Coupling of the biosynthesis of fatty acids and fatty alcohols.

A cell-free system for the biosynthesis of fatty alcohols in the pink portion of the rabbit harderian gland is described. The radiolabeled substrates for the fatty acid reductase were generated using soluble fatty acid synthase from the gland in the presence of acetyl-CoA, malonyl-CoA, and NADPH. Harderian gland microsomes, ATP, and Mg²⁺ were absolute requirements for the synthesis of fatty alcohols and if any of these components were deleted from the assay mixture, no alcohols were detected. We were also unable to detect formation of fatty alcohols if acyl-CoAs were substituted for fatty acid synthase with either NADPH or NADH as reducing agents. The reductase was localized in the microsomal fraction and appears to be on the cytosol-membrane interface of the vesicles, as indicated in experiments using detergents and trypsin. The fatty alcohols formed by the system had the same chain length distribution as the fatty acids made by the fatty acid synthase. The alkyl moieties of the ether lipids in the harderian gland are exclusively saturated and the properties of the alcohol-synthesizing system described in this report can account for the observed exclusion of unsaturated alkyl moieties from the ether lipids of this gland.     

Incomplete fatty acid oxidation. The production and epimerization of 3-hydroxy fatty acids.

3-Hydroxydicarboxylic acids are major urinary metabolites derived from fatty acid metabolism. These compounds are produced from the omega-oxidation of 3-hydroxy fatty acids. The production of the precursor 3-hydroxy fatty acids from incomplete beta-oxidation of fatty acids in rat liver mitochondria was investigated. Independent of the chain length or the concentration of fatty acid substrates, the accumulation of 3-hydroxyacyl intermediates was relatively constant at the concentration of 3-5 nmol/mg of mitochondrial protein. The extent of the incomplete oxidation was the same in Percoll gradient-purified mitochondria. Rotenone treatment increased the production of 3-hydroxy fatty acids. 3-Hydroxy fatty acids did not exist as pure L-enantiomer as expected from beta-oxidation. Instead, these metabolites were epimerized to a near racemic mixture of D- and L-isomers with a slightly dominant D-isomer (58 +/- 3%). By using deuterium-isotope labeling, the mechanism of epimerizartion was shown to be a rapid dehydration-rehydration through trans-2-enoyl-CoA. In addition, cis-3 and trans-3 fatty acids were produced; these metabolites were derived from the isomerization of trans-2-enoyl-CoA. Epimerase and isomerase were thought to be enzymes involved in the oxidation of unsaturated fatty acids. Current data have shown that the metabolism of these acids is actually through NADPH-dependent reduction pathways. The activities of epimerase and isomerase detected in rat liver mitochondria possibly function mainly in the metabolism of saturated fatty acids in a reverse role to the conventional concept.     

A fluorescently labeled intestinal fatty acid binding protein. Interactions with fatty acids and its use in monitoring free fatty acids.

The fatty acid-binding protein from rat intestine (I-FABP) has been covalently modified with the fluorescent compound Acrylodan. Acrylodan was found to label Lys27, one of the few amino acid residues found by x-ray diffraction studies to change orientation upon fatty acid (FA) binding to I-FABP. Binding of FA to this Acrylodan-modified I-FABP (ADIFAB) induces a large shift in fluorescence emission wavelength from 432 to 505 nm. As a consequence, the ratio of emission intensities provides a direct measure of the concentration of FA bound to the protein. Binding of FA is well described by single site equilibrium for FA concentrations below the critical micelle concentration. ADIFAB dissociation constants (Kd) determined at 37 degrees C and at concentrations below the critical micelle concentration for oleate, palmitate, linoleate, arachidonate, and linolenate were, respectively, 0.28, 0.33, 0.97, 1.6, and 2.5 microM. The variation of these Kd values with FA molecular species is highly correlated with the solubility of the FA in water, suggesting that all these FA bind with a similar conformation in the I-FABP binding site. The ADIFAB response together with the measured equilibrium constants allows a direct determination of the concentration of long chain free fatty acid (FFA) in the concentration range, depending upon the FA molecular species, between 1 nM and > 20 microM. As an example of its use as a probe to measure FFA levels, ADIFAB is used here to monitor the time course for FFA release from IgE receptor- and ionomycin-activated rat basophilic leukemia (RBL) cells.     

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 fatty acid-binding proteins. The binding properties of rat liver fatty acid-binding protein.

The inactive 2Fe species of the Fe protein of the nitrogenase of Klebsiella pneumoniae was generated by treating oxidized Fe protein (Kp2) with MgATP and chelator. Incubation of the 2Fe species of Kp2 with the sulphurtransferase rhodanese in the presence of thiosulphate, ferric citrate and reduced lipoate reproducibly restored activity. The extent of restoration of activity depended on the molar ratio of 2Fe Kp2 to rhodanese and was time-dependent. Re-activation did not occur in the reaction mixture lacking rhodanese.     

Acute fatty liver of pregnancy.

Acute fatty liver of pregnancy (AFLP) is rare and is peculiar to the latter half of pregnancy. Despite the high rates of death among affected mothers and their fetuses, early recognition of the disease and immediate delivery of the infant may improve the chances of survival. This paper describes a case of AFLP, characterized by a rapid decrease in the size of the liver, a greatly prolonged prothrombin time and minimal increases in the serum transaminase levels, in which an immediate cesarean section followed by vigorous supportive care led to survival of both mother and infant. It is clear that guidelines on treatment are necessary if the management of such cases is to be successful.     

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.     

Studies on fatty acid-binding proteins. The diurnal variation shown by rat liver fatty acid-binding protein.

The concentration of fatty acid-binding protein in rat liver was examined by SDS/polyacrylamide-gel electrophoresis, by Western blotting and by quantifying the fluorescence enhancement achieved on the binding of the fluorescent probe 11-(dansylamino)undecanoic acid. A 2-3-fold increase in the concentration of this protein produced by treatment of rats with the peroxisome proliferator tiadenol was readily detected; however, only a small variation in the concentration of the protein due to a diurnal rhythm was observed. This result contradicts the 7-10-fold variation previously reported for this protein [Hargis, Olson, Clarke & Dempsey (1986) J. Biol. Chem. 261, 1988-1991].