Erucic acid oxidation by beating heart cells in culture

Fatty acid oxidation in the heart

The heart is known for its ability to produce energy from fatty acids (FA) because of its important beta-oxidation equipment, but it can also derive energy from several other substrates including glucose, pyruvate, and lactate. The cardiac ATP store is limited and can assure only a few seconds of beating. For this reason the cardiac muscle can adapt quickly to the energy demand and may shift from a 100% FA-derived energy production (after a lipid-rich food intake) or any balanced situation (e.g., diabetes, fasting, exercise). These situations are not similar for the heart in terms of oxygen requirement because ATP production from glucose is less oxygen-consuming than from FA. The regulation pathways for these shifts, which occur in physiologic as well as pathologic conditions (ischemia-reperfusion), are not yet known, although both insulin and pyruvate dehydrogenase activation are clearly involved. It becomes of strategic importance to clarify the pathways that control these shifts to influence the oxygen requirement of the heart. Excess FA oxidation is closely related to myocardial contraction disorders characterized by increased oxygen consumption for cardiac work. Such an increased oxygen cost of cardiac contraction was observed in stunned myocardium when the contribution of FA oxidation to oxygen consumption was increased. In rats, an increase in n-3 polyunsaturated FA in heart phospholipids achieved by a fish-oil diet improved the recovery of pump activity during postischemic reperfusion. This was associated with a moderation of the ischemia-induced decrease in mitochondrial palmitoylcarnitine oxidation. In isolated mitochondria at calcium concentrations close to that reported in ischemic cardiomyocytes, a futile cycle of oxygen wastage was reported, associated with energy wasting (constant AMP production). This occurs with palmitoylcarnitine as substrate but not with pyruvate or citrate. The energy wasting can be abolished by CoA-SH and other compounds, but not the oxygen wasting. Again, the calcium-induced decrease in mitochondrial ADP/O ratio was reduced by increasing the n-3 polyunsaturated FA in the mitochondrial phospholipids. These data suggest that in addition to the amount of circulating lipids, the quality of FA intake may contribute to heart energy regulation through the phospholipid composition. On the other hand, other intervention strategies can be considered. Several studies have focused on palmitoylcarnitine transferase I to achieve a reduction in beta-oxidation. In a different context, trimetazidine was suggested to exert its anti-ischemic effect on the heart by interfering with the metabolic shift, either at the pyruvate dehydrogenase level or by reducing the beta-oxidation. Further studies will be required to elucidate the complex system of heart energy regulation and the mechanism of action of potentially efficient molecules.

Erucic acid metabolism in rat liver. A combined biochemical and radioautographical study

Metabolism of erucic acid was studied in rat liver in comparison with oleic acid in relation with diet lipids. Rats were fed for 3 or 60 days a balanced diet containing 30% of the calories of either rapeseed oil rich in erucic acid or sunflower seed oil rich in linoleic acid. They were intravenously injected with tritiated erucic or oleic acid. After 1 or 15 min, the radioactivity recovered in liver lipids was 9 to 26% whatever the diet or the acid injected. One minute after injection of erucic acid a high part of radioactivity was recovered in the free fatty acid fraction and as untransformed erucic acid. After 15 min the major part of radioactivity was recovered in the triacylglycerol fraction which contained a high proportion of labelled oleic acid formed by shortening of erucic acid. The autoradiography did not show any marked difference between the labelling of peroxisomes and mitochondria when tritiated erucic or oleic acid was injected. These data do not bring about arguments for a peroxisomal nor for a mitochondrial location of erucic acid shortening in liver.

