Alanine and inter-organ relationships in branched-chain amino and 2-oxo acid metabolism. Review

Comparison of chicken breast quality characteristics and metabolites due to different rearing environments and refrigerated storage

The objective of the present study was to compare the breast meat quality and metabolomic characteristics from broilers that were raised in conventional (conventional farm reared-broilers; CB, n = 20) and legally approved animal welfare farms (welfare farm reared-broilers; WB, n = 20) in aerobic cold storage (1, 3, 5, and 7 d). Compared to CB chickens, the WB chickens had a larger floor size as well as lower stocking density, atmospheric ammonia, and nipple-shared chicken counts. The results demonstrated significantly higher pH, L^⁎- and b^⁎-value, and lower shear force in CB compared to WB during cold storage. Using ¹H NMR analysis, 25 compounds were identified in the chicken breast meat. Partial least square-discriminant analysis (PLS-DA) was performed based on the identified metabolites. The content of 15 metabolites (1 di-peptide, 9 free amino acids, 2 glycolytic potential-related products, 2 nucleotide-related products, and 1 organic acid) was significantly different due to the rearing environment (CB vs. WB). Among them, all free amino acids were higher in CB than in WB. Six free amino acids (glycine, isoleucine, leucine, phenylalanine, valine, and β-alanine) had variable importance in projection (VIP) score >1, regardless of the number of cold storage days. Therefore, these compounds in the breast meat may be used as potential markers to determine the rearing environment of broilers. Also, this result might be an indication of stress-related meat quality changes in broilers.

Plasma Metabolomics Reveals Pathogenesis of Retained Placenta in Dairy Cows

The complex etiology and pathogenesis of retained placenta (RP) bring huge challenges for researchers and clinical veterinarians in investigating the pathogenesis and treatment schedule. This study aims to investigate the pathogenesis of RP in dairy cows by plasma metabolomics. As subjects, 10 dairy cows with RP and 10 healthy dairy cows were enrolled according to strict enrollment criteria. Imbalanced antioxidant capacity, reduced Th1/Th2 cytokine ratio, and deregulation of total bilirubin (T-bil), alkaline phosphatase (ALP), and reproductive hormones were shown in dairy cows with RP by detecting biochemical indicators, oxidation and antioxidant markers, and cytokines in serum. Plasma metabolites were detected and analyzed by a liquid chromatography–mass spectrometry (LC–MS) system coupled with multivariate statistical analysis software. A total of 23 potential biomarkers were uncovered in the plasma of dairy cows with RP. The metabolic pathways involved in these potential biomarkers are interconnected, and the conversion, utilization, and excretion of nitrogen were disturbed in dairy cows with RP. Moreover, these potential biomarkers are involved in the regulation of antioxidant capacity, inflammation, and autocrine or paracrine hormone. All of these findings suggest that an imbalance of these potential biomarkers might be responsible for the imbalanced antioxidant capacity, reduced Th1/Th2 cytokine ratio, and deregulation of reproductive hormones in dairy cows with RP. The regulation of metabolic pathways involved in these potential biomarkers represents a promising therapeutic strategy for RP.

Why Are Branched-Chain Amino Acids Increased in Starvation and Diabetes?

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.

Natural Isotope Abundances of Carbon and Nitrogen in Tissue Proteins and Amino Acids as Biomarkers of the Decreased Carbohydrate Oxidation and Increased Amino Acid Oxidation Induced by Caloric Restriction under a Maintained Protein Intake in Obese Rats

A growing body of evidence supports a role for tissue-to-diet ¹⁵N and ¹³C discrimination factors (Δ¹⁵N and Δ¹³C), as biomarkers of metabolic adaptations to nutritional stress, but the underlying mechanisms remain poorly understood. In obese rats fed ad libitum or subjected to gradual caloric restriction (CR), under a maintained protein intake, we measured Δ¹⁵N and Δ¹³C levels in tissue proteins and their constitutive amino acids (AA) and the expression of enzymes involved in the AA metabolism. CR was found to lower protein mass in the intestine, liver, heart and, to a lesser extent, some skeletal muscles. This was accompanied by Δ¹⁵N increases in urine and the protein of the liver and plasma, but Δ¹⁵N decreases in the proteins of the heart and the skeletal muscles, alongside Δ¹³C decreases in all tissue proteins. In Lys, Δ¹⁵N levels rose in the plasma, intestine, and some muscles, but fell in the heart, while in Ala, and to a lesser extent Glx and Asx, Δ¹³C levels fell in all these tissues. In the liver, CR was associated with an increase in the expression of genes involved in AA oxidation. During CR, the parallel rises of Δ¹⁵N in urine, liver, and plasma proteins reflected an increased AA catabolism occurring at the level of the liver metabolic branch point, while Δ¹⁵N decreases in cardiac and skeletal muscle proteins indicated increased protein and AA catabolism in these tissues. Thus, an increased protein and AA catabolism results in opposite Δ¹⁵N effects in splanchnic and muscular tissues. In addition, the Δ¹³C decrease in all tissue proteins, reflects a reduction in carbohydrate (CHO) oxidation and routing towards non-indispensable AA, to achieve fuel economy.

