Metabolism of Propionic Acid in Animal Tissues

Overcompensation of CoA Trapping by Di(2-ethylhexyl) Phthalate (DEHP) Metabolites in Livers of Wistar Rats

Di(2-ethylhexyl) phthalate (DEHP) is commonly used as a plasticizer in various industrial and household plastic products, ensuring widespread human exposures. Its routine detection in human bio-fluids and the propensity of its monoester metabolite to activate peroxisome proliferator activated receptor-α (PPARα) and perturb lipid metabolism implicate it as a metabolic disrupter. In this study we evaluated the effects of DEHP exposure on hepatic levels of free CoA and various CoA esters, while also confirming the metabolic activation to CoA esters and partial β-oxidation of a DEHP metabolite (2-ethyhexanol). Male Wistar rats were exposed via diet to 2% (^w/_w) DEHP for fourteen-days, following which hepatic levels of free CoA and various CoA esters were identified using liquid chromatography-mass spectrometry. DEHP exposed rats showed significantly elevated free CoA and increased levels of physiological, DEHP-derived and unidentified CoA esters. The physiological CoA ester of malonyl-CoA and DEHP-derived CoA ester of 3-keto-2-ethylhexanoyl-CoA were the most highly elevated, at eighteen- and ninety eight-times respectively. We also detected sixteen unidentified CoA esters which may be derivative of DEHP metabolism or induction of other intermediary metabolism metabolites. Our results demonstrate that DEHP is a metabolic disrupter which affects production and sequestration of CoA, an essential cofactor of oxidative and biosynthetic reactions.

The role of lipids in the central nervous system and their pathological implications in amyotrophic lateral sclerosis

Lipids play an important role in the central nervous system (CNS). They contribute to the structural integrity and physical characteristics of cell and organelle membranes, act as bioactive signalling molecules, and are utilised as fuel sources for mitochondrial metabolism. The intricate homeostatic mechanisms underpinning lipid handling and metabolism across two major CNS cell types; neurons and astrocytes, are integral for cellular health and maintenance. Here, we explore the various roles of lipids in these two cell types. Given that changes in lipid metabolism have been identified in a number of neurodegenerative diseases, we also discuss changes in lipid handling and utilisation in the context of amyotrophic lateral sclerosis (ALS), in order to identify key cellular processes affected by the disease, and inform future areas of research.

Neuronal Lipid Metabolism: Multiple Pathways Driving Functional Outcomes in Health and Disease

Lipids are a fundamental class of organic molecules implicated in a wide range of biological processes related to their structural diversity, and based on this can be broadly classified into five categories; fatty acids, triacylglycerols (TAGs), phospholipids, sterol lipids and sphingolipids. Different lipid classes play major roles in neuronal cell populations; they can be used as energy substrates, act as building blocks for cellular structural machinery, serve as bioactive molecules, or a combination of each. In amyotrophic lateral sclerosis (ALS), dysfunctions in lipid metabolism and function have been identified as potential drivers of pathogenesis. In particular, aberrant lipid metabolism is proposed to underlie denervation of neuromuscular junctions, mitochondrial dysfunction, excitotoxicity, impaired neuronal transport, cytoskeletal defects, inflammation and reduced neurotransmitter release. Here we review current knowledge of the roles of lipid metabolism and function in the CNS and discuss how modulating these pathways may offer novel therapeutic options for treating ALS.

Molecular chaperone TRAP1 regulates a metabolic switch between mitochondrial respiration and aerobic glycolysis

TRAP1 (TNF receptor-associated protein), a member of the HSP90 chaperone family, is found predominantly in mitochondria. TRAP1 is broadly considered to be an anticancer molecular target. However, current inhibitors cannot distinguish between HSP90 and TRAP1, making their utility as probes of TRAP1-specific function questionable. Some cancers express less TRAP1 than do their normal tissue counterparts, suggesting that TRAP1 function in mitochondria of normal and transformed cells is more complex than previously appreciated. We have used TRAP1-null cells and transient TRAP1 silencing/overexpression to show that TRAP1 regulates a metabolic switch between oxidative phosphorylation and aerobic glycolysis in immortalized mouse fibroblasts and in human tumor cells. TRAP1-deficiency promotes an increase in mitochondrial respiration and fatty acid oxidation, and in cellular accumulation of tricarboxylic acid cycle intermediates, ATP and reactive oxygen species. At the same time, glucose metabolism is suppressed. TRAP1-deficient cells also display strikingly enhanced invasiveness. TRAP1 interaction with and regulation of mitochondrial c-Src provide a mechanistic basis for these phenotypes. Taken together with the observation that TRAP1 expression is inversely correlated with tumor grade in several cancers, these data suggest that, in some settings, this mitochondrial molecular chaperone may act as a tumor suppressor.

Carbon-13 nuclear magnetic resonance study of metabolism of propionate by Escherichia coli

We have evaluated the use of [1,2-13C2]propionate for the analysis of propionic acid metabolism, based on the ability to distinguish between the methylcitrate and methylmalonate pathways. Studies using propionate-adapted Escherichia coli MG1655 cells were performed. Preservation of the 13C-13C-12C carbon skeleton in labeled alanine and alanine-containing peptides involved in cell wall recycling is indicative of the direct formation of pyruvate from propionate via the methylcitrate cycle, the enzymes of which have recently been demonstrated in E. coli. Additionally, formation of 13C-labeled formate from pyruvate by the action of pyruvate-formate lyase is also consistent with the labeling of pyruvate C-1. Carboxylation of the labeled pyruvate leads to formation of [1,2-13C2]oxaloacetate and to multiply labeled glutamate and succinate isotopomers, also consistent with the flux through the methylcitrate pathway, followed by the tricarboxylic acid (TCA) cycle. Additional labeling of TCA intermediates arises due to the formation of [1-13C]acetyl coenzyme A from the labeled pyruvate, formed via pyruvate-formate lyase. Labeling patterns in trehalose and glycine are also interpreted in terms of the above pathways. The information derived from the [1, 2-13C2]propionate label is contrasted with information which can be derived from singly or triply labeled propionate and shown to be more useful for distinguishing the different propionate utilization pathways via nuclear magnetic resonance analysis.

