Regulation of fatty acid synthesis.

Acetyl-CoA carboxylase and fatty acid synthetase are the two major enzymes involved in the synthesis of fatty acids in animals. The activities of both enzymes are affected by nutritional manipulations. Although acetyl-CoA carboxylase is considered generally to be the rate-limiting step in lipogenesis, there is evidence that suggests that fatty acid synthetase may become rate limiting under certain conditions. The principal support for the view that acetyl-CoA carboxylase is the rate-limiting enzyme for lipogenesis is that the activity of the enzyme is controlled by allosteric effectors that change the catalytic efficiency of the enzyme. Until recently, the only known control of fatty acid synthetase was through changes in rate of enzyme synthesis. Data are reviewed that show that fatty acid synthetase can exist in forms possessing different catalytic activities. Thus fatty acid synthetase appears to be subject to the type of control necessary for an enzyme to serve as a regulator of the rate of a biological process over a short term.     

Studies on the synthesis of rat liver fatty acid synthetase.

The synthesis of the multienzyme complex rat liver fatty acid synthetase was investigated utilizing modifications of methods developed in the laboratory of Schimke (Schimke, R. T. (1964) J. Biol. Chem. 239, 3808-3817 and Arias, I. M., Doyle, D., and Schimke, R. T. (1969) J. Biol. Chem. 244, 3303-3315). The relative amounts of radioactivity from a pulse of labeled lysine appearing in polypeptides derived from purified synthetase complex can be measured compensating for the varying amounts of lysine per polypeptide chain. The results show that labeled amino acid is incorporated into polypeptides derived from the complex at heterogeneous rates. However, 10 to 15 hours after the administration of a pulse, the amount of label per lysine residue in these polypeptides is identical. The results support the previously proposed model of this multienzyme complex (Tweto, J., Dehlinger, P., and Larrabee, A. R. (1972) Biochem. Biophys. Res. Commun. 48, 1371-1377). The previous work and that reported here suggests the existence of a pool of synthetase subunits which is an obligatory intermediate in both synthesis and turnover of the complex. The results obtained in this work are consistent with this model if the exchange of subunits into the intact complex is a relatively slow process requiring several hours to reach equilibrium.     

Inhibition of polyketide synthesis in Alternaria alternata by the fatty acid synthesis inhibitor cerulenin.

The fatty acid synthase inhibitor cerulenin (50 to 100 micrograms/ml) inhibited production of the polyketide mycotoxins alternariol (AOH) and alternariol monomethyl ether (AME) by the mold Alternaria alternata. The results suggested that AOH synthesis was inhibited by a direct mechanism by cerulenin, whereas production of AME was probably limited by a shortage of the precursor AOH.     

Inhibition of polyketide synthesis in Alternaria alternata by the fatty acid synthesis inhibitor cerulenin.

The fatty acid synthase inhibitor cerulenin (50 to 100 micrograms/ml) inhibited production of the polyketide mycotoxins alternariol (AOH) and alternariol monomethyl ether (AME) by the mold Alternaria alternata. The results suggested that AOH synthesis was inhibited by a direct mechanism by cerulenin, whereas production of AME was probably limited by a shortage of the precursor AOH.     

Cell-free synthesis of pigeon liver fatty acid synthetase.

Pigeon liver fatty acid synthetase proteins (apo- and holo-forms) have been synthesized in a cell-free system reconstituted from polysomes and a soluble enzyme fraction. Identification of the cell-free synthesized products as fatty acid synthetase was achieved by affinity chromatography, by immuno-precipitation and by the simultaneous conversion of both the authentic carrier protein and the $\text{in vitro}$ synthesized products from the holo- to the apo-form of the synthetase. The reverse conversion was also effected.     

Inhibition of plant fatty acid synthesis by nitroimidazoles.

1. The effect of the addition of a number of nitroimidazoles was tested on fatty acid synthesis by germinating pea seeds, isolated lettuce chloroplasts and a soluble fraction from pea seeds. 2. All the compounds tested had a marked inhibition on stearate desaturation by lettuce chloroplasts and on the synthesis of very-long-chain fatty acids by pea seeds. 3. In contrast, the effect of the drugs on total fatty acid synthesis from [14C]acetate in chloroplasts was related to the compound's electron reduction potentials. 4. Of the compounds used, only metronidazole had a marked inhibition on palmitate elongation in the systems tested. 5. The mechanism of inhibition of plant fatty acid synthesis by nitroimidazoles is discussed and the possible relevance of these findings to their neurotoxicity is suggested.     

Mercury inhibition of fatty acid synthesis in chicks.

