Lack of effect of oral L-carnitine treatment on lipid metabolism and cardiac function in chronically diabetic rats

L-Carnitine is necessary for the transfer of long-chain fatty acids into the mitochondrial matrix where energy production occurs. In the absence of L-carnitine, the accumulation of free fatty acids and related intermediates could produce myocardial subcellular alterations and cardiac dysfunction. Diabetic hearts have a deficiency in the total carnitine pool and develop cardiac dysfunction. This suggested that carnitine therapy may ameliorate alteration in cardiac contractile performance seen during diabetes. In this study, heart function was studied in streptozotocin diabetic rats given L-carnitine orally. Oral L-carnitine treatment (50-250 mg.kg-1.day-1) of 1- and 3-week diabetic rats increased plasma free and total carnitine and decreased plasma acyl carnitine levels. In both groups, myocardial total carnitine levels were increased. However, L-carnitine (200 mg.kg-1.day-1) treatment of diabetic rats for 6 weeks had no effect on plasma carnitine levels. Similarly, plasma lipids remained elevated whereas cardiac function was still depressed. These studies suggest that in the chronically diabetic rat, the route of administration of L-carnitine is an important factor in determining an effect.     

Lipid-mediated impairment of normal energy metabolism in the isolated perfused diabetic rat heart studied by phosphorus-31 NMR and chemical extraction

The relationship between extracellular palmitate and the accumulation of long-chain fatty-acyl coenzyme A with that of high-energy phosphate metabolism was investigated in the isolated perfused diabetic rat heart. Hearts were perfused with a glucose/albumin buffer supplemented with 0, 0.5, 1.2 or 2.0 mM palmitate. ³¹P-NMR was used to analyze phosphocreatine and ATP metabolism during 1 h of constant-flow recirculation perfusion. At the end of perfusion, frozen samples were taken for chemical analysis of high-energy phosphates and the free and acylated fractions of coenzyme A and carnitine. Perfusion of diabetic hearts with palmitate, unlike control hearts, caused a time-dependent and concentration-dependent reduction in ATP, despite normal and constant phosphocreatine. Concentrations of acid-soluble coenzyme A, long-chain-acyl coenzyme A and total tissue coenzyme A were elevated in palmitate-perfused diabetic hearts, while the total tissue carnitine pool was decreased. Increases in long-chain-acyl coenzyme A correlated with the reduction in myocardial ATP. This reduction in ATP could not be adequately explained by alterations in heart rate, perfusion pressure or vascular resistance.     

Carnitine stimulation of glucose oxidation in the fatty acid perfused isolated working rat heart.

The effects of L-carnitine on myocardial glycolysis, glucose oxidation, and palmitate oxidation were determined in isolated working rat hearts. Hearts were perfused under aerobic conditions with perfusate containing either 11 mM [2-3H/U-14C]glucose in the presence or absence of 1.2 mM palmitate or 11 mM glucose and 1.2 mM [1-14C]palmitate. Myocardial carnitine levels were elevated by perfusing hearts with 10 mM L-carnitine. A 60-min perfusion period resulted in significant increases in total myocardial carnitine from 4376 +/- 211 to 9496 +/- 473 nmol/g dry weight. Glycolysis (measured as 3H2O production) was unchanged in carnitine-treated hearts perfused in the absence of fatty acids (4418 +/- 300 versus 4547 +/- 600 nmol glucose/g dry weight.min). If 1.2 mM palmitate was present in the perfusate, glycolysis decreased almost 2-fold compared with hearts perfused in the absence of fatty acids. In carnitine-treated hearts this drop in glycolysis did not occur (glycolytic rates were 2911 +/- 231 to 4629 +/- 460 nmol glucose/g dry weight.min, in control and carnitine-treated hearts, respectively. Compared with control hearts, glucose oxidation rates (measured as 14CO2 production from [U-14C]glucose) were unaltered in carnitine-treated hearts perfused in the absence of fatty acids (1819 +/- 169 versus 2026 +/- 171 nmol glucose/g dry weight.min, respectively). In the presence of 1.2 mM palmitate, glucose oxidation decreased dramatically in control hearts (11-fold). In carnitine-treated hearts, however, glucose oxidation was significantly greater than control hearts under these conditions (158 +/- 21 to 454 +/- 85 nmol glucose/g dry weight.min, in control and carnitine-treated hearts, respectively). Palmitate oxidation rates (measured as 14CO2 production from [1-14C]palmitate) decreased in the carnitine-treated hearts from 728 +/- 61 to 572 +/- 111 nmol palmitate/g dry weight.min. This probably occurred secondary to an increase in overall ATP production from glucose oxidation (from 5.4 to 14.5% of steady state myocardial ATP production). The results reported in this study provide direct evidence that carnitine can stimulate glucose oxidation in the intact fatty acid perfused heart. This probably occurs secondary to facilitating the intramitochondrial transfer of acetyl groups from acetyl-CoA to acetylcarnitine, thereby relieving inhibition of the pyruvate dehydrogenase complex.     

