Metabolism of malonic semialdehyde in man

The presence of 3-hydroxypropionate and 1,3-propanediol suggests an alternative path for conversion of glycerol to Acetyl-CoA

BACKGROUND

In our recent study using [U-13C3]glycerol, a small subset of hamsters showed an unusual profile of glycerol metabolism: negligible gluconeogenesis from glycerol plus conversion of glycerol to 1,3-propanediol (1,3PDO) and 3-hydroxypropionate (3HP) which were detected in the liver and blood. The purpose of the current study is to evaluate the association of these unusual glycerol products with other biochemical processes in the liver.

METHODS

Fasted hamsters received acetaminophen (400 mg/kg; n = 16) or saline (n = 10) intraperitoneally. After waiting 2 h, all the animals received [U-13C3]glycerol intraperitoneally. Liver and blood were harvested 1 h after the glycerol injection for NMR analysis and gene expression assays.

RESULTS

1,3PDO and 3HP derived from [U-13C3]glycerol were detected in the liver and plasma of eight hamsters (two controls and six hamsters with acetaminophen treatment). Glycerol metabolism in the liver of these animals differed substantially from conventional metabolic pathways. [U-13C3]glycerol was metabolized to acetyl-CoA as evidenced with downstream products detected in glutamate and β-hydroxybutyrate, yet 13C labeling in pyruvate and glucose was minimal (p < 0.001, 13C labeling difference in each metabolite). Expression of aldehyde dehydrogenases was enhanced in hamster livers with 1,3PDO and 3HP (p < 0.05).

CONCLUSION

Detection of 1,3PDO and 3HP in the hamster liver was associated with unorthodox metabolism of glycerol characterized by conversion of 3HP to acetyl-CoA followed by ketogenesis and oxidative metabolism through the TCA cycle. Additional mechanistic studies are needed to determine the causes of unusual glycerol metabolism in a subset of these hamsters.

Role of the malonyl-CoA synthetase ACSF3 in mitochondrial metabolism

Malonyl-CoA is a central metabolite in fatty acid biochemistry. It is the rate-determining intermediate in fatty acid synthesis but is also an allosteric inhibitor of the rate-setting step in mitochondrial long-chain fatty acid oxidation. While these canonical cytoplasmic roles of malonyl-CoA have been well described, malonyl-CoA can also be generated within the mitochondrial matrix by an alternative pathway: the ATP-dependent ligation of malonate to Coenzyme A by the malonyl-CoA synthetase ACSF3. Malonate, a competitive inhibitor of succinate dehydrogenase of the TCA cycle, is a potent inhibitor of mitochondrial respiration. A major role for ACSF3 is to provide a metabolic pathway for the clearance of malonate by the generation of malonyl-CoA, which can then be decarboxylated to acetyl-CoA by malonyl-CoA decarboxylase. Additionally, ACSF3-derived malonyl-CoA can be used to malonylate lysine residues on proteins within the matrix of mitochondria, possibly adding another regulatory layer to post-translational control of mitochondrial metabolism. The discovery of ACSF3-mediated generation of malonyl-CoA defines a new mitochondrial metabolic pathway and raises new questions about how the metabolic fates of this multifunctional metabolite intersect with mitochondrial metabolism.

Inter-relations between 3-hydroxypropionate and propionate metabolism in rat liver: relevance to disorders of propionyl-CoA metabolism

Propionate, 3-hydroxypropionate (3HP), methylcitrate, related compounds, and ammonium accumulate in body fluids of patients with disorders of propionyl-CoA metabolism, such as propionic acidemia. Although liver transplantation alleviates hyperammonemia, high concentrations of propionate, 3HP, and methylcitrate persist in body fluids. We hypothesized that conserved metabolic perturbations occurring in transplanted patients result from the simultaneous presence of propionate and 3HP in body fluids. We investigated the inter-relations of propionate and 3HP metabolism in perfused livers from normal rats using metabolomic and stable isotopic technologies. In the presence of propionate, 3HP, or both, we observed the following metabolic perturbations. First, the citric acid cycle (CAC) is overloaded but does not provide sufficient reducing equivalents to the respiratory chain to maintain the homeostasis of adenine nucleotides. Second, there is major CoA trapping in the propionyl-CoA pathway and a tripling of liver total CoA within 1 h. Third, liver proteolysis is stimulated. Fourth, propionate inhibits the conversion of 3HP to acetyl-CoA and its oxidation in the CAC. Fifth, some propionate and some 3HP are converted to nephrotoxic maleate by different processes. Our data have implications for the clinical management of propionic acidemia. They also emphasize the perturbations of the liver intermediary metabolism induced by supraphysiological, i.e., millimolar, concentrations of labeled propionate used to trace the intermediary metabolism, in particular, inhibition of CAC flux and major decreases in the [ATP]/[ADP] and [ATP]/[AMP] ratios.