Male chicks were fed a commercial ration and were given drinking water which contained 0, 50, 100, 150, 200 or 300 mug of mercury/ml as mercuric chloride from hatching to 3 weeks of age. In one experiment, the mercuric chloride was administered by injection into the abdominal cavity rather than in the drinking water. At 3 weeks the chicks were killed, and the livers were removed and weighed. The activity of fatty acid synthetase in the 800 X gav supernatant fractions of the liver homogenates and in vivo incorporation of [14C]acetate into liver and carcass fatty acids and respiratory 14CO2 was determined as indicated. Administration of mercury at a treatment level of 300 mug/ml of drinking water depressed growth, feed and water consumption, liver weight, hepatic fatty acid synthetase activity, and in vivo incorporation of [14C]acetate into liver and carcass fatty acids, and increased the production of respiratory 14CO2 as compared with controls. In experiments in which graded doses of mercury were administered, body weights, liver weights, and feed and water intakes of the chicks receiving 0, 50 and 100 mug of mercury/ml of drinking water were similar to each other, but these parameters were severely depressed by 200 mug of mercury/ml of drinking water. Mercury caused a dose-related decrease of fatty acid synthetase activity. Incorporation of [14C]acetate into carcass fatty acid was depressed by 50 and 200 mug of mercury/ml of drinking water; incorporation into liver fatty acids and production of respiratory 14CO2 was not affected by mercury. Intra-abdominal injection of 6 mg of mercury/100 g body weight (as mercuric chloride) into well alimented chicks depressed hepatic fatty acid synthetase activity at 1 h post-injection. The data are consistent with the hypothesis that a portion of the effects of mercury on fatty acid synthesis are direct rather than a secondary effect of inanition.     

Medium-chain fatty acid synthesis by goat mammary-gland fatty acid synthetase. The effect of limited proteolysis.

Fatty acid synthetase from goat mammary gland was subjected to limited proteolysis by trypsin and elastase. Both proteolytic enzymes selectively cleaved the chain-terminating thioester hydrolase component from the enzyme complex, leaving all other partial activities intact in the core peptides. Trypsin, but not elastase, caused extensive degradation of the released thioester hydrolase. The released thioester hydrolase could be purified to homogeneity by gel filtration. The molecular weight was estimated as 29 000 and the enzyme showed only significant hydrolytic activity toward long-chain acyl-CoA esters. The core peptides retained the ability to synthesize medium-chain acyl-CoA esters in the presence of 2,6-di-O-methyl-alpha-cyclodextrin. The results conclusively show that the terminating thioester hydrolase of goat mammary-gland fatty acid synthetase is not involved in termination of medium-chain-length fatty acid synthesis by this enzyme.     

Carbon flux and fatty acid synthesis in plants.

The de novo synthesis of fatty acids in plants occurs in the plastids through the activity of fatty acid synthetase. The synthesis of the malonyl-coenzyme A that is required for acyl-chain elongation requires the import of metabolites from the cytosol and their subsequent metabolism. Early studies had implicated acetate as the carbon source for plastidial fatty acid synthesis but more recent experiments have provided data that argue against this. A range of cytosolic metabolites including glucose 6-phosphate, malate, phosphoenolpyruvate and pyruvate support high rates of fatty acid synthesis by isolated plastids, the relative utilisation of which depends upon the plant species and the organ from which the plastids are isolated. The import of these metabolites occurs via specific transporters on the plastid envelope and recent advances in the understanding of the role of these transporters are discussed. Chloroplasts are able to generate the reducing power and ATP required for fatty acid synthesis by capture of light energy in the reactions of photosynthetic electron transport. Regulation of chloroplast fatty acid synthesis is mediated by the response of acetyl-CoA carboxylase to the redox state of the plastid, which ensures that the carbon metabolism is linked to the energy status. The regulation of fatty acid synthesis in plastids of heterotrophic cells is much less well understood and is of particular interest in the tissues that accumulate large amounts of the storage oil, triacylglycerol. In these heterotrophic cells the plastids import ATP and oxidise imported carbon sources to produce the required reducing power. The sequencing of the genome of Arabidopsis thaliana has now enabled a number of aspects of plant fatty acid synthesis to be re-addressed, particularly those areas in which in vitro biochemical analysis had provided equivocal answers. Examples of such aspects and future opportunities for our understanding of plant fatty acid synthesis are presented and discussed.     

Propionate-induced synthesis of odd-chain-length fatty acids by Escherichia coli.

Exogenous propionate is incorporated in vivo by Escherichia coli as a primer to produce lipids with fatty acids of odd chain lengths. This provides a method for the specific labeling of the three terminal carbons in the fatty acyl chains of phospholipids.