Amino acid homeostasis and signalling in mammalian cells and organisms

Cells have a constant turnover of proteins that recycle most amino acids over time. Net loss is mainly due to amino acid oxidation. Homeostasis is achieved through exchange of essential amino acids with non-essential amino acids and the transfer of amino groups from oxidised amino acids to amino acid biosynthesis. This homeostatic condition is maintained through an active mTORC1 complex. Under amino acid depletion, mTORC1 is inactivated. This increases the breakdown of cellular proteins through autophagy and reduces protein biosynthesis. The general control non-derepressable 2/ATF4 pathway may be activated in addition, resulting in transcription of genes involved in amino acid transport and biosynthesis of non-essential amino acids. Metabolism is autoregulated to minimise oxidation of amino acids. Systemic amino acid levels are also tightly regulated. Food intake briefly increases plasma amino acid levels, which stimulates insulin release and mTOR-dependent protein synthesis in muscle. Excess amino acids are oxidised, resulting in increased urea production. Short-term fasting does not result in depletion of plasma amino acids due to reduced protein synthesis and the onset of autophagy. Owing to the fact that half of all amino acids are essential, reduction in protein synthesis and amino acid oxidation are the only two measures to reduce amino acid demand. Long-term malnutrition causes depletion of plasma amino acids. The CNS appears to generate a protein-specific response upon amino acid depletion, resulting in avoidance of an inadequate diet. High protein levels, in contrast, contribute together with other nutrients to a reduction in food intake.

The evolutionary benefit of insulin resistance

Insulin resistance is perceived as deleterious, associated with conditions as the metabolic syndrome, type 2 diabetes mellitus and critical illness. However, insulin resistance is evolutionarily well preserved and its persistence suggests that it benefits survival. Insulin resistance is important in various states such as starvation, immune activation, growth and cancer, to spare glucose for different biosynthetic purposes such as the production of NADPH, nucleotides in the pentose phosphate pathway and oxaloacetate for anaplerosis. In these conditions, total glucose oxidation by the tricarboxylic acid cycle is actually low and energy demands are largely met by fatty acid and ketone body oxidation. This beneficial role of insulin resistance has consequences for treatment and research. Insulin resistance should be investigated at the cellular, tissue and whole organism level. The metabolic pathways discussed here, should be integrated in the accepted and valid mechanistic events of insulin resistance before interfering with them to promote insulin sensitivity at any cost.

Disruption of BCAA metabolism in mice impairs exercise metabolism and endurance

Exercise enhances branched-chain amino acid (BCAA) catabolism, and BCAA supplementation influences exercise metabolism. However, it remains controversial whether BCAA supplementation improves exercise endurance, and unknown whether the exercise endurance effect of BCAA supplementation requires catabolism of these amino acids. Therefore, we examined exercise capacity and intermediary metabolism in skeletal muscle of knockout (KO) mice of mitochondrial branched-chain aminotransferase (BCATm), which catalyzes the first step of BCAA catabolism. We found that BCATm KO mice were exercise intolerant with markedly decreased endurance to exhaustion. Their plasma lactate and lactate-to-pyruvate ratio in skeletal muscle during exercise and lactate release from hindlimb perfused with high concentrations of insulin and glucose were significantly higher in KO than wild-type (WT) mice. Plasma and muscle ammonia concentrations were also markedly higher in KO than WT mice during a brief bout of exercise. BCATm KO mice exhibited 43-79% declines in the muscle concentration of alanine, glutamine, aspartate, and glutamate at rest and during exercise. In response to exercise, the increments in muscle malate and alpha-ketoglutarate were greater in KO than WT mice. While muscle ATP concentration tended to be lower, muscle IMP concentration was sevenfold higher in KO compared with WT mice after a brief bout of exercise, suggesting elevated ammonia in KO is derived from the purine nucleotide cycle. These data suggest that disruption of BCAA transamination causes impaired malate/aspartate shuttle, thereby resulting in decreased alanine and glutamine formation, as well as increases in lactate-to-pyruvate ratio and ammonia in skeletal muscle. Thus BCAA metabolism may regulate exercise capacity in mice.