Effects of L-carnitine on mechanical recovery of isolated rat hearts in relation to the perfusion with glucose and palmitate

The purpose of this study was to investigate the effects of L-carnitine on the hemodynamic parameters of Langendorff hearts. Isolated rat hearts were perfused with various solutions containing high or low concentrations of fatty acids, additional glucose or no glucose, and L-carnitine or no L-carnitine. The most interesting part of the experiments was the behaviour of the hearts in the reperfusion period after no-flow ischemia of 20 min. The results were: (1) With glucose and high fatty acid concentrations the hearts showed an improved recovery of the left ventricular functions in the reperfusion period compared with low fatty acid concentrations. Without glucose the left ventricular pressure is much lower in the reperfusion period. (2) Addition of L-carnitine improved the recovery of the ischemically damaged hearts. This improvement is especially impressive at low fatty acid concentrations. L-carnitine addition at high fatty acid concentrations but without glucose strongly improved reperfusion behaviour. (3) The coronary flow is increased by 2 experimental conditions: (i) perfusion at low levels of fatty acids, carnitine and with glucose and (ii) high levels of fatty acids and carnitine but without glucose. These findings suggest that supplementation of L-carnitine has a beneficial effect on the isolated heart under various conditions, and possibly on specific human heart diseases.     

Effects of SM-20550, a selective Na+-H+ exchange inhibitor,on the ion transport of myocardial mitochondria

The study investigated the influence of L-carnitine on the formation of malondialdehyde, an indicator of lipid peroxidation, in isolated Langendorff rat hearts. Earlier investigations of hemodynamic parameters and the recovery of ATP and creatine phosphate, carried out by means of ³¹P-NMR spectroscopy, had demonstrated that, depending on the composition of the perfusates (content of glucose, fatty acids, and carnitine), quite strong differences may occur in the reperfusion period after ischemia.In order to determine a possible relationship between these differences and the addition of carnitine, the study investigated whether carnitine penetrated into the tissue during the experiments, and whether it was able to reduce the concentration of detrimental substances. The concentrations of free and total carnitine as well as the malondialdehyde content as an indicator of ischemia/reperfusion damage were determined in different parts of the cardiac tissue as follows: After the Langendorff-experiments the hearts were dissected, homogenized and reconditioned; then carnitine and malondialdehyde were determined. The study included 63 hearts, which were divided into 8 different perfusion groups.Carnitine concentrations in heart tissue perfused with L-carnitine were much higher than those of the controls. Since exogenous L-carnitine and formed esters could be found in the tissue after the experiment, they must have permeated the cellular membrane rapidly. The concentrations of malondialdehyde behaved in an inverted way; as expected they were lower in carnitine-perfused hearts. The favourable effects of L-carnitine, expressed both by improved cardiac dynamics and ATP and CrP recovery in the reperfusion period, are obviously due to the fact that L-carnitine reduces ischemic damage.     

