Inhibition of unsaturated fatty acid synthesis in escherichia coli by the antibiotic cerulenin

Low concentrations of cerulenin inhibit the growth of Escherichia coli by selectively blocking unsaturated fatty acid synthesis. This inhibition was relieved by unsaturated fatty acid supplements alone but not by saturated fatty acid supplements. The utilization of exogenous unsaturated fatty acids to sustain growth in the presence of cerulenin was confirmed by the analysis of bulk lipid composition. The effects of cerulenin on fatty acid synthesis were examined in vivo by pulse labeling with [14C]acetate and in vitro using [14C]malonyl-coenzyme A. In both cases, unsaturated fatty acid synthesis was inhibited by low concentrations of cerulenin with a stimulation of saturated fatty acid synthesis. Using mutant strains deficient in fatty acid synthesis, the effects of cerulenin on beta-ketoacyl-[acyl-carrier-protein] synthetases I and II were examined. Our results indicate that beta-ketoacyl-[acyl-carrier-protein] synthetase I is more sensitive to inhibition by cerulenin than beta-ketoacyl-[acyl-carrier-protein] synthetase II.

The role of malonyl-coa in the coordination of fatty acid synthesis and oxidation in isolated rat hepatocytes

Fatty acid synthesis and fatty acid oxidation were examined in rat hepatocytes under a variety of experimental conditions. In cells from fed animals, glucagon acutely switched the direction of fatty acid metabolism from synthesis to oxidation. Addition of lactate plus pyruvate had the opposite effect. The inhibitory action of glucagon on fatty acid synthesis and its stimulatory effect on fatty acid oxidation were largely, but not completely, offset by the simultaneous addition of lactate plus pyruvate. Changes in cellular citrate and malonyl-CoA levels indicated that glucagon exerted its inhibitory effect on fatty acid synthesis at two levels: (i) blockade of glycolysis; and (ii) partial inhibition of a more distal step, probably acetyl-CoA carboxylase. Under all conditions, fatty acid oxidation was related in a linear and reciprocal fashion to the rate of fatty acid synthesis and the tissue malonyl-CoA content. The latter fluctuated through a range of 1 to 6 nmol per g wet weight of cells. Since malonyl-CoA inhibits carnitine acyltransferase I of liver mitochondria with a Ki in the region of 1 to 2 micron, the present studies support the concept that this compound plays a pivotal role in the coordination of hepatic fatty acid synthesis and oxidation. The ketogenic effect of glucagon on liver appears to be manifested in large part through the ability of the hormone to reduce the tissue malonyl-CoA concentration.

Hepatic malonyl-CoA levels of fed, fasted and diabetic rats as measured using a simple radioisotopic assay

A simple radioisotopic assay for malonyl-CoA is described. The method is based on the malonyl-CoA-dependent incorporation of labeled acetyl-CoA into palmitic acid catalyzed by fatty acid synthetase in the presence of NADPH. Its main advantages over the more conventional spectrophotometric procedure is that it is extremely sensitive and allows the simultaneous determination of picomole quantities of malonyl-CoA in multiple tissue extracts. It should prove particularly suitable for studies on the regulation of lipid metabolism in isolated hepatocytes where the quantity of tissue available for analysis is frequently very small. Application of the method to the measurement of malonyl-CoA in livers from fed, fasted, and diabetic rats yielded values that were consistent with the recently postulated role of malonyl-CoA in the regulation of hepatic ketone body production.

The effect of bovine serum albumin on partial reactions of palmitoyl-CoA chain elongation by rat liver microsomes

In the absence of albumin, v/s curves for both condensation and overall chain elongation demonstrated that the specific activity for overall chain elongation was 3.7 times that of condensation. When the molar ratio of palmitoyl-CoA to albumin was greater than 2 : 1, the specific activity of chain elongation exceeded that of condensation. At these low albumin concentrations, in the absence of NADPH, the beta-ketostearoyl-coA was converted back to palmitate. This cleavage reaction is inhibited by albumin in a concentration-dependent manner. When the palmitoyl-CoA to albumin molar ratio was less than 2 : 1, the specific activity for condensation exceeded that for overall chain elongation and some beta-ketostearate was shown to accumulate under chain elongation conditions. The specific activity for dehydration of beta-hydroxystearoyl-CoA was maximal when the acyl-CoA to albumin molar ratio was between 10 : 1 and 4 : 1 but the rate of this reaction was not markedly influenced by variations in albumin concentration. The specific activity for the NADPH-dependent reduction of 2-trans-octa-decenoyl-CoA was 18 nmol . min(-1) . mg(-1) in the absence of albumin and increased to a maximum of 112 when the substrate to albumin molar ratio was 2 : 1. At higher albumin concentrations the reductase reaction was inhibited. Conversely, the specific activity for the reverse dehydrase was maximal at low albumin concentrations and the rate of this reaction declined as the albumin concentration increased. Our results demonstrate that albumin not only alleviates a substrate induced inhibition but also regulates the metabolic fate of 2-trans-octadecenoyl-CoA and in this regard may possibly substitute for acyl-CoA binding proteins.