Uptake of 22-carbon fatty acids into rat retina and brain

Rat retina accumulates high levels of 22-carbon (C22) polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA, 22:6 omega 3), in rod outer segment (ROS) phospholipids (PL). However, plasma, the source of retina lipids, is enriched in 20-carbon (C20) fatty acids instead of C22 PUFA. This suggests that the retina has a mechanism(s) for selective uptake of C22 PUFA from the blood. It is not known if the selective uptake is specific for the carbon number alone, or if the number of double bonds is also important. To address this question, the following study was carried out using erucic acid (22:1 omega 9) as a metabolic marker molecule. Albino rats were raised from birth on a diet containing 10% (by weight) of either rapeseed oil (43% 22:1 omega 9) or blended canola oil (0.4% 22:1 omega 9). At 4 months of age, plasma, liver, adrenal gland, brain and retina were collected, lipids were extracted, and fatty acids were determined. In those rats fed rapeseed oil, 22:1 omega 9 was incorporated into the lipids of plasma (2.3%), liver (0.6%), and adrenal gland (17.6%), indicating that this fatty acid was absorbed, transported, and metabolized by the rats. However, 22:1 omega 9 was not incorporated into the lipids of retinal ROS or brain. Our results suggest that both the carbon number and degree of unsaturation are important determinants in the selective uptake of C22 fatty acids from plasma into both the brain and the retina.

Erucic acid metabolism in rat heart. A combined biochemical and radioautographical study

Metabolism of Erucic Acid was studied in rat heart in comparison with that of oleic acid, particularly in relation with diet lipids. Rats were fed for 3 or 60 days a diet containing 30% of the calories of either Rapessed Oil, rich in erucic acid or sunflower seed oil rich in linoleic acid. They were I.V. injected with tritiated erucic or oleic acid. After 1 or 15 min the radioactivity recovered in heart lipids was very low whatever the diet (1 to 2%). One minute after injection of erucic acid the radioactivity was mainly recovered in the free fatty acid fraction and as untransformed erucic acid. After 15 min the major part of radioactivity was recovered in the triacylglycerol fraction which contained a high proportion of labelled oleic acid formed by shortening of erucic acid. When oleic was injected, the radioactivity was principally recovered in triacylglycerols as untransformed oleic acid whatever the experimental conditions. Electron microscopy showed that a much higher proportion of peroxisomes, was present in heart cells, following sunflower seed oil diet as compared to rapeseed oil diet. In all cases mitochondria supported the greater part of radioactivity, especially when erucic acid was injected in rats fed rapeseed oil. After sunflower seed oil, a noticeable radioactivity was observed in peroxisomes, most of them containing silver grains, especially when oleic acid was injected. According to the data reported, peroxisomes do not seem more implicated than mitochondria in the metabolism of erucic acid in myocardium.

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.

Chain-shortening of erucic acid and microperoxisomal beta-oxidation in rat small intestine

The ability of rat small intestine to chain-shorten C22:1 fatty acids was investigated. Radioactive chain-shortened products, mainly C18:1, were demonstrated in intestinal-lymph lipids after intraluminal injection of [14-14C]erucic acid. Chain-elongation to C24:1 was also observed. Adaptation to a diet containing C22:1 fatty acids (partially hydrogenated-marine-oil diet) slightly increased the percentage of chain-shortened products. Microperoxisomal beta-oxidation activity, measured as CN(-)-insensitive palmitoyl-CoA-dependent NAD+ reduction, was detected in a microperoxisome-enriched fraction from mucosal scrapings. This activity was increased 1.9-fold by a soya-bean-oil diet, and 2.7-fold by a diet containing partially hydrogenated marine oil.

Preferential distribution of non-esterified fatty acids to phosphatidylcholine in the neonatal mammalian myocardium