Palmitic and erucic acid metabolism in isolated perfused hearts from weanling pigs

Hearts from 4 week-old weanling pigs were capable of continuous work output when perfused with Krebs-Henseleit buffer containing 11 mM glucose. Perfused hearts metabolized either glucose or fatty acids, but optimum work output was achieved by a combination of glucose plus physiological concentrations (0.1 mM) of either palmitate or erucate. Higher concentrations of free fatty acids increased their rate of oxidation but also resulted in a large accumulation of neutral lipids in the myocardium, as well as a tendency to increased acetylation and acylation of coenzyme A and carnitine. When hearts were perfused with 1 mM fatty acids, the work output declined below control values. Erucic acid is known to be poorly oxidized by isolated rat heart mitochondria and, to a lesser degree, by perfused rat hearts. In addition, it has been reported that erucic acid acts as an uncoupler of oxidative phosphorylation. In isolated perfused pig hearts used in the present study, erucic acid oxidation rates were as high as palmitate oxidation rates. When energy coupling was measured by 31P-NMR, the steady-state levels of ATP and phosphocreatine during erucic acid perfusion did not change noticeably from those during glucose perfusion. It was concluded that the severe decrease in oxidation rates and ATP production resulting from the exposure of isolated pig and heart mitochondria to erucic acid are not replicated in the intact pig heart.     

Carnitine protection against adriamycin-induced cardiomyopathy in rats

The effects of chronic adriamycin toxicity on myocardial carnitine content and contractile function were studied in rats, along with potential protective effects of L-carnitine administration. Cardiomyopathy was induced over a 6- to 7-week period by weekly intravenous injections of adriamycin, 2 mg/kg. In vivo myocardial tissue levels of carnitine were not significantly changed by adriamycin, but plasma levels were elevated. Cardiac output was depressed in isolated perfused hearts from adriamycin-treated rats perfused with 11 mM glucose. In a second experiment, 4-week-old male rats were divided into four groups: saline-treated control, L-carnitine-treated control, saline-treated adriamycin, and L-carnitine-treated adriamycin. L-Carnitine was given intraperitoneally each day at a dose of 500 mg/kg. Myocardial histology and ultrastructure were analyzed. Cardiac performance was determined in hearts perfused with 1.2 mM palmitate and 5.5 mM glucose. Hearts from saline-treated adriamycin rats showed histopathological changes and a significantly diminished cardiac output at various preloads when compared to saline-treated controls. Daily intraperitoneal L-carnitine reduced histopathological alterations and improved cardiac performance.     

Metoprolol improves cardiac function and modulates cardiac metabolism in the streptozotocin-diabetic rat

The effects of diabetes on heart function may be initiated or compounded by the exaggerated reliance of the diabetic heart on fatty acids and ketones as metabolic fuels. beta-Blocking agents such as metoprolol have been proposed to inhibit fatty acid oxidation. We hypothesized that metoprolol would improve cardiac function by inhibiting fatty acid oxidation and promoting a compensatory increase in glucose utilization. We measured ex vivo cardiac function and substrate utilization after chronic metoprolol treatment and acute metoprolol perfusion. Chronic metoprolol treatment attenuated the development of cardiac dysfunction in streptozotocin (STZ)-diabetic rats. After chronic treatment with metoprolol, palmitate oxidation was increased in control hearts but decreased in diabetic hearts without affecting myocardial energetics. Acute treatment with metoprolol during heart perfusions led to reduced rates of palmitate oxidation, stimulation of glucose oxidation, and increased tissue ATP levels. Metoprolol lowered malonyl-CoA levels in control hearts only, but no changes in acetyl-CoA carboxylase phosphorylation or AMP-activated protein kinase activity were observed. Both acute metoprolol perfusion and chronic in vivo metoprolol treatment led to decreased maximum activity and decreased sensitivity of carnitine palmitoyltransferase I to malonyl-CoA. Metoprolol also increased sarco(endo)plasmic reticulum Ca(2+)-ATPase expression and prevented the reexpression of atrial natriuretic peptide in diabetic hearts. These data demonstrate that metoprolol ameliorates diabetic cardiomyopathy and inhibits fatty acid oxidation in streptozotocin-induced diabetes. Since malonyl-CoA levels are not increased, the reduction in total carnitine palmitoyltransferase I activity is the most likely factor to explain the decrease in fatty acid oxidation. The metabolism changes occur in parallel with changes in gene expression.     