Membranes as Structural Antioxidants: RECYCLING OF MALONDIALDEHYDE TO ITS SOURCE IN OXIDATION-SENSITIVE CHLOROPLAST FATTY ACIDS

Genetic evidence suggests that membranes rich in polyunsaturated fatty acids (PUFAs) act as supramolecular antioxidants that capture reactive oxygen species, thereby limiting damage to proteins. This process generates lipid fragmentation products including malondialdehyde (MDA), an archetypal marker of PUFA oxidation. We observed transient increases in levels of endogenous MDA in wounded Arabidopsis thaliana leaves, raising the possibility that MDA is metabolized. We developed a rigorous ion exchange method to purify enzymatically generated (13)C- and (14)C-MDA. Delivered as a volatile to intact plants, MDA was efficiently incorporated into lipids. Mass spectral and genetic analyses identified the major chloroplast galactolipid: α-linolenic acid (18:3)-7Z,10Z,13Z-hexadecatrienoic acid (16:3)-monogalactosyldiacylglycerol (18:3-16:3-MGDG) as an end-product of MDA incorporation. Consistent with this, the fad3-2 fad7-2 fad8 mutant that lacks tri-unsaturated fatty acids incorporated (14)C-MDA into 18:2-16:2-MGDG. Saponification of (14)C-labeled 18:3-16:3-MGDG revealed 84% of (14)C-label in the acyl groups with the remaining 16% in the head group. 18:3-16:3-MGDG is enriched proximal to photosystem II and is likely a major in vivo source of MDA in photosynthetic tissues. We propose that nonenzymatically generated lipid fragments such as MDA are recycled back into plastidic galactolipids that, in their role as cell protectants, can again be fragmented into MDA.

Oral administration of Excitin-1 (beta-alanyl-L-leucine) alters behavior and brain monoamine and amino acid concentrations in rats

We previously demonstrated that beta-alanyl-branched chain amino acids have excitatory effects. Therefore, we named beta-alanyl-L-leucine, beta-alanyl-L-isoleucine and beta-alanyl-L-valine as Excitin-1, -2, and -3 , respectively. Since there is little known about the effects of Excitins, we clarified whether oral administration of Excitin-1 affects behavior in rats, alters the monoamine and amino acid levels in the central nervous system, whether Excitin-1 is incorporated into the brain, and how long it remains in the blood. Excitin-1 increased motor behavior, increasing the distance of path and number of rearings in the open field. Excitin-1 influenced some monoamine and amino acid levels in the cerebral cortex and hypothalamus. Following oral administration, Excitin-1 was detected in the cerebral cortex, hypothalamus, hippocampus and olfactory bulb. In the plasma, Excitin-1 and its metabolites beta-alanine and L-leucine were recorded. The present study demonstrated that Excitin-1 was incorporated in the brain and promoted behavioral changes in rats.

The mature size of rat 4-aminobutyrate aminotransferase is different in liver and brain

The amino acid sequence predicted from a rat liver cDNA library indicated that the precursor of beta-AlaAT I (4-aminobutyrate aminotransferase, beta-alanine-oxoglutarate aminotransferase) consists of a mature enzyme of 466 amino acid residues and a 34-amino acid terminal segment, with amino acids attributed to the leader peptide. However, the mass of beta-AlaAT I from rat brain was larger than that from rat liver and kidney, as assessed by Western-blot analysis, mass spectroscopy and N-terminal sequencing. The mature form of beta-AlaAT I from the brain had an ISQAAAK- peptide on the N-terminus of the liver mature beta-AlaAT I. Brain beta-AlaAT I was cleaved to liver beta-AlaAT I when incubated with fresh mitochondrial extract from rat liver. These results imply that mature rat liver beta-AlaAT I is proteolytically cleaved in two steps. The first cleavage of the motif XRX( downward arrow)XS is performed by a mitochondrial processing peptidase, yielding an intermediate-sized protein which is the mature brain beta-AlaAT I. The second cleavage, which generates the mature liver beta-AlaAT I, is also carried out by a mitochondrial endopeptidase. The second peptidase is active in liver but lacking in brain.

