Role of carnitine in hepatic ketogenesis

Palmitoyl-CoA stimulates cellular uptake and plasma membrane binding of carnitine

Carnitine cellular uptake and plasma membrane binding was investigated in S49 lymphoma cells. Palmitoyl-CoA was found to increase membrane binding of carnitine from 506 +/- 48 to 8,690 +/- 235 pmol/mg membrane protein. Palmitate and CoA acted synergistically and increased carnitine binding to plasma membranes but could not replace palmitoyl-CoA. The effect of palmitoyl-CoA on membrane binding of carnitine was maximal at 10 microM and required the presence of ATP. Palmitoyl-CoA increased the cellular uptake rate of carnitine from 181 +/- 5 to 884 +/- 25 amol/cell and h-1. We conclude that palmitoyl-CoA is a major regulator of cellular uptake of carnitine and, based on quantitative estimations, that the carnitine carrier binds more than one carnitine molecule.

Regulation of carnitine binding to plasma membranes by an ATP-dependent mechanism

This is the first demonstration of L-carnitine binding to plasma membranes. Plasma membranes derived from S49 lymphoma cells bound 40.6 +/- 5.7 pmol carnitine/ mg membrane protein under basal conditions whereas addition of ATP in the presence of magnesium ions increased the number of carnitine binding sites to 557 +/- 82 pmol/mg membrane protein, i.e., a 10-fold increase. Kinetic and equilibrium binding data indicated heterogeneity of carnitine binding sites. ATP modulated carnitine binding sites through a single class of sites at a KD of 20.7 +/- 3.5 microM. The ATP effect seemed mediated by a protein tyrosine kinase as judged from the observed noncompetive inhibition of carnitine binding induced by genistein with a Ki = 65 +/- 11 microM. Active cellular uptake of L-carnitine in S49 lymphoma cells was similarly reduced from 580 +/- 35 to 421 +/- 39 pmol/mg protein/h by genistein.

Acetyl-L-carnitine effects on nerve conduction and glycemic regulation in experimental diabetes

Acetyl-L-Carnitine (ALC), an activator of carnitine, can accelerate nerve regeneration after experimental surgical injury in rats. In this study, we examined the ability of ALC to improve nerve conduction velocity and its effect on intravenous glucose tolerance test in streptozotocin-induced diabetic rats. Diabetic (blood glucose > 200 mg%) and normal animals were treated intraperitoneally for four weeks with ALC, 50 mg/Kg/d and 150 mg/Kg/d. Nerve conduction velocity was measured by direct exposure of sural nerve. Two-hour IVGTT was studied by measuring plasma glucose, insulin and free fatty acids after intravenous injection of glucose, 1.75 gm/Kg/body weight in animals treated either with ALC 150 md/Kg/d or saline alone. Six weeks of STZ-induced diabetes resulted in impairment of nerve conduction velocity in animals injected with saline (16.05 +/- 1.09 m/s), as compared to saline-treated normals who did not receive streptozotocin (31.0 +/- 0.84 m/s, p<0.0005). Diabetic animals treated with ALC, 150 mg/Kg/d, preserved near normal nerve conduction (27.10 +/- 1.42 m/s), compared with the saline-treated diabetic animals (p < 0.0005), but diabetic animals treated with ALC, 50 mg/Kg/d, had a non-significant increase in nerve conduction (23.68 +/- 1.6). ALC treatment had no effect on fasting or post-intravenous plasma glucose in normal or diabetic rats, although it moderately reduced baseline and 40 minute insulin levels (p < 0.02) in normal rats as compared with their saline-treated counterparts. ALC treatment lowered baseline free fatty acids in normal (p < 0.04) and diabetic (p < 0.03) animals, and the 60 minute levels in the normal group only (p < 0.003).

CONCLUSION

ALC at a dose of 150 mg/Kg/d given for one month, produced near normalization of nerve conduction velocity in streptozotocin-induced diabetes with no adverse effects on glucose, insulin or free fatty acid levels.

