What Constitutes a Gluconeogenic Precursor?

Characterizing substrate utilization during the fasted state using plasma high-resolution metabolomics

OBJECTIVES

High-resolution metabolomics enables global assessment of metabolites and molecular pathways underlying physiologic processes, including substrate utilization during the fasted state. The clinical index for substrate utilization, respiratory exchange ratio (RER), is measured via indirect calorimetry. The aim of this pilot study was to use metabolomics to identify metabolic pathways and plasma metabolites associated with substrate utilization in healthy, fasted adults.

METHODS

This cross-sectional study included 33 adults (mean age 27.7 ± 4.9 y, mean body mass index 24.8 ± 4 kg/m2). Participants underwent indirect calorimetry to determine resting RER after an overnight fast. Untargeted metabolomics was performed on fasted plasma samples using dual-column liquid chromatography and ultra-high-resolution mass spectrometry. Linear regression and pathway enrichment analyses identified pathways and metabolites associated with substrate utilization measured with indirect calorimetry.

RESULTS

RER was significantly associated with 1389 metabolites enriched within 13 metabolic pathways (P < 0.05). Lipid-related findings included general pathways, such as fatty acid activation, and specific pathways, such as C21-steroid hormone biosynthesis and metabolism, butyrate metabolism, and carnitine shuttle. Amino acid pathways included those central to metabolism, such as glucogenic amino acids, and pathways needed to maintain reduction-oxidation reactions, such as methionine and cysteine metabolism. Galactose and pyrimidine metabolism were also associated with RER (all P < 0.05).

CONCLUSIONS

The fasting plasma metabolome reflects the diverse macronutrient pathways involved in carbohydrate, amino acid, and lipid metabolism during the fasted state in healthy adults. Future studies should consider the utility of metabolomics to profile individual nutrient requirements and compare findings reported here to clinical populations.

Inborn errors of amino acid metabolism - from underlying pathophysiology to therapeutic advances

Amino acids are organic molecules that serve as basic substrates for protein synthesis and have additional key roles in a diverse array of cellular functions, including cell signaling, gene expression, energy production and molecular biosynthesis. Genetic defects in the synthesis, catabolism or transport of amino acids underlie a diverse class of diseases known as inborn errors of amino acid metabolism. Individually, these disorders are rare, but collectively, they represent an important group of potentially treatable disorders. In this Clinical Puzzle, we discuss the pathophysiology, clinical features and management of three disorders that showcase the diverse clinical presentations of disorders of amino acid metabolism: phenylketonuria, lysinuric protein intolerance and homocystinuria due to cystathionine β-synthase (CBS) deficiency. Understanding the biochemical perturbations caused by defects in amino acid metabolism will contribute to ongoing development of diagnostic and management strategies aimed at improving the morbidity and mortality associated with this diverse group of disorders.

Roles of malate and aspartate in gluconeogenesis in various physiological and pathological states

Gluconeogenesis, a pathway for glucose synthesis from non-carbohydrate substances, begins with the synthesis of oxaloacetate (OA) from pyruvate and intermediates of citric acid cycle in hepatocyte mitochondria. The traditional view is that OA does not cross the mitochondrial membrane and must be shuttled to the cytosol, where most enzymes involved in gluconeogenesis are compartmentalized, in the form of malate. Thus, the possibility of transporting OA in the form of aspartate has been ignored. In the article is shown that malate supply to the cytosol increases only when fatty acid oxidation in the liver is activated, such as during starvation or untreated diabetes. Alternatively, aspartate synthesized from OA by mitochondrial aspartate aminotransferase (AST) is transported to the cytosol in exchange for glutamate via the aspartate-glutamate carrier 2 (AGC2). If the main substrate for gluconeogenesis is an amino acid, aspartate is converted to OA via urea cycle, therefore, ammonia detoxification and gluconeogenesis are simultaneously activated. If the main substrate is lactate, OA is synthesized by cytosolic AST, glutamate is transported to the mitochondria through AGC2, and nitrogen is not lost. It is concluded that, compared to malate, aspartate is a more suitable form of OA transport from the mitochondria for gluconeogenesis.

