched-chain alpha-keto acid dehydrogenase kinase activity and protein in mammalian tissues. Methods Enzymol 2001; 324:48-62. [PMID: 10989417 DOI: 10.1016/s0076-6879(00)24218-3] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
Affiliation(s)
- N Nakai
- Research Center of Health, Physical Fitness and Sports, Nagoya University, Japan
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Olde Damink SW, Dejong CH, Deutz NE, van Berlo CL, Soeters PB. Upper gastrointestinal bleeding: an ammoniagenic and catabolic event due to the total absence of isoleucine in the haemoglobin molecule. Med Hypotheses 1999; 52:515-9. [PMID: 10459831 DOI: 10.1054/mehy.1998.0026] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Upper gastrointestinal bleeding causes increased urea concentrations in patients with normal liver function and high ammonia concentrations in patients with impaired liver function. This ammoniagenesis may precipitate encephalopathy. The haemoglobin molecule is unique because it lacks the essential amino acid isoleucine and has high amounts of leucine and valine. Upper gastrointestinal bleeding therefore presents the gut with protein of very low biologic value, which may be the stimulus to induce a cascade of events culminating in net catabolism. This may influence the function of rapidly dividing cells and short half-life proteins. We hypothesize that, following a variceal bleed in a cirrhotic patient, the lack of isoleucine in blood protein is the cause of the exaggerated ammoniagenesis and catabolism. We propose that intravenous administration of isoleucine may serve as a simple therapeutic that transforms blood protein in a balanced protein, resulting in only a short-lived rise in ammonia and urea production, and preventing interference with protein synthesis.
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Affiliation(s)
- S W Olde Damink
- Department of Surgery, Academic Hospital Maastricht, Maastricht University, The Netherlands
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Abstract
A radiochemical assay was developed to measure pyruvate dehydrogenase complex (PDC) activity in liver and heart without interference by branched-chain 2-oxo acid dehydrogenase (BCODH). Decarboxylation of pyruvate by BCODH was eliminated by using low pyruvate concentration (0.5 mM), a preferred substrate for BCODH (3-methyl-2-oxopentanoate) that is not used by PDC, and a competitive inhibitor of BCODH, dichloroacetate. This method was validated by assaying a combination of both purified enzymes and tissue homogenates with known amounts of added BCODH. The actual percentage of active PDC decreased after 48 h starvation from 13.6 to 3.1 in liver and from 77.1 to 9.0 in heart. Total PDC activity (munits of PDC/units of citrate synthase) in starved rats was increased by 34% in liver and decreased by 23% in heart. Total PDC activity (munits/g wet wt.) in fed- and starved-rat liver was 0.8 and 1.3, and in heart was 6.6 and 5.8, respectively.
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Affiliation(s)
- R Paxton
- Department of Physiology and Pharmacology, Auburn University, AL 36849-5520
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