Non-esterified fatty acids are used to a limited extent as an energy source in the newborn-mammalian heart. Therefore additional roles for palmitic and oleic acids during this early period of growth and development were investigated in the cultured neonatal-rat heart cell model system. Our results indicate significant differences in nonesterified-fatty-acid metabolism exist in this system in comparison with the adult rat or embryonic chick heart. Initial rates of depletion of palmitate and oleate from serum-free growth medium by heart cells obtained from 2-day-old rats and maintained in culture for 10 or 11 days were 111 +/- 2 and 115 +/- 3 pmol/min per mg of protein respectively. In serum-containing medium, the initial depletion rates were 103 +/- 3 and 122 +/- 4 pmol/min per mg of protein respectively, when endogenous serum nonesterified-fatty-acid concentrations were included in rate calculations. Less than 1% of the intracellularly incorporated fatty acids were found in aqueous products at any time. After 25 h, 15.5% of the initial palmitate was deposited intracellularly in the phosphatidylcholine lipid fraction, 4.2% in the triacylglycerol + fatty-acid-ester fraction and 3.1% in the sphingomyelin fraction. These results contradict the classical view, based on findings with the lipid-dependent adult heart, that exogenous nonesterified fatty acids are directed intracellularly primarily to pathways of oxidation or to storage as triacylglycerol. More importantly, it underscores the significance of exogenous non-esterified fatty acids in membrane biosynthesis of the developing mammalian heart. Included here is a new method for one-dimensional t.l.c. separation of metabolically important polar lipids.

The effect of clofibrate on heart and plasma lipids in rats fed a diet containing rapeseed oil

The effect of clofibrate on heart and plasma lipids in rats fed a diet containing 30% of the calories as peanut oil (PO) or rapeseed oil (RSO) (42.7% erucic acid and 0.5% eicosenoic acid) was studied. A decrease of erucic acid content to one-third and concomitant increase in the content of 18:1, 16:1 and 16:0 fatty acids in plasma triacylglycerols were observed after administration of clofibrate to rats fed the RSO-diet. It is suggested that these changes reflect the increased capacity of the liver to chain-shorten very long chain length fatty acids. The extent of lipidosis in the heart of rats fed the RSO-diet was decreased by 50% by clofibrate. However, the concentration of erucic acid in heart triacylglycerols decreased much less (30%) than the concentration of all other fatty acids (50-65%). It is concluded that the clofibrate administration increased the oxidative capacity of the heart mitochondria and that the heart cell does not have an efficient system to handle very long chain length monounsaturated fatty acids as does the liver.

In vitro conversion of erucic acid by microsomes and mitochondria from liver, kidneys and heart of rats

Microsomes and mitochondria of liver, kidneys, and heart were incubated with [14-(14)C] erucic acid in three assay media: one favorable for chain elongation (NADPH + KCN), another favorable for beta-oxidation and the last one for shortening (NADP + KCN). Elongating reactions occurred mainly in microsomes, those of kidneys being very active; the mitochondria also showed some activity, heart mitochondria being, however, more active than the microsomes, when considering the amount of erucic acid activated. In the medium for beta-oxidation, practically no shortened fatty acids were found. On the contrary, when beta-oxidation was inhibited, and in the presence of NADP, the formation of shorter monoenes, probably in the outer membrane of the mitochondria, was observed, namely eicosenoic acid in high amount, oleic acid and hexadecenoic acid. Mitochondria from liver were very active as were those of heart, when compared with the quantity of activated erucic acid. In heart, the mitochondria shortened erucic acid into oleic acid and hexadecenoic acid, which were then probably used as energy substrates. With carnitine and without NADP, shortened fatty acids were formed in the mitochondria of liver, probably by the first reactions of beta-oxidation. In this case, the proportions of oleic acid and hexadecenoic acid were higher than with NADP alone. In the presence of carnitine and NADP, the level of the chain-shortening reaction did not differ from that observed with NADP alone. It appears, therefore, that the activated erucic acid is mainly directed towards shortening reactions and not towards transfer reactions across the mitochondrial membranes.