Branched-chain amino acid metabolon: interaction of glutamate dehydrogenase with the mitochondrial branched-chain aminotransferase (BCATm)

The catabolic pathway for branched-chain amino acids includes deamination followed by oxidative decarboxylation of the deaminated product branched-chain alpha-keto acids, catalyzed by the mitochondrial branched-chain aminotransferase (BCATm) and branched-chain alpha-keto acid dehydrogenase enzyme complex (BCKDC). We found that BCATm binds to the E1 decarboxylase of BCKDC, forming a metabolon that allows channeling of branched-chain alpha-keto acids from BCATm to E1. The protein complex also contains glutamate dehydrogenase (GDH1), 4-nitrophenylphosphatase domain and non-neuronal SNAP25-like protein homolog 1, pyruvate carboxylase, and BCKDC kinase. GDH1 binds to the pyridoxamine 5'-phosphate (PMP) form of BCATm (PMP-BCATm) but not to the pyridoxal 5'-phosphate-BCATm and other metabolon proteins. Leucine activates GDH1, and oxidative deamination of glutamate is increased further by addition of PMP-BCATm. Isoleucine and valine are not allosteric activators of GDH1, but in the presence of 5'-phosphate-BCATm, they convert BCATm to PMP-BCATm, stimulating GDH1 activity. Sensitivity to ADP activation of GDH1 was unaffected by PMP-BCATm; however, addition of a 3 or higher molar ratio of PMP-BCATm to GDH1 protected GDH1 from GTP inhibition by 50%. Kinetic results suggest that GDH1 facilitates regeneration of the form of BCATm that binds to E1 decarboxylase of the BCKDC, promotes metabolon formation, branched-chain amino acid oxidation, and cycling of nitrogen through glutamate.

Short-term fasting, seizure control and brain amino acid metabolism

The ketogenic diet is an effective treatment for seizures, but the mechanism of action is unknown. It is uncertain whether the anti-epileptic effect presupposes ketosis, or whether the restriction of calories and/or carbohydrate might be sufficient. We found that a relatively brief (24 h) period of low glucose and low calorie intake significantly attenuated the severity of seizures in young Sprague-Dawley rats (50-70 gms) in whom convulsions were induced by administration of pentylenetetrazole (PTZ). The blood glucose concentration was lower in animals that received less dietary glucose, but the brain glucose level did not differ from control blood [3-OH-butyrate] tended to be higher in blood, but not in brain, of animals on a low-glucose intake. The concentration in brain of glutamine increased and that of alanine declined significantly with low-glucose intake. The blood alanine level fell more than that of brain alanine, resulting in a marked increase ( approximately 50%) in the brain:blood ratio for alanine. In contrast, the brain:blood ratio for leucine declined by about 35% in the low-glucose group. When animals received [1-(13)C]glucose, a metabolic precursor of alanine, the appearance of (13)C in alanine and glutamine increased significantly relative to control. The brain:blood ratio for [(13)C]alanine exceeded 1, indicating that the alanine must have been formed in brain and not transported from blood. The elevated brain(alanine):blood(alanine) could mean that a component of the anti-epileptic effect of low carbohydrate intake is release of alanine from brain-to-blood, in the process abetting the disposal of glutamate, excess levels of which in the synaptic cleft would contribute to the development of seizures.

Functional glycerol kinase activity and the possibility of a major role for glyceroneogenesis in mammalian skeletal muscle

According to textbook descriptions of glycerol metabolism, liver and kidney are the only tissues that express significant glycerol kinase activity. Thus esterification of fatty acids to triglycerides in peripheral tissues such as skeletal muscle and adipose tissue is presumed to be dependent on the synthesis of glycerol-3-phosphate from glucose. This report describes exciting new data indicating that, although low, the glycerol kinase activity of skeletal muscle is functional. Interestingly, the results also suggest that neither glycerol nor glucose is the major substrate for the synthesis of muscle triglyceride glycerol. Rather, glyceroneogenesis, the synthesis of glycerol-3-phosphate from lactate, may play an as yet under-appreciated, but quantitatively important, role.

[The human kidney as an important producer of glucose]

BACKGROUND

According to current textbook knowledge the liver is the exclusive site of glucose production in postabsorptive humans. Although many animal and in-vitro data have documented that the kidney is capable of gluconeogenesis, production of glucose by the human kidney has been regarded as negligible to date. This traditional perception is based on methodologically inadequate net balance studies, which other than after a prolonged fast or during metabolic acidosis showed no significant net renal glucose release.

