Molecules Of acetyl-CoA Research Articles

Effects of palmitoyl CoA on citrate and malate transport by rat liver mitochondria.

Palmitoyl CoA inhibited citrate transport from isolated rat liver mitochondria. Under conditions described, 50% inhibition was observed at about 6-8 nmol/mg of mitochondrial protein per ml. The percentage inhibition was inversely proportional to the concentration of the counter-transporting anion in the medium. Although comparable levels of palmitoyl CoA had little effect on malate exit on the dicarboxylate carrier, higher concentrations inhibited both citrate and malate transport. The specificity of the inhibition by palmitoyl CoA was investigated by examination of the effects on the tricarboxylate transport system of fatty acids, CoASH, acetyl CoA, palmitoylcarnitine, deoxycholate, and other molecules with surface active properties. None of the above compounds, at sublytic concentrations, inhibited citrate transport appreciably. The inhibition of citrate transport by low concentrations of palmitoyl CoA was rapid and could be prevented or partially reversed by addition of albumin.

Biosynthesis of fatty acids in mammary tissue: II. Synthesis of butyrate in lactating rabbit mammary supernatant fraction by the reversal of β-oxidation

An enzyme fraction has been obtained from lactating rabbit mammary particlefree supernatant fraction which synthesizes butyrate from acetyl-CoA by the reversal of β-oxidation. This fraction has been shown to contain acetoacetyl-CoA thiolase, NADH-dependent acetoacetyl-CoA reductase, and crotonyl-CoA hydratase. The reduction of crotonyl-CoA appears to be brought about by the fatty acid synthetase using NADPH or NADH. The two reducing reactions are believed to overcome the thermodynamically unfavorable reaction involved in the condensation of two molecules of acetyl-CoA.

Studies on the synthesis of fatty acids by a beef heart mitochondrial enzyme system

Studies are reported on an enzyme system located on the outer membrane of beef heart mitochondria which synthesizes fatty acids by the elongation of pre-existing fatty acids (or acyl-CoA compounds). Proof that fatty acid primers (octanoate, palmitate and linoleate) are elongated by the sequential addition of one or more molecules of acetyl-CoA to the carboxyl end of the fatty acid was obtained by gas-liquid chromatography and decarboxylation of the biosynthesized fatty acids. Malonyl-CoA was not a substrate for this reaction and no evidence was obtained for the de novo synthesis of fatty acids. The elongation reaction requires only an acyl-CoA primer, acetyl-CoA and NADH. The potassium salt of a fatty acid plus ATP and Mg ++ can substitute for the acyl-CoA. The primer specificity is very broad—all primers tested from C 6 to C 20 stimulated the incorporation of acetyl-CoA—but the maximum incorporation of acetyl-CoA was observed with octanoic acid as a primer. When linoleic acid served as the primer arachidonic and other polyunsaturated C 20 acids were produced. The optimum pH of the reaction was 7.8-8.0 in glycylglycine buffer. NADPH inhibited the incorporation of acetyl-CoA into fatty acids. Both free and esterified fatty acids (saturated, unsaturated, and β-hydroxy acids) were identified as products of the enzyme reaction; presumably the relative amounts of each were determined by the amount of deacylase activity in the enzyme preparation. A 14-fold purification of enzyme activity was achieved through the preparation of an acetone powder of the mitochondria followed by ammonium sulfate fractionation. The enzyme system appeared to behave as a single unit during subsequent treatments designed to separate proteins on the basis of size and charge.

Effect of Divalent Metal Ions on Nucleotide Inhibition of Pig Heart Citrate Synthase

The effect of divalent metal ions on the reaction catalyzed by pig heart citrate synthase has been investigated. Concentrations of divalent metal ions (Mn++, Mg++, and Ca++) in 100-fold excess of acetyl coenzyme A concentration inhibit the enzymic reaction. It is suggested that the inhibition is due to chelate formation between the divalent metal ions and the acetyl-CoA molecule, most likely at the polyphosphate chain of the molecule. The inhibition of pig heart citrate synthase by adenosine triphosphate and other nucleotides, palmitoyl-CoA, reduced nicotinamide adenine dinucleotide, and reduced nicotinamide adenine dinucleotide phosphate was studied. Divalent metal ions (Mn++, Mg++, and Ca++) at low concentrations reduce the inhibition. We suggest that the effect is due to the chelation of the metal ions with the polyphosphate moiety of the inhibitor molecules.

In Vivo Synthesis of Certain Low Molecular Weight Milk Fatty Acids

Summary Ten lactating dairy cows were given 1-C 14 and 2-C 14 acetate and 1-C 14 , 2-C 14 , and 3-C 14 butyrate by intravenous injection in a series of separate investigations. Butyric, caproic, and caprylic acids were isolated from the milk fat produced at 3 and 10 hr after injection and were degraded to determine their intramolecular C 14 labeling patterns. Our results provide additional proof that low molecular weight milk fatty acids are synthesized via the successive addition of two carbon units. These condensation reactions appear to be highly organized and give rise to carbon 14 labeling patterns which suggest that the carboxyl carbon of the primary precursor—presumably Acetyl CoA—retains its identity as the carboxyl carbon of the higher milk fatty acids. Thus, carbon chain elongation seems to proceed at the methyl end of the primary precursor and of each successive two carbon homologue. Lastly, we obtained strong evidence that the four carbon skeleton of butyrate may be utilized as such for the synthesis of the lower milk fatty acids. However, it appears that this is a relatively minor pathway of butyrate metabolism—the major pathway being beta oxidation to Acetyl CoA molecules which are, in turn, used for fatty acid synthesis.

Enzymes of fatty acid metabolism

The intermediates in the biological breakdown and synthesis of fatty acids are S-acyl derivatives of coenzyme A. Fatty acid synthesis is accomplished through repetition of a cycle of four consecutive reactions: a. Condensation of two molecules of acetyl CoA to form acetoacetyl CoA and coenzyme A (CoASH); b. reduction of acetoacetyl CoA to β-hydroxybutyryl CoA; c. dehydration of β-hydroxybutyryl CoA to crotonyl CoA, and d. reduction of crotonyl CoA to butyryl CoA. A new cycle is started by the reaction of butyryl CoA with another molecule of acetyl CoA to form β-keto-caproyl CoA + CoA-SH, and so forth. The cycle is repeated eight times until stearyl CoA is formed. All four reactions of the fatty acid cycle are reversible and fatty acid oxidation, once the fatty acid is activated through conversion to the corresponding S-acyl CoA derivative, proceeds by a reversal of the above sequence. There are two main mechanisms for activation of fatty acids: (a) By a reaction with ATP and CoA to form S-acyl CoA, adenosine monophosphate and pyrophosphate, and (b) by transfer of CoA from certain acyl CoA compounds such as acetyl CoA or succinyl CoA. The isolation and identification of some of the key enzymes of fatty acid metabolism is outlined and their mechanism of action discussed.