The effect of essential fatty acid deficiency upon fatty acid uptake by the brain.

Young adult rats, either control or essential fatty acid deficient, were administered either [3-H] oleic acid or [3-H] arachidonic acid by stomach tube. In addition, a group of control rats was given [3-H] palmitic acid. The rats were killed at various times therafter, and the radioactivity of the lipids of brain and plasma was examined. In confirmation of previous work, the blood lipid label was found to rise rapidly and then fall, wheras the activity of brain lipids increased slowly and did not show a decline through the 24-h period studied. Analysis of the brain uptake data according to first-order kinetics confirmed the impressions gained from visual inspection of the data. The initial rate of uptake of arachidonic acid was about 4.5 times that of oleic acid in control animals and in deficient animals. Essential fatty acid deficiency, however, did not induce an altered rate of uptake for either oleic acid or arachidonic acid. The rate of uptake of palmitic acid by control rats was not significantly different from that of oleic acid. Even though the initial rates of incorporation of oleic and arachidonic acids were not changed during essential fatty acid deficiency, the final levels of radioactivity obtained in brain lipids were higher in deficient rats with both fatty acids. The plateau value obtained with oleic acid was 1.5 times higher in deficient animals, while the plateau value for arachidonic acid was 1.7 times higher. An experiment in which deficient animals were allowed access to a control diet for 12 or 24 h prior to the labeling experiment suggested that the higher levels of radioactivity found in brain lipids of deficient animals was not due to an isotope dilution effect. Such animals still displayed the labeling pattern of deficient animals with arachidonic acid, while the results with oleic acid varied somewhat. Our results suggest that essential fatty acid deficiency does not alter the ability of the brain to take up the fatty acids studied. However, the fatty acids, especially arachidonic, are retained in the brain to a greater extent in the deficient animals.     

Requirement for the heart-type fatty acid binding protein in cardiac fatty acid utilization.

Nonenzymatic cytosolic fatty acid binding proteins (FABPs) are abundantly expressed in many animal tissues with high rates of fatty acid metabolism. No physiological role has been demonstrated for any FABP, although these proteins have been implicated in transport of free long-chain fatty acids (LCFAs) and protection against LCFA toxicity. We report here that mice lacking heart-type FABP (H-FABP) exhibit a severe defect of peripheral (nonhepatic, non-fat) LCFA utilization. In these mice, the heart is unable to efficiently take up plasma LCFAs, which are normally its main fuel, and switches to glucose usage. Altered plasma levels of LCFAs, glucose, lactate and beta-hydroxybutyrate are consistent with depressed peripheral LCFA utilization, intensified carbohydrate usage, and increased hepatic LCFA oxidation; these changes are most pronounced under conditions favoring LCFA oxidation. H-FABP deficiency is only incompletely compensated, however, causing acute exercise intolerance and, at old age, a localized cardiac hypertrophy. These data establish a requirement for H-FABP in cardiac intracellular lipid transport and fuel selection and a major role in metabolic homeostasis. This new animal model should be particularly useful for investigating the significance of peripheral LCFA utilization for heart function, insulin sensitivity, and blood pressure.     

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.     

Self-association of the cardiac fatty acid binding protein. Influence on membrane-bound, fatty acid dependent enzymes

The present study on the fatty acid binding protein, purified from pig heart and studied by three independent techniques (electron spin resonance, circular dichroism, and polyacrylamide gel electrophoresis), suggests that the protein self-aggregates and exists in at least four distinct molecular species. This plurality is demonstrated by the presence of four bands after electrophoretic migration at pH 7.2 and by three transitions of molar ellipticity theta 225 that depend on protein concentration. A mathematical model is formulated to simulate the three transitions and to calculate the concentrations of the four species. The multistates manifest themselves in a complex binding capacity for fatty acid, with two sigmoidal components in the binding curve. A general equation for the curve is formulated, and the characteristic constants are evaluated by a nonlinear least-squares fit. The experimental results and their interpretation in quantitative terms lead to a theoretical evaluation of the importance of this new property of self-aggregation of the protein on the activity of membrane-bound model enzymes which are fatty acid or acyl coenzyme A dependent.     

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.     

Structure and dynamics of the fatty acid binding cavity in apo rat intestinal fatty acid binding protein.

The structure and dynamics of the fatty acid binding cavity in I-FABP (rat intestinal fatty acid binding protein) were analyzed. In the crystal structure of apo I-FABP, the probe occupied cavity volume and surface are 539+/-8 A3 and 428 A2, respectively (1.4 A probe). A total of 31 residues contact the cavity with their side chains. The side-chain cavity surface is partitioned according to the residue type as follows: 36-39% hydrophobic, 21-25% hydrophilic, and 37-43% neutral or ambivalent. Thus, the cavity surface is neither like a typical protein interior core, nor is like a typical protein external surface. All hydrophilic residues that contact the cavity-with the exception of Asp74-are clustered on the one side of the cavity. The cavity appears to expand its hydrophobic surface upon fatty acid binding on the side opposite to this hydrophilic patch. In holo I-FABP the fatty acid chain interactions with the hydrophilic side chains are mediated by water molecules. Molecular dynamics (MD) simulation of fully solvated apo I-FABP showed global conformational changes of I-FABP, which resulted in a large, but seemingly transient, exposure of the cavity to the external solvent. The packing density of the side chains lining the cavity, studied by Voronoi volumes, showed the presence of two distinctive small hydrophobic cores. The MD simulation predicts significant structural perturbations of the cavity on the subnanosecond time scale, which are capable of facilitating exchange of I-FABP internal water.     

Light control of fatty acid synthesis and diurnal fluctuations of fatty acid composition in leaves.

1. Although isolated spinach chloroplasts were almost entirely (greater than 99%) dependent on light for fatty acid synthesis, leaf discs were capable of fatty acid synthesis in the dark (up to 500nmol of 3H/h per mg of chlorophyll equivalent to approx. 400nmol of carbon/h per mg of chlorophyll), which represented 12-20% of the corresponding 'light rates'. 2. Net fatty acid accumulation by greening maize leaves occurred largely or entirely during the light period. 3. There was a diurnal fluctuation in the proportions of C18 unsaturated fatty acids in the lipids of developing spinach leaves, where an increase in the concentration of oleate during the day and a subsequent decline at night was observed; a complementary change occurred in the concentration of alpha-linolenate. The rhythm is interpreted as reflecting the continuation of oleate and linoleate desaturation at high rates when oleate synthesis is markedly decreased at night. 4. Changes in the fatty acid composition of 3-sn-phosphatidylcholine accounted for at least 60% of the total decrease in oleate over the dark period. This result is consistent with suggestions that this lipid is the substrate for the leaf microsomal oleate desaturase and an intermediate in leaf glycerolipid biosynthesis.