Thia fatty acids, metabolism and metabolic effects

(1) The chemical properties of thia fatty acids are similar to normal fatty acids, but their metabolism (see below: points 2-6) and metabolic effects (see below: points 7-15) differ greatly from these and are dependent upon the position of the sulfur atom. (2) Long-chain thia fatty acids and alkylthioacrylic acids are activated to their CoA esters in endoplasmatic reticulum. (3) 3-Thia fatty acids cannot be beta-oxidized. They are metabolized by extramitochondrial omega-oxidation and sulfur oxidation in the endoplasmatic reticulum followed by peroxisomal beta-oxidation to short sulfoxy dicarboxylic acids. (4) 4-Thia fatty acids are beta-oxidized mainly in mitochondria to alkylthioacryloyl-CoA esters which accumulate and are slowly converted to 2-hydroxy-4-thia acyl-CoA which splits spontaneously to an alkylthiol and malonic acid semialdehyde-CoA ester. The latter presumably is hydrolyzed and metabolized to acetyl-CoA and CO2. (5) Both 3- and 4-thiastearic acid are desaturated to the corresponding thia oleic acids. (6) Long-chain 3- and 4-thia fatty acids are incorporated into phospholipids in vivo, particularly in heart, and in hepatocytes and other cells in culture. (7) Long-chain 3-thia fatty acids change the fatty acid composition of the phospholipids: in heart, the content of n-3 fatty acids increases and n-6 fatty acids decreases. (8) 3-Thia fatty acids increase fatty acid oxidation in liver through inhibition of malonyl-CoA synthesis, activation of CPT I, and induction of CPT-II and enzymes of peroxisomal beta-oxidation. Activation of fatty acid oxidation is the key to the hypolipidemic effect of 3-thia fatty acids. Also other lipid metabolizing enzymes are induced. (9) Fatty acid- and cholesterol synthesis is inhibited in hepatocytes. (10) The nuclear receptors PPAR alpha and RXR alpha are induced by 3-thia fatty acids. (11) The induction of enzymes and of PPAR alpha and RXR alpha are increased by dexamethasone and counteracted by insulin. (12) 4-Thia fatty acids inhibit fatty acid oxidation and induce fatty liver in vivo. The inhibition presumably is explained by accumulation of alkylthioacryloyl-CoA in the mitochondria. This metabolite is a strong inhibitor of CPT-II. (13) Alkylthioacrylic acids inhibits both fatty acid oxidation and esterification. Inhibition of esterification presumably follows accumulation of extramitochondrial alkylthioacryloyl-CoA, an inhibitor of microsomal glycerophosphate acyltransferase. (14) 9-Thia stearate is a strong inhibitor of the delta 9-desaturase in liver and 10-thia stearate of dihydrosterculic acid synthesis in trypanosomes. (15) Some attempts to develop thia fatty acids as drugs are also reviewed.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

The disposition of acrylic acid in the male Sprague-Dawley rat following oral or topical administration

The disposition of [1-14C] acrylic acid (AA) was characterized in the male Sprague-Dawley rat following oral administration, by gavage in water, at 400 mg/kg and topical application, in acetone, at 501 micrograms/cm2. The oral dose was well absorbed, rapidly and extensively metabolized, and excreted primarily (approx. 80%) as 14CO2 within 24 hr of administration. The rate and extent of 14CO2 evolution from [14C]AA was greater for [1-14CAA] while a significantly lower proportion of the dosed radioactivity remained in the tissue of animals than that reported for [2,3-14C]AA (Winter et al., Drug Metabolism and Disposition 1992, 20, 665). This is consistent with incorporation of AA into a minor beta-oxidation pathway of mitochondrial propionate metabolism by which AA may be metabolized to CO2 or incorporated into cellular constituents. Approximately 5% of the dosed radioactivity was excreted in the urine. The disposition of [1-14C]AA following dermal application was studied using charcoal-containing traps attached to the back of the rats to trap volatilized AA from the dosing sites. Following application of 100 microliters AA [4% (v/v) in acetone] to an area of 8.4 cm2 of the skin of a rat (501 micrograms/cm2), the majority (about 73%) of the dose volatilized and was recovered in the charcoal trap. Percutaneous absorption of AA that did not volatilize was rapid and appeared to have the same metabolic fate as AA administered orally with about 75% of the absorbed dose excreted as 14CO2 within 24 hr.

