Enzymes in peroxisomes

The enzymes in peroxisomes do not attack shorter-chain fatty acids the P-oxidation sequence ends at oc-tanoyl-CoA. Octanoyl and acetyl groups are both further oxidized in mitochondria. Another role of peroxisomal P-oxidation is to shorten the side chain of cholesterol in bile acid formation (Chapter 26). Peroxisomes also take part in the synthesis of ether glycerolipids (Chapter 24), cholesterol, and dolichol (Figure 26-2). 

Chloroplasts contain a specific glyoxylate reductase active with NADPH as coenzyme an enzyme in peroxisomes that catalyzes glyoxylate reduction by NADH is not specific and probably functions mainly as hydroxypyruvate reductase (Tolbert et al., 1970) (this is discussed in Section II,A,2). These enzymes could conceivably be involved in the oxidation of glycolate to provide glyoxylate for synthesis of glycine. 

From this study we can conclude that the two forms of the COT enzyme in peroxisomes may be a result of the fra -splicing mechanism in the COT pre-mRNAs. The strength of the 3 acceptor splice site, the presence of several ESEs in exon 2 and the fact that exon 1 is untranslated with only 30 nt, may be responsible of this process (Fig. 5). 

In these experiments cultured glioma cells were exposed to TTA, and key enzymes in peroxisomal (fatty acyl-CoA oxidase—FAO) and mitochondrial (carnitine palmitoyl transferase II—CPT II) fatty acid oxidation were studied. 

Kovacs et al. (2002) reviewed some of the recent findings related to the localization of cholesterol biosynthetic enzymes in peroxisomes and discussed the impairment of cholesterol biosynthesis in peroxisomal deficiency disease. 

B. Gerhardt, Localization of B-oxidation enzymes in peroxisomes isolated from nonfatty plant tissues. Planta 159 238 (1983). 

Normally, these reactive species are destroyed by protective enzymes, such as superoxide dismutase in mitochondria and cytosol and catalase in peroxisomes, but if a tissue has been anoxic the respiratory chain is very reduced and reoxygenation allows dangerous amounts to be formed. Muscle also contains significant quantities of the dipeptide, camosine ((J-alanylhistidine) (10—25 mM). The functions of camosine are obscure although it has been suggested to be an effective antioxidant (Pavlov et al., 1993). 

A modified form of P-oxidation is found in peroxisomes and leads to the formation of acetyl-CoA and H2O2 (from the flavoprotein-linked dehydrogenase step), which is broken down by catalase. Thus, this dehydrogenation in peroxisomes is not linked directly to phosphorylation and the generation of ATP. The system facilitates the oxidation of very long chain fatty acids (eg, Cjq, C22). These enzymes are induced by... 

Figure26-2. Biosynthesis of squalene, ubiquinone, dolichol, and other polyisoprene derivatives. (HMG, 3-hydroxy-3-methylglutaryl x, cytokinin.) A farnesyl residue is present in heme a of cytochrome oxidase. The carbon marked with asterisk becomes C or C,2 in squalene. Squalene synthetase is a microsomal enzyme all other enzymes indicated are soluble cytosolic proteins, and some are found in peroxisomes.

M (32% increase in liver weights 178% increase in peroxisomal beta-oxidation enzyme activity)... 

A 32% increase in liver weights, along with a 187% increase in peroxisomal beta-oxidation activity, was noted in rats exposed to 1,000 mg/kg/day MIL-H-5606 for 26 days (Mattie et al. 1993). The toxicological significance of the changes in peroxisomal enzyme activities is unclear. 

The chain shortening pathway has not been characterized in detail at the enzymatic level in insects. It presumably is similar to the characterized pathway as it occurs in vertebrates. These enzymes are a partial P-oxidation pathway located in peroxisomes [29]. The key enzymes involved are an acyl-CoA oxidase (a multifunctional protein containing enoyl-CoA hydratase and 3-hy-droxyacyl-CoA dehydrogenase activities) and a 3-oxoacyl-CoA thiolase [30]. These enzymes act in concert to chain shorten acyl-CoAs by removing an acetyl group. A considerable amount of evidence in a number of moths has accumulated to indicate that limited chain shortening occurs in a variety of pheromone biosynthetic pathways. 

Fig. 4. Modification of plant metabolic pathways for the synthesis of poly(3HAMCL) in peroxisomes. The pathways created or enhanced by the expression of transgenes (P. aeruginosa PHA synthase and C. lanceolata decanoyl-ACP thioesterase) and of mutant alleles of plant fatty acid desaturase genes are highlighted by bold arrows and the enzymes involved underlined...