Carnitine and physical exercise

Carnitine plays a central role in fatty acid (FA) metabolism. It transports long-chain fatty acids into mitochondria for beta-oxidation. Carnitine also modulates the metabolism of coenzyme-A (CoA). It is not surprising that the use of supplementary carnitine to improve physical performance has become widespread in recent years, although there is no unequivocal support to this practice. However, critical reflections and current scientific-based knowledge are important because the implications of reduced or increased carnitine concentrations in vivo are not thoroughly understood. Several rationales have been forwarded in support of the potential ergogenic effects of oral carnitine supplementation. However, the following arguments derived from established scientific observations may be forwarded: (i) carnitine supplementation neither enhances FA oxidation in vivo or spares glycogen or postpones fatigue during exercise. Carnitine supplementation does not unequivocally improve performance of athletes; (ii) carnitine supplementation does not reduce body fat or help to lose weight; (iii) in vivo pyruvate dehydrogenase complex (PDC) is fully active already after a few seconds of intense exercise. Carnitine supplementation induces no further activation of PDC in vivo; (iv) despite an increased acetyl-CoA/free CoA ratio, PDC is not depressed during exercise in vivo and therefore supplementary carnitine has no effect on lactate accumulation; (v) carnitine supplementation per se does not affect the maximal oxygen uptake (VO2max); (vi) during exercise there is a redistribution of free carnitine and acylcarnitines in the muscle but there is no loss of total carnitine. Athletes are not at risk for carnitine deficiency and do not have an increased need for carnitine. Although there are some theoretical points favouring potential ergogenic effects of carnitine supplementation, there is currently no scientific basis for healthy individuals or athletes to use carnitine supplementation to improve exercise performance.

Effects of L-carnitine supplementation on physical performance and energy metabolism of endurance-trained athletes: a double-blind crossover field study

A double-blind crossover field study was performed to investigate the effects of acute L-carnitine supplementation on metabolism and performance of endurance-trained athletes during and after a marathon run. Seven male subjects were given supplements of 2 g L-carnitine 2 h before the start of a marathon run and again after 20 km of the run. The plasma concentration of metabolites and hormones was analysed 1 h before, immediately after and 1 h after the run, as well as the next morning after the run. In addition, the respiratory exchange ratio (R) was determined before and at the end of the run, and a submaximal performance test was completed on a treadmill the morning after the run. The administration of L-carnitine was associated with a significant increase in the plasma concentration of all analysed carnitine fractions (i.e. free carnitine, short-chain acylcarnitine, long-chain acylcarnitine, total acid soluble carnitine, total carnitine) but caused no significant change in marathon running time, in R, in the plasma concentrations of carbohydrate metabolites (glucose, lactate, pyruvate), of fat metabolites (free fatty acids, glycerol, beta-hydroxybutyrate), of hormones (insulin, glucagon, cortisol), and of enzyme activities (creatine kinase, lactate dehydrogenase). Moreover, there was no difference in the result of the submaximal performance test the morning after the run. In conclusion, acute administration of L-carnitine did not affect the metabolism or improve the physical performance of the endurance-trained athletes during the run and did not alter their recovery.

Effects of short- and long-term feeding of L-carnitine and congeners on the production of eicosanoids from rat peritoneal leucocytes

The effect of short- and long-term feeding with L-carnitine, L-acetyl carnitine and L-propionyl carnitine on the production of eicosanoids from in vitro stimulated carrageenan-induced rat peritoneal macrophages was investigated. Both young (4 weeks) and old (18 months) rats were used. A lower number of cells was isolated from the peritonea of treated than control young rats after 4 d feeding, but after 60 d no differences were observed. A similar reduction in cell number was found when old animals were given L-acetyl carnitine or L-propionyl carnitine (acutely) or L-acetyl carnitine or L-carnitine (chronically). Plasma carnitine levels were higher in young rats given carnitine both chronically and acutely. Carnitine derivatives were without effect. In contrast, levels of total carnitine in the plasma of old rats given L-carnitine and L-acetyl carnitine for 4 d and 60 d were higher than in controls. There was no correlation between total plasma carnitine level and effects on prostaglandin, thromboxane and leukotriene B4 (LTB4) production. In young rats the most important changes were observed in relation to the production of prostacyclin (PGI2), measured as 6 keto-prostaglandin F1 alpha. Prostacyclin production was higher in the groups given carnitine or its derivatives. The net result of the changes in PGI2 was that the 6 keto-prostaglandin F1 alpha: thromboxane B2 and the 6-keto-prostaglandin F1 alpha: LTB4 ratios tended to be higher in cells from young animals following short-term feeding with L-carnitine.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of medium-chain and long-chain triglycerides in surgical patients

Available lipid emulsions made from soybean or safflower oil are classified as long-chain triglycerides (LCT). In contrast, medium-chain triglyceride (MCT) emulsions have different physical properties and are metabolized by other biochemical pathways. To compare the differences between these two fat emulsions, the authors studied 12 surgical patients and 6 volunteers. These subjects were randomly assigned to receive parenteral nutrition with MCT or LCT emulsion. Measurement of arterial and venous concentration differences across the forearm demonstrated that muscle utilization was significantly improved with MCT administration. There was also a trend toward improved nitrogen balance in the MCT group, and less weight loss in the postoperative period also was observed in this group. During the fat clearance test, the serum ketone concentrations were significantly higher in the MCT than the LCT group. The improvement in nitrogen retention may be associated with increasing ketone and insulin levels. Fat emulsions containing 50% MCT are safe for use in parenteral nutrition and may provide an alternate fuel that improves protein metabolism.