Methylmalonic/beta-hydroxy-n-valeric aciduria due to methylmalonyl-CoA mutase deficiency

A patient with methylmalonic and beta-hydroxy-n-valeric aciduria, apparently due to deficiency of methylmalonyl-CoA mutase, is described. The excretion of beta-hydroxy-n-valerate did not parallel that of beta-hydroxypropionate and methylmalonate but was observed, together with beta-keto-n-valerate, only during ketosis. beta-Hydroxy-n-valerate excretion thus correlates primarily not with the pool size of propionyl-CoA but with that of acetyl-CoA, and may occur during ketosis in any disorder causing accumulation of propionyl-CoA.

Role of phosphatidylcholine on microsomal chain elongation and the fate of stearoyl-CoA in rat liver microsomes

The effect of phosphatidylcholine dispersion on the chain elongation of palmitoyl-CoA in rat liver microsomes has been investigated. Addition of phosphatidylcholine increased the formation of stearic acid and its incorporation into phosphatidylcholine. In the presence or absence of phosphatidylcholine, newly-formed stearic acid was preferentially incorporated into the C-1 position of phosphatidylcholine. The incorporation of newly formed stearic acid into microsomes was much higher than that into the added phosphatidylcholine dispersion.

Ketogenesis and malonyl coenzyme A content of isolated rat hepatocytes

We have measured rates of ketogenesis and malonyl-CoA contents of hepatocytes isolated from meal-fed rats under a variety of incubation conditions in order to determine the relationship between the intracellular malonyl-CoA level and the rate of ketogenesis. Evidence obtained from rat liver homogenates suggested that malonyl-CoA, which is a major determinant of fatty acid synthesis in vivo, also inhibits carnitine acyltransferase I (EC 2.3.1.21) and thereby decreases the rate of ketogenesis (McGarry, J.D., Mannaerts, G.P., and Foster, D.W. (1977) J. Clin. Invest. 60, 265-270). In hepatocytes from meal-fed rats, malonyl-CoA could be increased by glucose or lactate plus pyruvate and decreased by glucagon, oleic acid and the fatty acid synthesis inhibitor 5-(tetradecyloxy)-2-furoic acid. Malonyl-CoA varied from 14.8 +/- 1.2 to 1.4 +/- 0.1 nmol/g wet weight of cells. Rates of ketone body production varied from 0.10 +/- 0.01 to 0.96 +/- 0.06 mumol/min/g wet weight of cells and varied inversely with the malonyl-CoA content. Dixon plots and Cornish-Bowden plots of data suggest that malonyl-CoA is a competitive inhibitor of ketogenesis with a Ki of 2 nmol/g wet weight of cells. We conclude that in hepatocytes from meal-fed rats the cellular content of malonyl-CoA and the concentration of long chain fatty acid available to the cells are major determinants of the rate of ketogenesis.

Some characteristics of soluble fatty acid synthesis in germinating pea seeds

Soluble fractions from germinating pea synthesize palmitic acid de novo and stearic acid by elongation. Malonyl CoA, acyl carrier protein and NADPH are required for both reactions. In contrast to some other plant systems, no requirement was found for divalent cations. On the other hand, the formation of both stearate and palmitate was inhibited by sulphydryl reagents and palmitate elongation was sensitive to arsenite. The products of the reactions were examined and found to be principally acyl-acyl carrier proteins and unesterified fatty acids. Unlike the pea microsomal fractions, the soluble enzymes are stimulated only slightly by the addition of exogenous lipids. The substrate for palmitate elongation is palmitoylacyl carrier protein, which is quantitatively elongated to stearate. Comparisons are made with membrane-localised fatty acid synthesis from the same tissue.