Control of fatty acid metabolism in ischemic and hypoxic hearts

The effects of whole heart ischemia on fatty acid metabolism were studied in the isolated, perfused rat heart. A reduction in coronary flow and oxygen consumption resulted in lower rates of palmitate uptake and oxidation to CO2. This decrease in metabolic rate was associated with increased tissue levels of long chain acyl coenzyme A and long chain acylcarnitine. Cellular levels of acetyl-CoA, acetylcarnitine, free CoA, and free carnitine decreased. These changes in CoA and its acyl derivatives indicate that beta oxidation became the limiting step in fatty acid metabolism. The rate of beta oxidation was probably limited by high levels of NADH and FADH2 secondary to a reduced supply of oxygen. Tissue levels of neutral lipids showed a slight increase durning ischemia, but incorporation of [U-14C]palmitate into lipid was not altered significantly. Although both substrates for lipid synthesis were present in higher concentrations during ischemia, compartmentalization of long chain acyl-CoA in the mitochondrial matrix and alpha-glycerol phosphate in the cytosol may have accounted for the relatively low rate of lipid synthesis.     

The properties and regulation of pantothenate kinase from rat heart

Pantothenate kinase (ATP:D-pantothenate 4'-phosphotransferase, EC 2.7.1.33), the first enzyme in the pathway of CoA synthesis, was partially purified from rat heart. A study of the properties of the kinase showed that it possesses a broad pH optimum between 6 and 9, is activated or inhibited nonspecifically by various anions, and has MgATP as the nucleotide substrate. The Km for MgATP is 0.6 mM and that for pantothenate is 18 microM. CoA and acyl esters of CoA are inhibitors of the kinase with the inhibition by acetyl-CoA being only slightly greater than that by free CoA. The inhibition by free CoA is uncompetitive with respect to pantothenate concentration, with a Ki for inhibition of 0.2 microM. L-Carnitine was found to be a nonessential activator of the kinase. This compound had no effect by itself but specifically reversed the inhibition of the kinase by CoA. The Ka for deinhibition by L-carnitine is 0.27 mM. Free carnitine content was measured in perfused hearts and is found to vary in correlation with perfusion conditions that are known to alter rates of intracellular phosphorylation of pantothenate. These properties of pantothenate kinase provide a potential mechanism for the control of CoA synthesis. The enzyme is regulated by feedback inhibition by CoA and its acyl esters and this inhibition is modified by changes in the concentration of free carnitine.     

Changes in tissue levels of carnitine and other metabolites during myocardial ischemia and anoxia

The induction of ischemia in the open chest dog, or anoxia in the perfused rat heart, causes dramatic changes in the tissue levels of free acyl carnitine and related metabolites. During the early phase of ischemia or anoxia the tissue levels of free carnitine decline, while acetyl carnitine rapidly increases. These changes are accompanied by elevation in long-chain acyl carnitine, long-chain acyl CoA, and lactate and by decreases in acetyl CoA, CoA, ATP, and creatine phosphate. As the degree of ischemia becomes more severe, carnitine appears to be lost from the myocardium. A scheme is presented which relates carnitine-linked mitochondrial metabolism to the activity of carnitine acyl transferase, ANT, carnitine/acyl carnitine translocase, creatine phosphokinase, and pyruvate dehydrogenase. It is suggested that the conversion of carnitine to acyl carnitine during the onset of ischemia may play an important role, by virtue of its effect on these enzymes, in the regulation of metabolism during the early or reversible phase of ischemia.     

