Fat metabolism in higher plants. The elongation of saturated and unsaturated acyl-CoAs by a stromal system from isolated spinach chloroplasts

Purification and characterization of 2-enoyl-CoA reductase from bovine liver

Mitochondrial 2-enoyl-CoA reductase from bovine liver was purified and characterized. A simple three-step purification was developed, involving ion-exchange chromatography to separate the bulk of the NADPH-dependent 2,4-dienoyl-CoA reductase, followed by chromatography on Blue Sepharose and adenosine 2',5'-bisphosphate-Sepharose. Homogeneous enzyme with a subunit Mr of 35 500 is obtained in 35% yield. The Mr of the native enzyme, determined by three different methods, yielded values that suggest that the enzyme is dimeric. NADPH is required as cofactor, and cannot be replaced by NADH. The activity of the purified enzyme towards 2-trans-double bonds in 2-monoene and 2,4-diene structures was investigated. 2-Enoyl-CoA reductase reduced the double bonds in a series of 2-trans-monoenoyl-CoA esters with different chain lengths, but did not exhibit significant activity towards 2-trans-double bonds of 2,4-dienoyl-CoA esters. This result is discussed in the light of analogous observations with enoyl-CoA hydratase.

The physiological role of oxygen-sensitive pyruvate dehydrogenase in mitochondrial fatty acid synthesis in Euglena gracilis

In Euglena gracilis a malonyl-CoA-independent fatty acid-synthetic system, in which fatty acids are synthesized directly from acetyl-CoA as both primer and C2 donor, occurs in mitochondria, and the system contributes to the wax ester fermentation. The activity of fatty acid synthesis in the mitochondrial system was enhanced about six times when an artificial acetyl-CoA-regenerating system was present, indicating that the fatty acid-synthetic activity is controlled by the ratio of acetyl-CoA against CoA. When fatty acids were synthesized using pyruvate instead of acetyl-CoA as substrate, a high activity, about 30 times higher than that from acetyl-CoA, was found under anaerobic conditions (below 10(-5) M oxygen), while in aerobiosis fatty acids were not synthesized at all. CoA, NADH, and NADP+ were required as cofactors for fatty acid synthesis from pyruvate. It was indicated that high activity of fatty acid synthesis from pyruvate due to the high ratio of acetyl-CoA against CoA was maintained by the action of the oxygen-sensitive pyruvate dehydrogenase found in Euglena mitochondria. When [2-14C]pyruvate was fed into intact mitochondria under anaerobic conditions, radioactive fatty acids were formed in the presence of malate, which provided reducing power for the matrix.

Fatty acid synthesis in mitochondria of Euglena gracilis

A malonyl-CoA-independent fatty acid synthetic system, different from the systems in other subcellular fractions, occurred in mitochondria of Euglena gracilis. The system had ability to synthesize fatty acids directly from acetyl-CoA as both primer and C2 donor using NADH as an electron donor. Fatty acids were synthesized by reversal of beta-oxidation with the exception that enoyl-CoA reductase functioned instead of acyl-CoA dehydrogenase in degradation system. A fairly high activity of enoyl-CoA reductase was found on various enoyl-CoA substrates (C4-C12) with NADH or NADPH. Three species of enoyl-CoA reductase, distinct from each other by their chain-length specificity, were found in Euglena mitochondria, and one of them was highly specific for crotonyl-CoA. It is also discussed that the mitochondrial fatty-acid synthetic system contributes to wax ester fermentation, the anaerobic energy-generating system found in the organism.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Incorporation of [2(14)C]malonyl CoA into fatty acids by a cell-free extract of Catharanthus roseus suspension culture cells

A cell-free extract containing the enzymes for de-novo synthesis, elongation and desaturation of fatty acids was prepared from cultured cells of Catharanthus roseus G. Don. (14)C-Fatty acids synthesized by the extract from [2-(14)C]malonyl CoA substrate were palmitic (16:0), stearic (18:0) and oleic (18:1). Dialyzed extract was active and stable at room temperature and at 4° C, but was inactivated on boiling. There was an absolute requirement for NADPH for incorporation of [2-(14)C]malonyl CoA into total fatty acids. Escherichia coli acyl carrier protein stimulated total fatty-acid synthesis without affecting the relative ratio of individual fatty acids. Total fatty-acid synthesis at a rate of 45 nmol·mg(-1) protein·h(-1) occurred at a substrate level of 73 μM malonyl CoA, cofactor levels of 500 μM NADPH, 30 μg·ml(-1) E. coli ACP, and 1.0 mg·ml(-1) extract protein. Total fatty acid synthesis was also sensitive to cerulenin and CoA levels. Variations in the relative abundance of individual (14)C-fatty acids were regulated by concentrations of [(14)C]malonyl CoA. NADPH and ferredoxin, as well as by pH, temperature and length of incubation. Fatty-acid synthetase enzymes responsible for [(14)C]palmitic acid were rapidly saturated at a low substrate level (0.3 μM malonyl CoA). Increasing the level of [2-(14)C]malonyl CoA permitted further synthesis of [(14)C]stearate and [(14)C]oleate. Desaturation of [(14)C]stearate to [(14)C]oleate was stimulated by increasing the levels of NADPH and ferredoxin. The desaturase and elongase enzymes were sensitive to acidic pH. The desaturase was also unstable at 41° C, although fatty acid synthetase and elongase were unaffected by this temperature.

Methyl-directed desaturation of arachidonic to eicosapentaenoic acid in the fungus, Saprolegnia parasitica

The lipids of Saprolegnia parasitica contain 5,8,11,14,17-eicosapentaenoic acid as major constituent. No other acid having (n-3) structure was detected, but 5,8,11,14-eicosatetraenoic (arachidonic) acid and its common precursors of (n-6) structure are present in significant amounts. During rapid growth of the organism, [1-14C]acetate was efficiently incorporated into fatty acids. Arachidonic acid was labeled after 2 h to nearly the same extent as any precursor acid and 14C in eicosapentaenoic acid reached this level within 6 h. Results of incubations with labeled fatty acids indicated that, in S. parasitica, oleic, linoleic, (6,9,12)-linolenic and arachidonic acids are major intermediates in the pathway to eicosapentaenoic acid. Methyl-directed desaturation of (n-6) to (n-3) acids does not occur with C18 acids but is specific for the polyunsaturated C20 chain length. Arachidonic acid is the direct precursor of eicosapentaenoic acid.