Polyunsaturated fatty microsomal enzymes

Vitamin E (a-tocopherol), a potent antioxidant, appears to offer protection against injuries caused by 02, 03, and N02, and nitrosamine formation. Male rats supplemented with daily doses of 100 mg tocopheryl acetate and exposed to 1.0 ppm 03 have been shown to survive longer than vitamin E-deficient rats. The action of 03 is attributed in part at least to free radical formation. In addition, there is sufficient evidence that vitamin E protects phospholipids of microsomal and mitochondrial membranes from perox-idative damage by reacting with free radicals. Because lipid peroxidation is associated with decrease in oxidase activities, it is expected that the enzyme activity is affected by dietary vitamin E. Maximum activity has been observed when diets included both polyunsaturated fatty acids and vitamin E. 

Monooxygenase metabolism of essential fatty acids has received comparatively little attention. DiAugustine and Fonts found that in liver microsomes, linoleic acid, linolenic acid and arachidonic acid induced spectral changes of type I, which are associated with metabolism, but the products were not identified [394]. These polyunsaturated fatty acids could be expected to be wl- and w2-hydroxylated like their saturated counterparts and to be transformed into hydroxylated cis,trans conjugated products by chemical peroxidation (autooxidation) as discussed above. Recent studies show that cytochrome P-450 can metabolise polyunsaturated fatty acids to a large extent by epoxidation. The epoxides are rapidly hydrolysed to vicinal diols by microsomal or soluble enzymes. 

On the contrary, the microsomes of plant cells proved to contain all the enzymes necessary for the biosynthesis of either polyunsaturated fatty acids (at least linoleic acid (Vijay and Stumpf, 1971 Ben Abdelkader et al., 1973 Kader, 1977a) or the m or phospholipids of plant membranes i.e., phosphatidylcholine and phosphatidylethariolamine (Devor and Mudd, 1971 Moore et al., 1973). Therefore one was forced to the conclusion that there must be a transfer of lipids from microsomes to other intracellular organelles to correlate the limited synthetic capabilities of the various organelles with the uniformity of their lipid composition. We proposed, in 1968 (Mazliak et al., 1968), that an intermembrane phospholipid exchange was implied in this necessary cooperation between microsomes and other cellular organelles. 

Ferrous ion-induced Hpid peroxidation of rat liver mitochondria was accelerated by phosphate (Yamamoto et al. 1974). Preincubation of rat liver microsomes with iron (Fe)/ascorbate (50 pM/ 200 pM), known to induce peroxidation, resulted in a significant inhibition of (i) the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (46 %, P <0.01, (ii) the crucial enzyme control-Hng the conversion of cholesterol in bile acids, cholesterol 7a-hydroxylase (48%, P <0.001), and (iii) the central enzyme for cholesterol esterification, acyl-CoAxholesterol acyltransferase (ACAT, 80%, P <0.0001) (Brunet etal. 2000). The disturbances of these key enzymes coincided with a high rate of malondialdehyde production (350%, P <0.007) and the loss of polyunsaturated fatty adds (36.19 1.06% vs. 44.24 0.41% in controls, P <0.0008). While a-tocopherol simultaneously neutrahsed lipid peroxidation, preserved microsomal fatty acid status, and restored ACAT activity, it was not effective in preventing Fe/ascorbate-induced inactivation of both HMG-CoA reductase (44%, P <0.01) and cholesterol 7a-hydroxylase (71%, P< 0.0001). 

Microsomal (0-6 desaturases use cytochrome b5 as electron donor to introduce a double bond into the co-6 position of monounsaturated oleic acid to produce polyunsaturated linoleic acid. Thus microsomal -6 desaturases play a vital role in the polyunsaturated fatty acid synthesis in angiosperms. It has been estimated that these enzymes are responsible for more than 90% of the polyunsaturated fatty acid synthesis in non-photosynthetic tissues and developing seeds of oil crops (1). 

Little change occurred in the 18 2. Again, this raises further questions about the specificity of desaturase enzymes for complex lipid substrate. Are the same desaturases involved irrespective of the headgroup composition of the complex lipid It should be noted, however, that PE in microsomes from oil-seeds has never, in our experience, been associated with the desaturation of exogenously supplied acyl-CoA and appears therefore not to be a prime substrate in the production of polyunsaturated fatty acids for other lipids. 

In plants, the polyunsaturated fatty acids linoleate (octadeca-A9,12-dienoic acid) and a-linolenate (octadeca-A6,9,15-trienoic acid) are synthetized by A12- and A15-desaturases, respectively. They are essential fatty acid components of human nutrition as precursors of prostaglandins. Plant A12-desaturase (oleic acid - linoleic acid) is a well known membrane-bound enzyme present in endoplasmic reticulum of developing seeds. A12-desaturase employs NADH and molecular oxygen and requires cytochrome b5 and NADH-cytochrome b5 reductase as intermediate electron carrier (Demandre etaLy 1986). In contrast, in vitro A15-desaturase activity (linoleic acid — a-linolenic acid) is actually not observed in plant microsomes. 

The fatty acid desaturases are exclusively microsomal. Three types of desaturation enzymes involved in polyunsaturated acid biosynthesis have been characterized the A6, the A5 and the A4. 

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