The effects of pantothenic acid,cysteine and dithiothreitol in intact,reperfused pig hearts

The objective of this study was to augment myocardial tissue levels of amphiphiles using a treatment protocol of pantothenic acid, cysteine and dithiothreitol (DTT) in 24hr fasted pigs and to test their influence on mechanical recovery in reperfusion. Eighteen pig hearts were extracorporeally perfused aerobically, subjected to regionally reversible ischemia in the left anterior descending perfusion system and reperfused. Nine hearts served as a placebo group; nine hearts were treated. All hearts received trace-labeled palmitate to measure fatty acid oxidation and were perfused with an infusion of 20% Intralipid to augment perfusate levels of fatty acids. Fasting alone in the presence of carbon substrates in the coronary perfusate was not sufficient to de-inhibit pantothenic acid kinase such that CoA synthesis was not enhanced. Tissue contents of triacylglycerols and phospholipids in reperfused myocardium were no different than in aerobic heart muscle but free CoA and free and total carnitine were reduced, suggesting a leakage of cytosolic contents across injured sarcolemma. Treatment significantly impaired mechanical recovery during reflow, presumable due to the noxious properties of DTT whose reported effects in heart muscle are wide ranging, difficult to predict in intact hearts and may be harmful.     

Salient effects of l-carnitine on adenine-nucleotide loss and coenzyme A acylation in the diabetic heart perfused with excess palmitic acid: A phosphorus-31 NMR and chemical extract study

The beneficial effects of l-carnitine perfusion on energy metabolism and coenzyme A acylation were studied in isolated hearts from control and diabetic rats. All hearts were perfused at a constant flow rate with a glucose/albumin buffer which contained 2.0 mM palmitate. ³¹P-NMR was utilized to assess sequential phosphocreatine and ATP metabolism during 1 h of recirculation perfusion. l-Carnitine (5.0 mM final concentration) was added after 12 min of baseline recirculation perfusion. Frozen samples were taken after 1 h of recirculation perfusion for spectrophotometric analysis of high-energy phosphates and the free and acylated fractions of coenzyme A. l-Carnitine perfusion of diabetic hearts attenuated or prevented the reduction of ATP observed in untreated diabetic hearts. It also attenuated the accumulation of long-chain fatty-acyl coenzyme A. Although l-carnitine improved myocardial function in diabetic hearts, this was independent of any direct effect on physiological indices. Thus, the salutory effect of acute perfusion with l-carnitine on energy metabolism in the isolated perfused diabetic rat heart appears to be a direct effect on lipid metabolism.     

Effect of ischemia on fatty acid metabolism in fetal lung

The effects of ischemia on in vivo fatty acid metabolism in fetal lung were studied using rabbit fetuses of 25 to 28 gestational age. Ischemia was produced by inflating the aortic balloon thereby reducing the uterine blood flow. Ischemic insult resulted significant increase in lactate/pyruvate and NADH/NAD ratios and decrease in ATP/ADP ratio in fetal lung. Levels of CoA, acetyl CoA, carnitine and acetyl carnitine decreased while those of long chain acyl CoA and long chain acyl carnitine enhanced. Tissue content of these metabolites returned to normal after 2 hr stabilization following 20 min of ischemic insult. Ischemia also caused small increase in lipogenesis and neutral lipid content of fetal lungs. Our results thus suggest that β-oxidation in fetal lung is inhibited and becomes rate-limiting for fatty acid oxidation during ischemia.Sudden occurrence of hypoxia or ischemia in the fetus is a typical challenge for the obstetricians. The patients occasionally suffer from neurological injury following cerebral hypoxemia. The hypoxic insult may also affect the respiratory activity significantly. For example, acute alveolar hypoxia causes pulmonary vasoconstriction by damaging pulmonary vascular smooth muscle (1) and results in reduction of fatty acid oxidation by limiting the ATP supply required for metabolic processes (2). Hypoxia has also been shown to decrease the rate of palmitate incorporation into phospholipids (3), inhibit rate of fatty acid synthesis (3) and depress rate of incorporation of fatty acid and phosphatidic acid into lipids (4). Despite the fact that fatty acids represent a major substrate for energy metabolism in lung, no work has been done on the fatty acid metabolism in fetal lung. The present study was designed to determine the fate of fatty acid oxidation in fetal lung during ischemic challenge. The levels of acyl CoA and acylcarnitine intermediates were also measured in order to determine the rate-controlling steps of fatty acid metabolism in the fetal lung.     