Recent Advances in Synthesis of Graphite from Agricultural Bio-Waste Material: A Review

Graphitic carbon is a valuable material that can be utilized in many fields, such as electronics, energy storage and wastewater filtration. Due to the high demand for commercial graphite, an alternative raw material with lower costs that is environmentally friendly has been explored. Amongst these, an agricultural bio-waste material has become an option due to its highly bioactive properties, such as bioavailability, antioxidant, antimicrobial, in vitro and anti-inflammatory properties. In addition, biomass wastes usually have high organic carbon content, which has been discovered by many researchers as an alternative carbon material to produce graphite. However, there are several challenges associated with the graphite production process from biomass waste materials, such as impurities, the processing conditions and production costs. Agricultural bio-waste materials typically contain many volatiles and impurities, which can interfere with the synthesis process and reduce the quality of the graphitic carbon produced. Moreover, the processing conditions required for the synthesis of graphitic carbon from agricultural biomass waste materials are quite challenging to optimize. The temperature, pressure, catalyst used and other parameters must be carefully controlled to ensure that the desired product is obtained. Nevertheless, the use of agricultural biomass waste materials as a raw material for graphitic carbon synthesis can reduce the production costs. Improving the overall cost-effectiveness of this approach depends on many factors, including the availability and cost of the feedstock, the processing costs and the market demand for the final product. Therefore, in this review, the importance of biomass waste utilization is discussed. Various methods of synthesizing graphitic carbon are also reviewed. The discussion ranges from the conversion of biomass waste into carbon-rich feedstocks with different recent advances to the method of synthesis of graphitic carbon. The importance of utilizing agricultural biomass waste and the types of potential biomass waste carbon precursors and their pre-treatment methods are also reviewed. Finally, the gaps found in the previous research are proposed as a future research suggestion. Overall, the synthesis of graphite from agricultural bio-waste materials is a promising area of research, but more work is needed to address the challenges associated with this process and to demonstrate its viability at scale.

Effect of macronutrient and micronutrient manipulation on avian blood glucose concentration: A systematic review

Animals with natural protections against diabetes complications may provide clues to improve human health. Birds are unique in their ability to avoid hyperglycemia-associated complications (e.g., glycation and oxidative stress) despite having naturally high blood glucose (BG) concentrations. This makes them useful models to elucidate strategies to prevent and/or treat diabetes-related complications in mammals. As diet plays a key role in BG concentration and diabetes risk, this systematic review aimed to summarize the effects of macro and micronutrient manipulation on avian BG. Three databases were searched (PubMed, SCOPUS, and Web of Science) for articles that met inclusion criteria: altered at least one nutrient and measured BG in at least one avian species. The search yielded 91 articles that produced 128 datasets (i.e., one nutrient manipulation in one sample). Across all macronutrient manipulations (n = 69 datasets), 62% reported no change in BG and 23% measured an increase (p < 0.001). Within the macronutrient groups (carbohydrate, lipid, protein, and mixed) most datasets showed no change in BG (67%, 62%, 52%, and 86%, respectively). Across micronutrient manipulations (n = 59 datasets), 51% demonstrated no change and 41% decreased BG (p < 0.001). While manipulations that altered vitamin intake largely produced no change in BG (62%), 48% of datasets examining altered mineral intake found no change and 46% decreased BG. Chromium was the most studied micronutrient (n = 24 datasets), where 67% of datasets reported a decrease in BG. These results suggest birds are largely able to maintain blood glucose homeostasis in response to altered nutrient intake indicative of dietary flexibility.

