Fatty acids, long-chain microsomal

Many of these disorders are associated with the urinary excretion of acylcarnitines, acyl conjugates of glycine, and dicarboxylic acids that are characteristic of the metabolic block. A general conclusion derived from studies of these disorders is that an impairment of p-oxidation makes fatty acids available for microsomal (o-oxidation by which fatty acids are oxidized at their terminal (to) methyl group or at their penultimate (o) - 1) carbon atom. Molecular oxygen is required for this oxidation and the hydroxylated fatty acids are further oxidized to dicarboxylic acids. Long-chain dicarboxylic acids can be... 

FAAH was originally purified and cloned from rat liver microsomes and is able to catalyse the hydrolysis of anandamide and 2-AG, in addition to other long-chain fatty acid amides [25]. Studies into the structure and role of this enzyme have generated interest in the potential therapeutic applications of FAAH inhibitors [26-28]. FAAH knock-out mouse brains contained 15-fold higher levels of anandamide than their wild-type counterparts and these animals have also been shown to be more responsive to exogenously administered anandamide [29]. These animals also showed a reduced response to painful stimuli, supporting the hypothesis that FAAH inhibition may provide novel analgesics. Levels of 2-AG were not elevated in the FAAH knock-out animals, apparently due to the existence of alternative metabolic fates for this compound [30]. 

The biosynthesis of hydrocarbons occurs by the microsomal elongation of straight chain, methyl-branched and unsaturated fatty acids to produce very long-chain fatty acyl-CoAs (Figure 11.1). The very long chain fatty acids are then reduced to aldehydes and converted to hydrocarbon by loss of the carboxyl carbon. The mechanism of hydrocarbon formation has been controversial. Kolattukudy and coworkers have reported that for a plant, an algae, a vertebrate and an insect, the aliphatic aldehyde is decarbonylated to the hydrocarbon and carbon monoxide, and that this process does not require cofactors (Cheesbrough and Kolattukudy, 1984 1988 Dennis and Kolattukudy, 1991,1992 Yoder et al., 1992). In contrast, the Blomquist laboratory has presented evidence that the aldehyde is converted to hydrocarbon and carbon dioxide in a process that... 

The most common desaturase in most organisms, including insects, is stearoyl Co A desaturase, which introduces a double bond in the 9-10 position of long-chain fatty acids (2JL). Similarities between this enzyme and the All desaturase from cabbage looper include location in the microsomal fraction, lack of sensitivity to carbon monoxide, inhibition by cyanide, use of a reduced nicotine-adenine nucleotide cofactor as an electron source and use of 16 and 18 carbon acids as preferred substrates. 

Long chain fatty acids are are bound to Fatty acid binding protein for transport within the cytosol. They are impermeable to the inner mitochondrial membrane. They are thus esterified in the cytosol by microsomal Fatty acyl CoA synthetase in a reaction identical to the one shown above. Again the reaction is driven by the hydrolysis of pyrophosphate. The enzyme involves an acyl AMP intermediate ... 

By feeding nutritionally adequate diets, dietary intake of 18 2n-6, 18 3n-3, or the proportion of 18 2n-6 to 18 3n-3, particularly during development, has been shown to influence the content of long-chain polyunsaturated fatty acids in membrane lipids by changing the composition of the whole brain, oligodendrocytes, myelin, astrocytes mitochondrial, microsomal, and synaptosomal membrane (Bourre et al., 1984 Foot et al., 1982 Lamptey Walker, 1976 Tahin et al., 1981). Feeding diets with a 18 2n-6 to 18 3n-3 fatty acid ratio between 4 1 and 7 1 to rats from birth to 1, 2, 3, and 6 wk of... 

Acyl-CoA molecules are desaturated in ER membrane in the presence of NADH and 02. All components of the desaturase system are integral membrane proteins that are apparently randomly distributed on the cytoplasmic surface of the ER. The association of cytochrome b5 reductase (a flavoprotein), cytochrome b5, and oxygen-dependent desaturases constitutes an electron transport system. This system efficiently introduces double bonds into long-chain fatty acids (Figure 12.15). Both the flavoprotein and cytochrome b5 (found in a ratio of approximately 1 30) have hydrophobic peptides that anchor the proteins into the microsomal membrane. Animals typically have A9, A6, and A5 desaturases that use electrons supplied by NADH via the electron transport system to activate the oxygen needed to create the double bond. Plants contain additional desaturases for the A12 and A15 positions. 