Tissue specific depletion of L-carnitine in rat heart and skeletal muscle by D-carnitine

L-carnitine deficiency in heart and skeletal muscle was induced by intraperitoneal injection of D-carnitine into starved or fed rats. Carnitine levels in kidney were slightly lowered, but liver, brain and plasma were unaffected. L-carnitine deficient hearts were unable to maintain normal cardiac function when perfused in an isolated working heart apparatus with palmitate as the only perfused substrate. These findings indicate that tissue levels of carnitine in heart and skeletal muscle are maintained $\text{in vivo}$ by an exchange transport mechanism. It is postulated that the depletion of L-carnitine from these tissues occurs by an exchange of the D- and L-isomer across the cell membrane. The technique may be useful for estimating the levels of carnitine required for fatty acid oxidation and normal cardiac and skeletal muscle function; however, interpretation of such tests may be complicated by the inhibitory effects of the D-isomer upon carnitine transferase enzymes.     

Carnitine transport and exogenous palmitate oxidation in chronically volume-overloaded rat hearts

L-Carnitine transport and free fatty acid oxidation have been studied in hearts of rats with 3-month-old aorto-caval fistula. For carnitine transport experiments, the hearts were perfused via the ascending aorta with a bicarbonate buffer containing 11 mM glucose and variable concentrations L-[14C]carnitine (10-200 microM). In some experiments, the active component of carnitine transport was suppressed by the adjunction of 0.05 mM mersalyl acid. The subtraction of passive from total transport allowed reconstruction of the saturation curves of the carrier-mediated transport of L-carnitine. Our data suggest that at a physiological carnitine concentration (50 microM), the rate of [14C]carnitine accumulation was significantly depressed in mechanically overloaded hearts. In addition, according to Lineweaver-Burk analysis, the affinity of the membrane carrier for L-carnitine was considerably diminished (Km carnitine 125 instead of 83 microM, Vmax unchanged). The above alterations of L-carnitine transport did not result from a decrease of the transmembrane gradient of sodium, since the intracellular Na+ content of the hypertrophied hearts was quite similar to that of control hearts. The ability of atrially perfused, working hearts to oxidize the exogenous free fatty acids was assessed from 14CO2 production obtained in the presence of [U-14C]palmitate or [1-14C]octanoate. The total 14CO2 production, expressed per min per g dry weight, was significantly diminished in hearts from rats with the aorto-caval fistula if 1.2 mM palmitate was used. On the other hand, in the presence of 2.4 mM octanoate, a substrate which circumvents the carnitine-acylcarnitine translocase, no such reduction of the 14CO2 production could be detected. Our results suggest that the decrease of L-carnitine transport, resulting in a significant depression of tissue carnitine, may impair long-chain fatty acid activation and/or translocation into mitochondria. In contrast, the oxidation of short-chain fatty acids, the activation of which takes place directly in mitochondrial matrix, is not limited in volume-overloaded hearts.     

Effects of in vivo and in vitro treatment with L-carnitine on isolated hearts from chronically diabetic rats

The effects of in vivo and in vitro L-carnitine administration on cardiac function were studied in isolated perfused working hearts from control and diabetic rats. Injection of L-carnitine (3 g.kg-1.day-1, i.p.) for 2 weeks into rats previously diabetic for 6 weeks partially reversed the adverse effects of chronic diabetes on heart function. In a second experiment, a lower dose of L-carnitine (0.5 g.kg-1.day-1, i.p.) injected for 6 weeks prevented the onset of heart dysfunction in chronically diabetic rats. The protective action of L-carnitine in the myocardium appeared to be independent of any direct pharmacological effects. In both studies, L-carnitine was a potent lipid-lowering agent. The data suggests that L-carnitine administration at either dose had a protective effect against myocardial damage seen during diabetes. The mechanism(s) underlying these effects remains to be elucidated but are discussed.     