Intracellular lipase activities in heart and skeletal muscle homogenates. The absence of trierucin cleavage by the heart: a possible biochemical basis for erucic acid lipidosis

Rat heart and skeletal muscle homogenates were compared for their intracellular lipolytic activity towards a series of saturated and unsaturated triglycerides from trilaurin (C12:0) to trierucin (C22:1). It is shown that for all triglycerides esterified with fatty acids from C12 to C18, lipolytic activity in heart homogenates was higher than in skeletal muscle homogenates. For these triglycerides there was no relationship between the fatty acid chain length and the lipolytic activity. In both homogenates cleavage of unsaturated triglycerides was higher than cleavage of the homologous saturated triglyceride. Lipolysis of tri-delta-11-eicosenoin (C20:1) was similar in both homogenates but much lower than lypolysis of other triglycerides. Although cleavage of trierucin (C22:1) was very low in skeletal muscle homogenates, it was undetectable in heart homogenates, even when enzyme concentration was increased. A mixture of triglycerides did not show preferential hydrolysis of any simple triglyceride. Trierucin was the only triglyceride that did not complete for lipolytic activity and only with heart homogenates, which shows that that lipase(s) do not cleave trierucin. The absence of lipolytic activity towards trierucin in heart homogenates could explain the selective accumulation of erucic acid-rich triglycerides in hearts of animals fed a diet with a high erucic acid content.

The effects of clofibrate feeding on the metabolism of palmitate and erucate in isolated hepatocytes

The metabolism of palmitate and erucate has been investigated in hepatocytes isolated from control rats and from rats fed 0.3% clofibrate. Clofibrate increased the oxidation of [1-14C]palmitate 1.5 to 2-fold while the esterification was decreased. At a high concentration of palmitate (1.5 mM), the total rate of fatty acid metabolism was stimulated. Clofibrate stimulated both the oxidation (3.5 to 5-fold) and the esterfication (1.7-fold) of [14-14C]erucate. Erucate undergoes chain-shortening in isolated liver cells. This chain-shortening was stimulated at least 2-fold by clofibrate feedings. The isolated mitochondrial fraction from clofibrate-fed rats showed an increased capacity for oxidation of short-chain acylcarnitines (including acetylcarnitine), while the oxidation of palmitoyl- and erucoylcarnitine showed little change. It is suggested that erucate is shortened by the recently detected beta-oxidation system of peroxisomes.

Monoethlenic C20 and C22 fatty acids in marine oil and rapeseed oil. Studies on their oxidation and on their relative ability to inhibit palmitate oxidation in heart and liver mitochondria

1. Carnitine esters of erucic acid (22:1 n-9 cis), cetoleic acid (22:1 n-11 cis), brassidic acid (22:1 n-9 trans), gadoleic acid (20:1 n-9 cis) and oleic acid (18:1 n-9 cis) have been compared as mitochondrial substrates and as inhibitors of palmitoylcarnitine oxidation in heart and liver mitochondria. 2. Both the rate of intramitochondrial-CoA acylation and the rate of beta-oxidation decreases as the chain length increases from C18 to C22. There are no significant differences among the three C22 isomers as oxidizable substrates. 3. All the tested acylcarnitines inhibit palmitoylcarnitine oxidation. The C18 and C20 acylcarnitines inhibit by virtue of being competing substrates; i.e. the respiration is not inhibited. The C22-isomers inhibit also respiration; this shows that the inhibition of palmitolycarnitine oxidation is not compensated for by oxidation of C22-acylcarnitines. Brassidoylcarnitine inhibits the oxidation of palmitoylcarnitine and respiration less than erucoyl-and cetoleoylcarnitine. The different behaviour of the C22-isomers is probably due to the difference in their competitive properties with respect to long-chain acyl-CoA dehydrogenase. 4. All C22 acylcarnitines seem to be relatively better oxidized in the liver than in the heart mitochondria while their inhibitory effect on the usage of the radioactive palmitoylcarnitine is very similar. 5. Palmitoylcarnitine inhibits almost completely the "endogenous" formation of acetyl-CoA presumably from malate via pyruvate in the liver mitochondria while the C22-acylcarnitines cause only a partial inhibiton of this acetyl-CaO formation.