Long-distance transit alters liver and skeletal muscle physiology of beef cattle

Transportation of cattle is necessary but negatively impacts animal health and production efficiency. To gain a better understanding of the physiological responses to long-distance road transit, 36 crossbred beef steers (324 ± 36 kg) were randomly assigned to treatments (n = 12 steers/treatment): no transit and ad libitum access to feed and water (CON), no transit but deprived of feed and water for 18 h (DEPR), or road transit and no access to feed or water for 18 h (1 790 km; TRANS). Blood, liver, and muscle (longissimus dorsi) samples were collected pre- and post-treatment for analysis of blood metabolites, blood leukocyte profiles, blood markers of oxidative stress, and tissue antioxidant enzyme activity. Additionally, discovery-based metabolomics and proteomics analyses were performed on tissue samples collected immediately post-treatment (d 1). Data (except for omics) were analyzed using ProcMixed of SAS 9.4 with the fixed effect of treatment and steer as the experimental unit. Omics data were analyzed using MetaboAnalyst; metabolites and proteins of interest were identified based on a fold change threshold of 1.20 and t-test P-value of 0.10. On d 1, percent of pretreatment BW and DM intake were least for TRANS steers (P ≤ 0.06). Percent of pretreatment BW remained lesser for TRANS steers on d 8 (P = 0.05). Serum haptoglobin was greatest for TRANS steers immediately post-treatment (P = 0.02). Additionally, TRANS steers exhibited the greatest increase in the neutrophil to lymphocyte ratio and serum non-esterified fatty acids during the treatment period (P < 0.01), indicating TRANS steers experienced a more robust inflammatory and neuroendocrine response. Immediately post-treatment, liver superoxide dismutase activity tended to be greatest for both DEPR and TRANS (P = 0.07) while muscle superoxide dismutase activity was only greatest for TRANS (P = 0.02), suggesting TRANS steers may have experienced more oxidative stress due to the additional physical effort required to stand and maintain balance during transit. The abundance of several proteins (alpha-2-HS-glycoprotein) and metabolites (lactate, citrate, tri-hydroxybutyric acid, and leucine) associated with energy metabolism were altered in the liver and muscle of TRANS. The differential responses for DEPR versus TRANS steers indicate muscle plays an important role in how cattle respond to and recover from transportation stress.

Branched-chain Amino Acids: Catabolism in Skeletal Muscle and Implications for Muscle and Whole-body Metabolism

Branched-chain amino acids (BCAAs) are critical for skeletal muscle and whole-body anabolism and energy homeostasis. They also serve as signaling molecules, for example, being able to activate mammalian/mechanistic target of rapamycin complex 1 (mTORC1). This has implication for macronutrient metabolism. However, elevated circulating levels of BCAAs and of their ketoacids as well as impaired catabolism of these amino acids (AAs) are implicated in the development of insulin resistance and its sequelae, including type 2 diabetes, cardiovascular disease, and of some cancers, although other studies indicate supplements of these AAs may help in the management of some chronic diseases. Here, we first reviewed the catabolism of these AAs especially in skeletal muscle as this tissue contributes the most to whole body disposal of the BCAA. We then reviewed emerging mechanisms of control of enzymes involved in regulating BCAA catabolism. Such mechanisms include regulation of their abundance by microRNA and by post translational modifications such as phosphorylation, acetylation, and ubiquitination. We also reviewed implications of impaired metabolism of BCAA for muscle and whole-body metabolism. We comment on outstanding questions in the regulation of catabolism of these AAs, including regulation of the abundance and post-transcriptional/post-translational modification of enzymes that regulate BCAA catabolism, as well the impact of circadian rhythm, age and mTORC1 on these enzymes. Answers to such questions may facilitate emergence of treatment/management options that can help patients suffering from chronic diseases linked to impaired metabolism of the BCAAs.

Central regulation of glucose metabolism in an insulin-dependent and -independent manner

The central nervous system (CNS) contributes significantly to glucose homeostasis. The available evidence indicates that insulin directly acts on the CNS, in particular the hypothalamus, to regulate hepatic glucose production, thereby controlling whole-body glucose metabolism. Additionally, insulin also acts on the brain to regulate food intake and fat metabolism, which may indirectly regulate glucose metabolism. Studies conducted over the last decade have found that the CNS can regulate glucose metabolism in an insulin-independent manner. Enhancement of central leptin signalling reverses hyperglycaemia in insulin-deficient rodents. Here, I review the mechanisms by which central insulin and leptin actions regulate glucose metabolism. Although clinical studies have shown that insulin treatment is currently indispensable for managing diabetes, unravelling the neuronal mechanisms underlying the central regulation of glucose metabolism will pave the way for the design of novel therapeutic drugs for diabetes.