Mechanism of fatty acid synthesis

Mitochondrial Quality Control via Mitochondrial Unfolded Protein Response (mtUPR) in Ageing and Neurodegenerative Diseases

Mitochondria play a key role in cellular functions, including energy production and oxidative stress regulation. For this reason, maintaining mitochondrial homeostasis and proteostasis (homeostasis of the proteome) is essential for cellular health. Therefore, there are different mitochondrial quality control mechanisms, such as mitochondrial biogenesis, mitochondrial dynamics, mitochondrial-derived vesicles (MDVs), mitophagy, or mitochondrial unfolded protein response (mtUPR). The last item is a stress response that occurs when stress is present within mitochondria and, especially, when the accumulation of unfolded and misfolded proteins in the mitochondrial matrix surpasses the folding capacity of the mitochondrion. In response to this, molecular chaperones and proteases as well as the mitochondrial antioxidant system are activated to restore mitochondrial proteostasis and cellular function. In disease contexts, mtUPR modulation holds therapeutic potential by mitigating mitochondrial dysfunction. In particular, in the case of neurodegenerative diseases, such as primary mitochondrial diseases, Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), Amyotrophic Lateral Sclerosis (ALS), or Friedreich's Ataxia (FA), there is a wealth of evidence demonstrating that the modulation of mtUPR helps to reduce neurodegeneration and its associated symptoms in various cellular and animal models. These findings underscore mtUPR's role as a promising therapeutic target in combating these devastating disorders.

Peroxisomes and beta-oxidation of long-chain unsaturated carboxylic acids

Degradation of unsaturated long-chain carboxylic acids by beta-oxidation, which is a compartmentalized process occurring in both mitochondria and peroxisomes in mammalian cells, was studied using rat liver as a model tissue. Inclusion of (poly)unsaturated fatty acids in the perfusion medium resulted in an increased concentration of catalase-H2O2 complex indicating on-going peroxisomal beta-oxidation. For this to occur, an active peroxisomal delta 3, delta 2-enoyl-CoA isomerase was required for metabolism of the double bonds at odd-numbered positions in acyl-CoA. Experiments with isolated subcellular organelles from rat liver confirmed the presence of isomerase in the peroxisomes, and the enzyme was subsequently isolated with apparent homogeneity. Comparison of amino acid sequences from the enzyme with a published sequence for a bifunctional protein from rat liver identified them representing the same molecule. The peroxisomal bifunctional protein can thus act as a multifunctional hydratase-dehydrogenase-isomerase enzyme. Examination of the mitochondrial isomerase revealed that rat liver mitochondria possess two isoenzymes: a long-chain isomerase not induced by clofibrate-treatment and showing a preference for C10-C12 substrates, and a clofibrate-inducible short-chain isomerase which gave the highest catalytic activity with C6 substrates. Experiments with isolated peroxisomes and unsaturated acyl-CoAs demonstrated that the beta-oxidation of fatty acids having double bonds at even-numbered positions was dependent on 2,4-dienoyl-CoA reductase in peroxisomes, as in mitochondria. Immunocytochemical experiments using the protein A-gold labelling technique, and comparison of their physicochemical properties indicated that all the mammalian reductases purified so far are mitochondrial isoenzymes. It turned out during the isolation of 3-hydroxyacyl-CoA epimerase that there was no monofunctional epimerase at all in the rat liver. Instead, the epimerization reaction occurred via dehydration-hydration catalyzed by two distinct stereospecific hydratases, 2-enoyl-CoA hydratase 1 (the classic hydratase) and 2-enoyl-CoA hydratase 2 (a novel hydratase). The present data demonstrate that peroxisomes contain all the enzymes required for the beta-oxidation of unsaturated fatty acids and support the notion that one of the physiological functions of peroxisomal beta-oxidation is to metabolize long-chain hydrophobic carboxylic acids to shorter, more polar metabolites which are then either metabolized further in the body or excreted.

Inhibition of the condensing component of chicken liver fatty acid synthase by iodoacetamide and 5,5'-dithiobis-(2-nitrobenzoic acid)

Chicken liver fatty acid synthase is inhibited by the thiol-modifying reagents 5,5'-dithiobis-(2-nitrobenzoic acid) and iodoacetamide. Total inactivation of the activity for fatty acid synthesis requires the modification of about 8 of the nearly 50 freely accessible thiol groups per molecule. The differential binding of iodo[14C]acetamide to phenylmethylsulphonyl fluoride-modified enzyme in the absence and in the presence of excess acetyl-CoA shows complete modification of one cysteine-SH site of the condensing enzyme and partial modification of the pantetheine-SH site for a total of approx. 1.4 mol of iodoacetamide bound per mol of enzyme. The reaction of the enzyme with 5,5'-dithiobis-(2-nitrobenzoic acid) generates disulphide cross-links for each molecule of the reagent added, but 95% of these cross-links are intrasubunit. Both the iodoacetamide- and 5,5'-dithiobis-(2-nitrobenzoic acid)-modified species catalyse all the component partial reactions of fatty acid synthesis except the condensation reaction. The results obtained with iodoacetamide show that in the dimeric fatty acid synthase modification of one cysteine-SH condensing site and/or one pantetheine-SH site per dimer is sufficient to affect inhibition of condensing activity and the activity for fatty acid synthesis, and are in accord with a recently proposed model for the mechanism of action of animal fatty acid synthases [Kumar (1982) J. Theor. Biol. 95, 263-283].

Degradation of pigeon liver fatty acid synthetase in the absence of exogenous proteinases

The homogeneity of pigeon liver fatty acid synthetase has been rigorously tested by physicochemical techniques and crossed-rocket immunoelectrophoresis. The enzyme has also been incubated for 1 h at 100 degrees C in 2% sodium dodecyl sulfate and 0.1 M dithiothreitol. The number of protein components on gel electrophoresis and of dansylated amino acids increased as a function of incubation time. Furthermore, the minor proteins observed after gel electrophoresis cross-reacted with antibody raised to the synthetase. Proteolysis was not chemically mediated by the detergent, the reducing agent or the buffer conditions chosen. Several commercially prepared proteins were not degraded by this procedure, and two proteins were recalcitrant to hydrolysis when included in the same incubation mixture as the synthetase. The inclusion of certain microbial proteinase inhibitors decreased the amount of degradation. This demonstrated that hydrolysis of the synthetase is mediated by a specific vertebrate enzyme which retains activity under denaturing conditions at 100 degrees C. Further degradation is also observed after individual treatment of four limited digestion products from the pigeon liver fatty acid synthetase, suggesting the possibility of an inherent proteolytic activity within the complex.

Changes in rat liver and adipose tissue lipogenesis in the course of continuous gamma radiation

Lipogenesis in liver of continuously irradiated rats (dose rate of 0.57 Gy per day for up to 25 or 30 days) was increased, in comparison with control animals, between 7 and 30 days. In the liver of fed rats it was approximately five times higher than animals fasted for 16 hours before sacrifice. Lipogenesis in adipose tissue was decreased on the third day of irradiation and its value was higher than in controls between 7 and 25 days. Glyceride glycerol synthesis in irradiated rats was increased compared with controls on the first, seventh and between 21 and 25 days of continuous irradiation. We suggest that the increase in lipogenesis in the liver of continuously irradiated rats is an important factor in lipid metabolism disorders caused by the continuous irradiation.