Carnitine and derivatives in rat tissues

1. Free carnitine, acetylcarnitine, short-chain acylcarnitine and acid-insoluble carnitine (probably long-chain acylcarnitine) have been measured in rat tissues. 2. Starvation caused an increase in the proportion of carnitine that was acetylated in liver and kidney; at least in liver fat-feeding had the same effect, whereas a carbohydrate diet caused a very low acetylcarnitine content. 3. In heart, on the other hand, starvation did not cause an increase in the acetylcarnitine/carnitine ratio, whereas fat-feeding caused a decrease. The acetylcarnitine content of heart was diminished by alloxan-diabetes or a fatty diet, but not by re-feeding with carbohydrate. 4. Under conditions of increased fatty acid supply the acid-insoluble carnitine content was increased in heart, liver and kidney. 5. The acylation state of carnitine was capable of very rapid change. Concentrations of carnitine derivatives varied with different methods of obtaining tissue samples, and very little acid-insoluble carnitine was found in tissues of rats anaesthetized with Nembutal. In liver the acetylcarnitine (and acetyl-CoA) content decreased if freezing of tissue samples was delayed; in heart this caused an increase in acetylcarnitine. 6. Incubation of diaphragms with acetate or dl-β-hydroxybutyrate caused the acetylcarnitine content to become elevated. 7. Perfusion of hearts with fatty acids containing an even number of carbon atoms, dl-β-hydroxybutyrate or pyruvate resulted in increased contents of acetylcarnitine and acetyl-CoA. Accumulation of these acetyl compounds was prevented by the additional presence of propionate or pentanoate in the perfusion medium; this prevention was not due to extensive propionylation of CoA or carnitine. 8. Perfusion of hearts with palmitate caused a severalfold increase in the content of acid-insoluble carnitine; this increase did not occur when propionate was also present. 9. Comparison of the acetylation states of carnitine and CoA in perfused hearts suggests that the carnitine acetyltransferase reactants may remain near equilibrium despite wide variations in their steady-state concentrations. This is not the case with the citrate synthase reaction. It is suggested that the carnitine acetyltransferase system buffers the tissue content of acetyl-CoA against rapid changes.     

Inhibitory effect of long chain fatty acyl CoAs on RNA polymerase from Escherichia coli

RNA polymerase from Escherichia coli was inhibited by long chain fatty acyl CoAs, such as myristoyl CoA (Ki = 17.2 microM), palmitoyl CoA (Ki = 8.9 microM), oleoyl CoA (Ki = 5.5 microM), and stearoyl CoA (Ki = 0.94 microM). The inhibition by these CoA thioesters was non-competitive against nucleoside triphosphates. Short chain fatty acyl CoAs, such as acetyl CoA, propionyl CoA, acetoacetyl CoA, butyryl CoA, and decanoyl CoA, failed to inhibit RNA polymerase. CoA, Na-myristate, Na-palmitate, Na-oleate, Na-stearate, palmitoyl carnitine, and carnitine did not inhibit the enzyme. The inhibition of RNA polymerase by long chain fatty acyl CoAs was competitive against template DNA.     

L-acetylcarnitine, a substrate for chloroplast fatty acid synthesis

Abstract. H¹⁴CO₃ was not incorporated into fatty acids by isolated pea leaf chloroplasts, which, therefore, do not possess a self-contained pathway for the synthesis of fatty acids from early intermediates of the Calvin cycle. Citrate, pyruvate, acetate and L-acetylcarnitine were all shown to act as sources of acetyl groups for fatty acid synthesis by pea leaf chloroplasts. L-acetylcarnitine was the best substrate, being incorporated into fatty acids at rates that were at least five-fold higher than those achieved with the other substrates. Citrate was incorporated into fatty acids at the lowest rate, followed by pyruvate, with acetate being incorporated at the second highest rate of all. When the isolated chloroplasts were ruptured, an inhibition of L-acetylcarnitine incorporation into fatty acids was noted, whilst acetate incorporation remained unaffected. L-acetylcarnitine also increased the ratio of monoenoic: saturated fatty acids synthesized, compared with a 1:1 ratio observed when citrate, pyruvate and acetate were supplied as substrates. It is suggested that L-carnitine and carnitine acyltransferases play a central role in plant acyl CoA metabolism by facilitating the transfer of activated acyl groups across membranes (acyl CoA barriers).