Odd-numbered medium-chain triglycerides (trinonanoin) in total parenteral nutrition: effects on parameters of fat metabolism in rabbits

Odd-numbered medium-chain triglycerides (MCTs) might combine the advantages of "usual" MCTs applied in clinical nutrition with lower ketogenic action and the release of three carbon units. To test subacute toxicity, trinonanoin/long-chain triglyceride (LCT) (7/3 wt/wt) fat emulsions were given to rabbits (n = 8) for 11 days (7 h/d) within a total parenteral nutrition regimen at a dose of 46.5% of total daily energy. Comparisons were made with rabbits receiving equicaloric amounts of MCT/LCT (7/3, wt/wt) or pure LCT fat emulsions, as well as with orally fed controls. The trinonanoin/LCT emulsion was well tolerated by all animals. Body weight changes showed no statistically significant differences between groups. The enzymatic determination of triglycerides, non-esterified fatty acids, and free glycerol concentrations in plasma samples revealed similar results for both MCT groups. However, ketone body concentrations (3-hydroxybutyrate) were significantly lower after trinonanoin/LCT emulsion administration. In the trinonanoin/LCT group, the plasma concentrations of propionic acid as well as of other short-chain fatty acids continuously increased; on days 10 and 11, elevated amounts of propionic acid were also detected in the urine. The histologic examination of the gut mucosa revealed no distinct differences between groups. On the basis of the presented data, the trinonanoin/LCT emulsion showed no inferiority to "usual" MCT/LCT emulsions. The lower ketogenic effect as well as the marked increase in plasma short-chain fatty acid concentrations may encourage further testing of this substrate for total parenteral nutrition.

[Methylmalonic aciduria. Classification, diagnosis and therapy (author's transl)]

Congenital methylmalonic aciduria (MMA) is a metabolic disorder inherited by an autosomal recessive trait. The metabolic block is located in the catabolic pathway of propionyl-CoA to succinyl-CoA. Biochemically, four enzymatic defects have been recognized, i.e.: 1. Methylmalonyl-CoA racemase. 2. Methylmalonyl-CoA mutase apoenzyme. 3. Synthesis of desoxyadenosyl-cobalamine. 4. Disturbance at an earlier level of cobalamine metabolism which causes defective synthesis of both vitamin B12-coenzymes. These four enzymatic defects express themselves in three ways: non-vitamin B12-dependent MMA (defects 1 and 2); vitamin B12-dependent MMA (defect 3); MMA associated with homocystinuria (defect 4). The various forms of MMA cannot be distinguished clinically from one another. The disorder manifests itself during the first few days to weeks of life. Principal symptoms and signs are: anorexia, vomiting, muscular hypotonia and metabolic acidosis. The diagnosis is established by determination of methylmalonic acid in plasma, cerebrospinal fluid and urine, as well as by assay of enzyme activities in leukocytes, liver tissue or cultured fibroblasts (from biopsied skin). A prenatal diagnosis is feasible by the examination of cultured amnion cells, amniotic fluid and maternal urine. Therapy of non vitamin B12-dependent MMA calls for reduction of protein intake, particularly that of precursors of methylmalonic acid, such as methionine, threonine, isoleucine and valine. The treatment of vitamin B12-dependent forms is accomplished by i.m. injection of high doses of vitamin B12. No definite statement can be made as yet with regard to long-term prognosis and normalcy of mental development in treated children.

Resistance to ketosis during prolonged fasting by rats fed on a diet containing undecanoic acid, an odd-carbon-number fatty acid

1. Sixty-four male rats were fed on a nutritionally complete diet containing 30% of the dietary energy as fat. For thirty-two rats (control) the fat source was maize oil; for the remaining thirty-two rats (experimental) the fat source was triundecanoin-maize oil (7:3, w/w). 2. After 6 weeks, groups of control and experimental rats were fasted and killed on days 0, 1, 2, and 4 of the fasting period. In the experimental group, adipose-tissue fatty acids contained, on average, 246 mmol undecanoic acid/mol total fatty acids. In contrast, the adipose tissue of the control group did not contain odd-carbon-number fatty acids. 3. Plasma glucose levels were significantly higher for all fasted experimental groups and blood ketone levels were significantly lower for these groups. 4. Thus, during prolonged starvation, animals which accumulated odd-C-number fatty acids resisted the development of ketosis. It appeared that odd-C-number fatty acids mobilized from fat depots during fasting may provide their terminal three-C-residues for gluconeogenesis rather than for acetyl-CoA production, thereby allowing for better maintenance of blood glucose and insulin levels than in control animals. 5. It is concluded that the lower acetyl-CoA production and higher blood insulin levels reduce ketogenesis.

Rats enriched with odd-carbon fatty acids: maintenance of liver glycogen during starvation

In young rats a diet containing triundecanoin as the major source of fat produces substantial enrichment of adipose tissue triglycerides with undecanoate and higher fatty acids with odd-numbered carbons. The terminal three-carbon residues arising from beta-oxidation of these acids are glucogenic and help to counteract the decreases in liver glycogen and serum glucose ordinarily induced by prolonged fasting.