STUDIES

Recent tracer studies, however, showing a renal glucose production accounting for 25% of systemic glucose production, have refuted this view. glucose production by the human kidney is stimulated by epinephrine and inhibited by insulin. Glucagon stimulates hepatic but not renal glucose production. The most important renal gluconeogenic precursors are lactate, glutamine and glycerol. The implications of these recent findings on the understanding of the physiology and pathophysiology of human glucose metabolism are discussed.

CONCLUSION

Magnitude and regulation of renal glucose production have important consequences for the intermediary metabolism, counterregulation of hypoglycemia, glucose metabolism of uremia and the pathophysiology of type I and type II diabetes.

Estimation of glucose-alanine-lactate-glutamine cycles in postabsorptive humans: role of skeletal muscle

To evaluate transfer of carbon between plasma glucose and plasma alanine (glucose-alanine cycle) and lactate (Cori cycle), to assess the contribution of skeletal muscle to these cycles, and to determine whether a glucose-glutamine cycle exists in postabsorptive humans, we infused 11 normal overnight-fasted volunteers with [2-3H]glucose, [6-14C]glucose, and [3-13C]alanine to isotopic steady state and in 7 of these simultaneously measured forearm net balance, uptake, and release of labeled and unlabeled glucose, lactate, and alanine. We found that 40.9 +/- 3.3, 66.8 +/- 3.2, and 13.4 +/- 1.1%, respectively, of plasma alanine, lactate, and glutamine carbon came from plasma glucose. More plasma glucose was converted to plasma alanine than could be derived from plasma alanine (1.89 +/- 0.20 vs. 1.48 +/- 0.15 mumol.kg-1.min-1, P < 0.001). A similar direction of net carbon flux was found for lactate (8.5 vs. 4.2 mumol.kg-1.min-1), with only glutamine adding more carbon to plasma glucose than was received from it (1.0 vs. 0.75 mumol.kg-1.min-1). Skeletal muscle accounted for 50.2 +/- 3.9 and 45.5 +/- 5.7% of the overall appearance of alanine and lactate in plasma and 54.2 +/- 5.4 and 36.4 +/- 4.2% of their respective origins from plasma glucose. Skeletal muscle release of alanine and lactate that had been formed from plasma glucose accounted for 19.1 +/- 2.1 and 48.4 +/- 4.8%, respectively, of muscle glucose uptake and 42.4 +/- 5.5 and 49.9 +/- 5.8% of the overall release of alanine and lactate from muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Endurance training alters alanine and glutamine release from muscle during contractions

The release of alanine and glutamine from perfused muscle of trained and control animals was investigated. Release rates did not differ between trained and control muscle at rest. During contractions in trained muscle, alanine release was higher than at rest, while glutamine release was transiently increased. Phenylalanine release did not differ between trained and control muscle, implying that protein degradation was not increased in trained muscle. The muscle cellular adaptations to training include a selective modification of amino acid output, which could potentially influence gluconeogenic flux and alter muscle ammonia levels during contractions.

Amino acid metabolism during exercise and following endurance training

Exercise results in marked alterations in amino acid metabolism within the body. The branched-chain amino acids, especially leucine, are particularly important since they contribute as energy substrates and as nitrogen donors in the formation of alanine, glutamine and aspartate. Leucine oxidation increases during whole-body exercise. Nonetheless, leucine's contribution as a muscle energy substrate is amll, being 3 to 4% at rest, and even lower (1%) during exercise. Traditional energy substrates (carbohydrates, lipid) remain most important. These rates of leucine oxidation can be readily attributed to skeletal muscle. Following endurance training, whole-body leucine oxidation is increased at rest and during exercise. Since its oxidation by muscle is not augmented, this whole-body increase is not due to muscle. Thus, other tissues within the body (i.e. liver) must account for this. Comparisons of leucine oxidation in rats and humans indicate that species differences exist. Much larger increases in leucine oxidation are brought about by exercise in humans. Calculations based on steady-state rates of leucine oxidation at rest and during exercise indicate that the recommended dietary intake of leucine is inadequate, since it is lower than measured whole-body rates of leucine oxidation. This inadequacy is exacerbated in individuals who are physically active.