Stegink, SJ, Vaughn KC, Kunce CM, Trelease RN. Biochemical, electrophoretic and immunological characterization of peroxisomal enzymes in three soybean organs. Plant Physiol 1987 69 211-220. 

Aoyama, T., Peters, J.M., Iritani, N., Nakajima, T., Furihata, K., Hashimoto, T. Gonzalez, F.J. (1998) Altered constitutive expression of fatly acid-metabohzing enzymes in mice lacking the peroxisome proliferator-activated receptor a (PPARa). J. biol. Chem., 213, 5678-5684... 

Marayama, H., Tanaka, T. Williams, GM. (1990) Effects of the peroxisome proliferator di(2-ethylhexyl)phthalate on enzymes in rat liver and on carcinogen-induced liver altered foci in comparison to the promoter phenobarbital Toxicol. Pathol., 18, 257-267 Matsushima, T, Muramatsu, M. Haresaku, M. (1985) Mutation tests on Salmonella typhimurium by the preincubation method. Prog. Mutat. Res., 5, 181-186 Matthews, E.J., DelBalzo, T. Rundell, J.O. (1985) Assays for morphological transformation and mutation to ouabain resistance of Balb/c-3T3 cells in culture. Prog. Mutat. Res., 5, 639-650... 

Viswalingam, A. Caldwell, J. (1997) Cinnamyl anthranilate causes co-induction of hepatic microsomal and peroxisomal enzymes in mouse but not rat. Toxicol, appl. Pharmacol., 142, 338-347... 

Catalase A haem-based enzyme which catalyzes the decomposition of hydrogen peroxide into oxygen and water. It is found e.g. in peroxisome located in the liver. 

In order to carry out all of these different functions, peroxisomes are equipped with a unique set of enzyme proteins, catalysing the different reactions involved. In addition, the peroxisomal membrane contains specific transporters in order to take up substrates from the cytosol and release the end products of peroxisomal metabolism. Since peroxisomes lack a citric acid cycle as well as a respiratory chain, the end products of peroxisomal metabolism, such as acetyl-CoA, propionyl-CoA and a range of other acyl-Co A esters predominantly derived from fatty acid beta-oxidation, are exported from the peroxisomal interior and shuttled to mitochondria for full oxidation to C02 and H20. The same applies to the NADH produced during beta-oxidation, which is reoxidised via redox-shuttles so that the NADH generated in peroxisomes is ultimately reoxidised in the mitochondrial respiratory chain at the expense of molecular oxygen. 

Peroxisomal disorders (Zellweger syndrome, Refsum s disease, neonatal adre-noleukodystrophy) are characterised by defective peroxisome biogenesis, or, being present, peroxisomes lacking / -oxidative enzymes. In the BA biosynthetic pathway, dihydroxycoprostanic acid (DHCA) and trihydroxycoprostanic acid (THCA) are /1-oxidised in peroxisomes to produce CA and CDCA, respectively, whereas peroxisomal disorders cause a defective oxidation of the BA precursor side chain, which leads to an accumulation of C27 bile acids, notably 3 ,7 -dihydroxy-5/3-cholesta-noic acid (DHCA) and 3a,7a,12a-trihydroxy-5/l-cholestanoic acid (THCA), in the plasma and urine of affected patients. 

With the hypolipidemic drug-inducible cytochromes P-450 (CYP4A), there is transcriptional activation within 1 hour of administration of clofibrate. The levels of CYP4A1 are very low in the liver and kidney, but are markedly increased by the inducers. As well as cytochromes P-450, a number of other enzymes involved in peroxisomal p-oxidation reactions are also induced. 

The evidence for DNA damage in peroxisomal proliferator-treated animals is more equivocal with some studies, reporting no change in markers such as 8-hydroxydeoxyguanosine residues, whereas other studies indicate otherwise. Also, increased expression of DNA repair enzymes has been reported. The relationship between oxidative stress and signaling for cell proliferation however is not clear. 

In mammals, high concentrations of fats in the diet result in increased synthesis of the enzymes of peroxisomal /3 oxidation in the liver. Liver peroxisomes do not contain the enzymes of the citric acid cycle and cannot catalyze the oxidation of acetyl-CoA to C02. Instead,... 

RGURE17-17 The a oxidation of a branched-chain fatty acid (phytanic add) in peroxisomes Phytanic acid has a methyl-substituted /3 carbon and therefore cannot undergo /3 oxidation. The combined action of the enzymes shown here removes the carboxyl carbon of phytanic acid, to produce pristanic acid, in which the /3 carbon is unsubstituted, allowing oxidation. Notice that /3 oxidation of pristanic acid releases propionyl-CoA, not acetyl-CoA. This is further catabolized as in Figure 17-11. (The details of the reaction that produces pristanal remain controversial.)... 

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