Oxidase unsaturated fatty acid

Figure 9.2. Mechanisms of aminoglycoside toxicity. This schematic representation summarizes the principles of aminoglycoside toxicity discussed in the text. Treatment with the drugs leads to the formation of reactive oxygen species through a redox-active complex with iron and unsaturated fatty acid or by triggering superoxide production by way of NADPH oxidase. An excess of reactive oxygen species, not balanced by intracellular antioxidant systems, will cause an oxidative imbalance potentially severe enough to initiate cell death pathways. Augmenting cellular defenses by antioxidant therapy can reverse the imbalance and restore homeostasis to protect the cell.

Very long chain fatty acids are initially oxidized in the peroxisome where the initial oxidation step is catalyzed by acyl-CoA oxidase and the subsequent steps in fS-oxidation are catalyzed by a multi-enzyme complex with hydratase, dehydo-genase, and thiolase activities. Unsaturated fatty acids require additional enzymatic activities, including enoyl-CoA isomerase and dienoyl-CoA reductase. Readers are directed to Vance and Vance (2) for additional details regarding fi-oxidation, including the details of the metabolic reactions. 

The renal cytochrome P-450 enzyme system is involved in oxidative reactions in which an atom of molecular oxygen is inserted in an organic molecule. The flavoprotein NADPH-cytochrome P-450 reductase is an essential component of the mixed-function oxidase systems (MFO). Microsomal membranes appear to be particularly subject to attack by reactive oxygen radicals due to their high content of unsaturated fatty acids and the presence of the cytochrome P-450 system [40]. Cephaloridine-induced peroxidation of membrane lipids is decreased by the cytochrome P-450 inhibitor cobalt chloride [31], suggesting a role for a cytochrome P-450 reductase in the P-lactam-induced generation of reactive oxygen species and subsequent peroxidation products. 

The NADPH oxidase complex is dormant in resting phagocytes and becomes assembled and activated for superoxide formation upon bacterial invasion. The respiratory burst is stimulated in vitro as well as in vivo by a variety of reagents, among which are phorbol esters (PMA, phorbol 12-myristate 13-acetate), heat-aggregated IgG, unsaturated fatty acids and analogues of bacterial peptides (FMLP, formylmethyonyl-leucyl phenylalanine). 

The ready oxidation of ascorbic acid will catalyze chemical changes in a number of other substances. Thus, unsaturated fatty acids in lecithins and tissues are catalytically oxidized in the presence of ascorbic acid to a substance producing color with thiobarbiturate (B21). The product of the ascorbic acid-catalyzed oxidation is malonaldehyde, which can also inhibit L-gulonolactone oxidase, the enzyme forming ascorbic acid (Cl). It has been suggested that this enzyme inhibition may occur in vivo in animals deficient in vitamin E, a compound believed to have antioxidant actions which would prevent the ascorbic acid-catalyzed lipid oxidation from giving rise to malonaldehyde. It is quite probable that the active intermediate in the formation of malonaldehyde is the monodehydroascorbate radical which initiates the lipid oxidation. 

CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increased the activity of 2,4-dienoyl-CoA reductase and A, A -enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for p-oxidation of unsaturated fatty acids [246], On the other hand, the results obtained by Fukuda et al. [247] suggest that increased fatty acid oxidation by dietary sesamin leads to decreased esterification of fatty acids and reduces the synthesis and secretion of triacylglycerol. 

Perhaps of most Importance Is yeast s unusual ability among eukaryotes to be able to live anaerobically and to derive Its ATP by nonresplratory glycolysis. Under these conditions, as seen under the electron microscope, the mitochondria change morphologically to what are called promltochondrla (14), and the cells become auxotrophic for sterol (11) and unsaturated fatty acid such as oleate (15). Aerobically yeast derives Its sterol and unsaturates by biosynthesis, and for both types of compound there are biosynthetic steps Involving mixed function oxidases,, for the formation of squalene oxide... 