Fuel selection and carbon flux during the starved-to-fed transition

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The effect of ketone bodies on alanine and glutamine metabolism in isolated skeletal muscle from the fasted chick

The effects of ketone bodies on the metabolism of alanine and glutamine were studied in isolated extensor digitorum communis (EDC) muscles from 24 h-fasted chicks. (1) Acetoacetate and DL-beta-hydroxybutyrate (4 mM) markedly inhibit branched-chain amino acid (BCAA) transamination and alanine formation. (2) Ketone bodies (1 and 4 mM) increase the intracellular concentration and release of glutamate and glutamine, suggesting that inhibition of BCAA transamination does not limit intracellular availability of glutamate for alanine synthesis. (3) Ketone bodies (1 and 4 mM) do not affect glucose uptake by muscles, but decrease the rate of glycolysis as well as the intracellular concentration and release of pyruvate in muscles. (4) Addition of 12 mM-glucose increases the formation of alanine in muscles incubated in the absence of ketone bodies, but has no effect in muscles incubated in the presence of 4 mM ketone bodies. (5) Addition of 5 mM-pyruvate to the media prevents the inhibiting effect of ketone bodies on BCAA transamination and alanine synthesis. These results suggest that ketone bodies decrease alanine synthesis by limiting the intracellular availability of pyruvate, owing to inhibition of glycolysis, and inhibit BCAA transamination by decreasing the intracellular concentration of amino-group acceptors such as pyruvate in EDC muscles from fasted chicks.

Carcass glycogen repletion on carbohydrate re-feeding after starvation

In mice, the response of carcass glycogen to glucose re-feeding after starvation is biphasic. The initial repletive phase is followed by partial (greater than 50%) glycogen mobilization. This turnover of carcass glycogen in response to carbohydrate re-feeding may play an important role in the provision of C3 precursors for hepatic glycogen synthesis.

3-Hydroxyisobutyrate dehydrogenase, an impurity in commercial 3-hydroxybutyrate dehydrogenase

The enzymic determination of D-3-hydroxybutyrate and acetoacetate normally involves the use of 3-hydroxybutyrate dehydrogenase (HBDH, EC 1.1.1.30) of bacterial origin. We show that HBDH from Rhodopseudomonas spheroides (BCL, grade II) contains a 3-hydroxyisobutyrate dehydrogenase (HIBDH) activity: activity with 3-hydroxyisobutyrate as substrate was greater than 10% of that with 3-hydroxybutyrate. However, HBDH could be prepared essentially free of HIBDH activity by incubation at 37 degrees C in the presence of 1 mM-CaCl2, to produce an enzyme preparation that may be used for the specific determination of 3-hydroxybutyrate. Use of the purified enzyme preparations indicated that a major product of valine metabolism in hemidiaphragms from 40 h-starved rats was 3-hydroxyisobutyrate rather than 3-hydroxybutyrate.

Effect of exercise intensity and starvation on activation of branched-chain keto acid dehydrogenase by exercise

Branched-chain keto acid (BCKA) dehydrogenase activity was examined in rat skeletal muscle as a function of exercise intensity and nutritional status. The activity of BCKA dehydrogenase increased with increasing exercise intensity, showing increases over resting values of 76, 172, and 245% at 10, 20, and 30 m X min-1. The exercise-induced increase in BCKA dehydrogenase activity was the same in the gastrocnemius and in the quadriceps muscles. Rapid removal of the muscle after death is essential because the activity of BCKA dehydrogenase decreased rapidly after death. Thus the likely reasons Wagenmakers et al. (Biochem. J. 223: 815-821, 1984) found exercise caused a much smaller increase in BCKA dehydrogenase activity than Kasperek et al. [Am. J. Physiol. 248 (Regulatory Integrative Comp. Physiol. 17): R166-R171, 1985] are differences in muscle removal time and the duration of exercise. Starvation for 24 h before exercise increased the exercise-induced activation of BCKA dehydrogenase by 160%, which suggests that the increased BCKA dehydrogenase activity is in response to an increased requirement for citric acid cycle intermediates.

Amino acid catabolism by perfused rat hindquarters: degradation of threonine and isoleucine

Rat hindquarters were perfused without added substrate other than trace amounts of [U-14C]threonine or [U-14C]isoleucine. Comparison of incorporation of radiolabel into some nonessential amino acids, citrate cycle intermediates, and lactate is presented. Activities of three enzymes for the initial reactions in threonine degradation are reported. It is concluded that skeletal muscle catabolizes threonine, and that the latter is a potential source of carbon for glucogenic precursors for the liver. In contrast, label from isoleucine was incorporated into glutamate, glutamine, and alanine much more than was that from threonine. Large amounts of organic acids accumulated, and more than 60% of total radioactivity was lost as CO2 during a 2-h perfusion period.