Erucic acid and phospholipids of newborn rat heart cells in culture

Erucic acid (delta 13-docosenoic acid), labeled with 14C in the 1- or 14-position, was incorporated into fetal calf serum and fed to beating, neonatal rat myocardial cell in culture. Uptake of the docosenoic acid during the first 6 hr of incubation was 41 nM/hr/mg protein in 7-day old cells and 29 nM/hr/mg protein in 14-day old cells. Fifty-seven percent of the 14C-activity was taken up from the medium in 24 hr, of which 77% was in the cells and 23% was unaccounted for. Of the 14C-activity taken up, 26% was in extractable lipid, with two-thirds in neutral lipid and one-third in phospholipid. Within the neutral lipid fraction, 88% of the 14C-activity was present in triglycerides; while in phospholipids, 66% of the 14C-activity was in phosphatidylcholine (PC); 14% in phosphatidylethanolamine (PE); 6% in sphinogomyelin (SPH) and 1% or less in cardiolipin (DPG). PC had the highest specific activity, followed by SPH and PE. The specific activity of PE was one-half that of SPH when the 14C-erucic acid substrate was labeled at the carboxyl position, but increased to equal that of SPH when the substrate was labeled at the double bond. The fatty acids of PC, PE, and SPH were influenced by erucic acid in the growth medium, but the amounts of each phospholipid were not affected. It is proposed that the altered fatty acid composition associated with incorporation of erucic acid or its metabolites into PC, PE, and SPH may affect integrity and function of heart cell membranes.

Studies on the mechanism of the inhibitory effects of erucylcarnitine in rat heart mitochondria

1. The mechanism of the inhibitory effect of erucylcarnitine on palmityl-carnitine oxidation in rat heart mitochondria was studied. 2. Erucylcarnitine inhibited in the same time the oxidation of [U-14-C]-palmitylcarnitine and the total rate of oxygen uptake. Other acylcarnitines competed as well for the oxidation with radioactive palmitylcarnitine, but they were well oxidized themselves, so that the total oxygen uptake did not decrease. 3. The presence of erucylcarnitine did not change the distribution pattern of Krebs cycle intermediates derived from [U-minus 14 C] palmitylcarnitine except that succinate/malate ratio increased. 4. The presence of erucylcarnitine did not lead to the formation of any beta-oxidation cycle intermediates from [U-minus 14 C] palymitylcarnitine. The formation of beta-hydroxy-palmityl derivative when rotenon was included into the incubation medium, decreased in the presence of erucylcarnitine. 5. It is postulated, that the inhibited entrance of palmityl groups into the beta-oxidation cycle is due to the fact that erucylcarnitine and palmitylcarnitine behave as substrate-competitive inhibitors for long chain acyl-CoA dehydrogenase. 6. There was observed a latency of 1-2 min in the effect of erucylcarnitine on the palmitylcarnitine oxidation, which seems to correspond to the time required for the formation of high amounts of intramitochondrial erucyl-CoA. 7. Erucylcarnitine inhibited the total oxygen uptake with long, medium and short chain acylcarnitines, pyruvate and alpha-ketoglutarate as substrates, while the oxidation of succinate was not affected. 8. Sequestration of free CoA in the form of very slowly metabolized erucyl-CoA is proposed as the partial explanation of the observed inhibitory effects of erucylcarnitine on the oxidation of CoA-dependent substrates (alternatively to the inhibition at the level of acyl-CoA dehydrogenases in case of acylcarnitines).

[Oxidation of erucic acid and erucyl-CoA by isolated rat heart mitochondria: comparison to oleic acid]

The oxidation of [14 14-C] or [1 14-C] erucic acid by isolated mitochondria from Rat heart has been studied and compared with that of [10 14-C] oleic acid in varying conditions of incubation. Erucic acid is converted to CO2 and acid-soluble compounds much more slowly than oleic acid. The acid-soluble compounds which have been identified are acylcarnitines, ketone bodies and intermediates from the Krebs cycle; they are found in similar proportions for both substrates. Moreover, the oxidation rate of erucyl-CoA is comparable, if not equal, to that of oleyl-CoA in the same conditions. These results are discussed here. They lead to the conclusion that erucic acid is oxidized by isolated Rat heart mitochondria through the beta oxidation pathway, and that its oxidation is limited owing to its slow activation rate.