Fatty acid auxotrophs of cerevlslae have been extensively utilized to probe relationships between fatty acid unsaturation and enzymic activity. A major area of Investigation has been elucidation of the bases for loss of oxidative phosphorylation by yeast cultures characterized by less than ca. 20-30% unsaturated fatty acid (42-44). Much reduced or no activity In mitochondrial membranes with highly saturated fatty acyl compositions has been documented for cytochromes a, a3, b and c, mitochondrial ATPase, succinate oxidase and NADH oxidase (45-47). Fatty acids apparently mediate many mitochondrial functions... 

When measuring the mitochondrial P-oxidation in liver, 2-methyl EPA did not cause any effect compared to EPA or control, while the other EPA-derivatives increased the fatty acid oxidation in mitochondria. We measured the mitochondrial activity and gene expression of an enzyme involved in the oxidation of unsaturated fatty acids, the 2,4-dienoyl-CoA reductase. Both the activity and gene expression seemed to increase in rats fed 2,2-dimethyl EPA. We also measured the total activity of CPT in liver, and found an increased activity in rats fed 2,2-dimethyl EPA. The increase in total CPT-activity after administration of 2,2-dimethyl EPA seemed to be due to the observed increase in CPT-II transcription, as the mRNA level of CPT-I was unchanged (data to be published). The peroxisomal P-oxidation, the activity and gene e q)ression of fatty acyl-CoA oxidase, the rate-limiting enzyme of peroxisomal P-oxidation, and the gene expression of the peroxisomal multifunctional protein were increased after administration of the EPA-derivatives, as shown in Table 2. 

The synthesis of unsaturated fatty acids (Fig. 17.19) requires the participation of molecular oxygen and involves a mixed function oxidase (NADP-dependent) [72]. 

The ability of LOX to catalyze cooxidation reactions has long been recognized (e.g., its carotene oxidase activity see Section I) and has been used as the basis for some LOX assays (see Section IV) and in commercial applications e.g., soybean or Vida faba flours (both rich in LOX activity) are added to bleach wheat flour pigments in white bread production. Cooxidation is clearly manifested in the bleaching of pigments (chlorophyll, carotenoids, etc.) but also results in the oxidation of protein-SH groups and of unsaturated fatty acids, including substrates for LOX. 

In cyclophorase-mitochondrial preparations of kidney, propionate is formed from the terminal 3 carbons of straight-chain fatty acids of the odd series. Crotonic, vinylacetic, and a series of 8-keto acids, all of which are perhaps intermediates of j8-oxidation, are completely oxidized. The oxidase apparently does not distinguish between ct and trans forms of A -unsaturated fatty acids or between the n- and L-forms of j8-hydroxy-acids. The manner in which fatty acids are activated in order to initiate their oxidation is discussed in a later section (VII). 

Peroxisomes appear to be capable of degrading long-chain, saturated, even-numbered fatty acids completely to their constituent acetyl units. Medium- and short-chain acyl-CoAs, intermediates in long-chain acyl-CoA oxidation, are oxidized by the acyl-CoA oxidase and serve as substrates for the overall in vitro 6-oxidation bv peroxisomes. However, storage triacylglycerols, the substrate source for 6-oxidation in fatty tissues, and membrane lipids, a possible substrate source for 6-oxidation in nonfatty tissues, contain unusual and unsaturated fatty acids. Additional enzyme reactions are required to link the catabolism of these fatty acids to the 6-oxidation sequence. 

A more direct pathway for unsaturated fatty acid biosynthesis was first demonstrated in Saccharomyces cerevisiae by Bloomfield and Bloch (1960). Their studies revealed that the formation of unsaturated fatty acid CoA esters could proceed by a desaturation of the corresponding long-chain fatty acid CoA ester, as shown in Fig. 7. The particulate enzyme involved was found to have characteristics typical of mixed function oxidases, requiring molecular oxygen and ITNH. In contrast, the p,y dehydration mechanism outlined above is essentially... 

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