The metabolism of tetradecylthiopropionic acid, a 4-thia stearic acid, in the rat. In vivo and in vitro studies

The metabolism of [1-14C]tetradecylthiopropionic acid (TTP), a 4-thia stearic acid, and its sulphoxide, [1-14C]texadecylsulphoxypropionic acid (TTP-SO), has been studied in intact rats, in isolated rat hepatocytes, and in rat liver mitochondria. Two pathways of oxidation (beta-oxidation and omega-oxidation) have been demonstrated. TTP is incorporated, in vivo, into tissue triacylglycerol and phospholipids, it is oxidized to CO2, and it is excreted in urine, mainly as carboxypropylsulphoxypropionic acid and a little as carboxymethylsulphoxypropionic acid. TTP-SO is metabolized, in vivo, more rapidly to the same two omega-oxidation products. In hepatocytes TTP is incorporated into triacylglycerol and phospholipids even more rapidly than stearic acid. It is recovered mainly in the 1-position of phosphatidylcholine. Some is oxidized to CO2 and acid-soluble products. TTP-SO is mainly omega-oxidized to the same metabolites as are found in urine. A small fraction is incorporated into phospholipids or oxidized to CO2. In isolated mitochondria [1-14C]TTP is converted into 14CO2, radioactive malonic semialdehyde, and addition products of malonic semialdehyde. In the presence of phenylhydrazine, malonic semialdehyde phenylhydrazone is the dominating product. In soluble extracts of mitochondria [1-14C]malonic semialdehyde is oxidized directly to 14CO2 in the presence of CoA and NAD+, probably by the (methyl)malonic acid semialdehyde dehydrogenase (EC 1.2.1.27).

Methylmalonic semialdehyde dehydrogenase deficiency: demonstration of defective valine and beta-alanine metabolism and reduced malonic semialdehyde dehydrogenase activity in cultured fibroblasts

Intact cultured fibroblasts from a child with a new metabolic disorder, thought to be due to a deficiency of methylmalonic semialdehyde dehydrogenase, produced labeled CO2 normally from [1-14C]valine but not from [2-14C]valine. CO2 production from labeled beta-alanine was also much reduced, confirming the suspicion that malonic semialdehyde dehydrogenase is also deficient in this condition. An assay for malonic semialdehyde dehydrogenase in cell homogenates showed low activity but it was impossible to assess the degree of reduction.

Rapid inactivation of brain glutamate decarboxylase by aspartate

In the absence of its cofactor, pyridoxal 5'-phosphate (pyridoxal-P), glutamate decarboxylase is rapidly inactivated by aspartate. Inactivation is a first-order process and the apparent rate constant is a simple saturation function of the concentration of aspartate. For the beta-form of the enzyme, the concentration of aspartate giving the half-maximal rate of inactivation is 6.1 +/- 1.3 mM and the maximal apparent rate constant is 1.02 +/- 0.09 min-1, which corresponds to a half-time of inactivation of 41 s. The rate of inactivation by aspartate is about 25 times faster than inactivation by glutamate or gamma-aminobutyric acid (GABA). Inactivation is accompanied by a rapid conversion of holoenzyme to apoenzyme and is opposed by pyridoxal-P, suggesting that inactivation results from an alternative transamination of aspartate catalyzed by the enzyme, as previously observed with glutamate and GABA. Consistent with this mechanism pyridoxamine 5'-phosphate, an expected transamination product, was formed when the enzyme was incubated with aspartate and pyridoxal-P. The rate of transamination relative to the rate of decarboxylation was much greater for aspartate than for glutamate. Apoenzyme formed by transamination of aspartate was reactivated with pyridoxal-P. In view of the high rate of inactivation, aspartate may affect the level of apoenzyme in brain.

Tracer studies of the interconversion of R- and S-methylmalonic semialdehydes in man

Two human subjects were given separate oral doses of sodium [2H6]isobutyrate and [methyl-2H3]thymine and the labelling patterns of urinary metabolites were determined. Ingestion of deuterated isobutyrate resulted in the excretion of 2H5-labelled S-3-hydroxyisobutyric acid, formed on the direct catabolic pathway, and of S- and R-[2H4]-3-hydroxyisobutyric acids, formed by the reduction of S- and R-methylmalonic semialdehydes respectively. Only the R-enantiomer of urinary 3-hydroxyisobutyric acid was labelled by thymine. This labelling pattern indicates a flow from S- to R-methylmalonic semialdehyde, suggesting that the R-enantiomer is the substrate of methylmalonic semialdehyde dehydrogenase.