Fig. 12. Cleavage of the O-alkyl linkage in glycerolipids (A) is catalyzed by (1) a Pte-H4-dependent alkyl monooxygenase, a microsomal enzyme found primarily in liver and intestinal tissues. The hemiacetal shown in this reaction has not been isolated because of its instability. The fatty aldehyde product can be either reduced to a long-chain fatty alcohol by (II) a reductase or oxidized to a fatty acid by (III) an oxidoreductase. Removal of the 0-alk-l -enyl moiety from plasmalogens (B) is catalyzed by plasmalogenase. As with the O-alkyl monooxygenase, the fatty aldehyde can be converted to either the corresponding fatty alcohol or the fatty acid. GPE, sn-glycero-3-phosphoethanolamine.

Acylation of Phospholipids - The esterification of long-chain, unsaturated fatty acids in the sn-2-position could be explained by different enzyme activities such as specific long-chain acyl-CoA synthetases and CoA-dependent or CoA-independent transacylases.6-11 For example, a long-chain acyl-CoA synthetase has been demonstrated in Isolated platelet membranes that is specific for arachidonate and other long-chain unsaturated fatty acids.6,7 Transfer of arachidonic acid between phospholipids has been observed by the action of CoA-dependent and CoA-independent transacylases. The CoA-dependent transacylases were demonstrated in lymphocytes,8 pancreatic acini9 and liver microsomes.10 A CoA-independent transacylase, recently observed in platelets, catalyzes the synthesis of arachidonoyl-plasmenyl-ethanolamine by acylation of lysoplasmenylethanolamine with arachidonic acid derived from phosphatidylcholine. 1... 

It also appears that tiaprofenic acid, an NSAID that also undergoes inversion in rats, is not a substrate for purified microsomal rat liver long-chain acyl-CoA synthetase for which R-ibuprofen is a substrate [25]. This data may suggest that metabolic pathways involved in the inversion of tiaprofenic acid and possibly other 2-APA NSAIDs are different from those known for R-ibuprofen. It has been recently reported that in both an in vitro cell-free system and in rat liver homogenates the chiral inversion of ibuprofen was apparent when both CoA and ATP were present however, the NSAID KE-748 was not inverted [26]. To induce hepatic microsomal and outer mitochondrial long-chain fatty acid CoA ligase, rats were treated with clofibric acid [27]. Whereas chiral inversion of ibuprofen was enhanced significantly compared to controls, this was not the case for R(—)-KE-748. 

Knights, K.M. Jones, M.E. Inhibition kinetics of hepatic microsomal long chain fatty acid-CoA ligase by 2-arylpropionic acid non-steroidal antiinflammatory drugs. Biochem. Pharmacol. 1992, 44, 2415-2417. 

LCFA oxidation occurs mairily in mitochondria but rat liver microsomes and peroxisomes contain also both membrane-bound/malonyl-CoA-sensitive and soluble/ malonyl-CoA-insensitive (luminal) CPT-like enzymes. " Thus, a similar fatty acid transport system operates in mitochondria, peroxisomes and microsomes, but it seems that the components involved in these systems are all different. The physiological role of these fatty acid transport systems in microsomes and peroxisomes remains unclear. The microsomal CPTs may have a role in providing fatty acids for transport of proteins through the Golgi apparatus and for acylation of secreted proteins. Since oxidation of very long-chain fatty acids is confined to peroxisomes, a possible role for the peroxisomal CPTs may be to shuttle chain-shorted products out of peroxisomes for further oxidation in mitochondria. 

In Escherichia coli an acyl-acyl carrier protein synthetase that uses acyl carrier protein instead of CoA for fatty acid activation has been described (Ray and Cronan, 1976). The hydrocarbon utilizing yeast, Candida lipolyti-ca fabricates two distinct long chain acyl-CoA synthetases one of them activates fatty acids exclusively for lipid synthesis, while the other one does so for p-oxidation (Numa, 1981). Comparisons of the mitochondrial and microsomal long chain acyl-CoA synthetases of rat liver have shown, however, that the two enzymes are very similar (Philipp and Parsons, 1979 Tanaka et al., 1979). 

Figure 7. Enzymatic steps in long-chain fatty acid elongation. Enzymatic steps of microsomal fatty acyl chain elongation. ELOVL, elongation of very-long-chain fatty acids KAR, 3-ketoacyl-CoA reductase HADC, 3-hydroxyacyl-CoA dehydratase TER, /rowi-2,3-enoyl-CoA reductase [108].

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