Effect of carnitine supplement to the dam on plasma carnitine concentration in the sucking foal

The changes in carnitine in plasma and milk during the first 3 months of lactation were studied in 14 broodmares and their foals. Six of the mares (Group S) were given a supplement of 10 g carnitine split between the morning and evening feeds, starting 2 weeks before birth. At birth the plasma carnitine concentration in Group S mares was about twice that in Group NS mares (no supplement). In both groups the concentration initially declined in the days after birth. Whilst this trend was reversed in Group S mares, the concentration in Group NS mares remained at a reduced level for the remainder of the study. Milk concentrations declined continuously over the monitoring period in both groups. There was no apparent relationship between milk and plasma concentrations. Despite this the milk concentration tended to be higher in Group S than in Group NS mares although differences were not significant. There was an immediate drop in the plasma concentration in foals after birth which was reversed in foals of Group S mares but not in those of Group NS mares. There were no apparent side effects of carnitine supplementation.

Plasma glucose, ketone bodies, insulin, glucagon and enteroglucagon in cows: diurnal variations related to ketone levels before feeding and to the ketogenic effects of feeds

Ingestions of a moderately ketogenic silage twice daily were followed by transient increments in plasma insulin and ketone bodies and decreases in plasma glucose. Ketone bodies and glucose were negatively correlated throughout the day, but the insulin elevations culminated before the maximal effects on ketone bodies and glucose were established. Cows with varying glucose levels before morning feeding reacted to a highly ketogenic silage by decreasing their glucose level uniformly to about 3 mmol/l, in spite of a widely varying feeding-induced insulin increment. Hay-feeding caused insulin increments of the same magnitude as silage-feeding, but the glucose decrease and the ketone increment was much smaller. The results indicate some direct action of ketone bodies on blood sugar regulation, in addition to effects mediated by insulin. The role of ketone bodies as the insulinotropic factor was not confirmed. The insulin level after feeding seems to be determined by the carbohydrate status of the animal before feeding. No significant changes in plasma glucagon were observed after feeding, and no consistent differences in plasma levels of this hormone were found when non-ketonemic, ketonemic, and clinically ketotic cows were compared. The plasma level of enteroglucagon (GLI) was positively correlated to the relative amount of concentrates consumed, but no relation to plasma glucose was found.

Plasma carnitine levels in liver cirrhosis: relationship with nutritional status and liver damage

The plasma level of carnitine, a co-factor involved in many metabolic reactions, is high in alcoholic liver cirrhosis, due to an increased amount of esterified carnitine. To determine if this alteration is linked to alcoholic liver disease or to liver cirrhosis per se. total carnitine, free carnitine, total esterified carnitine, short chain acylcarnitine and long chain acylcarnitine were measured in 41 patients suffering from liver cirrhosis of different aetiology and severity. In 19 of these patients, acetylcarnitine was also measured. Moreover, multivariate analysis was performed to assess the association of carnitine plasma levels with nutritional and liver disease indices. Of the nutritional indices (creatinine/height ratio, mid upper arm muscle circumference and triceps skinfold) only triceps skinfold appeared to be weakly correlated with carnitine (with long chain acylcarnitine). Significantly high levels of acetylcarnitine, short chain acylcarnitine, total esterified carnitine and total carnitine were found in cirrhotics independently of the aetiology of cirrhosis, even though a trend towards higher levels of acetylcarnitine was evident in heavy drinkers. Direct correlations of gamma-glutamyltransferase with acetylcarnitine, acetylcarnitine/free carnitine, short chain acylcarnitine/free carnitine and total esterified carnitine/free carnitine were found. Carnitine plasma levels did not differ in the three Pugh-Child's classes; however, a trend towards higher levels of acetylcarnitine was found in Pugh-Child's class C. In conclusion, the high levels of acetylcarnitine, short chain acylcarnitine, total esterified carnitine and total carnitine found in cirrhosis were linked to liver disease. Alcohol abuse seemed only to be an exacerbating factor.

Plasma glucose, ketone bodies, insulin, glucagon and enteroglucagon in cows: diurnal variations related to ketone levels before feeding and to the ketogenic effects of feeds

Ingestions of a moderately ketogenic silage twice daily were followed by transient increments in plasma insulin and ketone bodies and decreases in plasma glucose. Ketone bodies and glucose were negatively correlated throughout the day, but the insulin elevations culminated before the maximal effects on ketone bodies and glucose were established. Cows with varying glucose levels before morning feeding reacted to a highly ketogenic silage by decreasing their glucose level uniformly to about 3 mmol/l, in spite of a widely varying feeding-induced insulin increment. Hay-feeding caused insulin increments of the same magnitude as silage-feeding, but the glucose decrease and the ketone increment was much smaller. The results indicate some direct action of ketone bodies on blood sugar regulation, in addition to effects mediated by insulin. The role of ketone bodies as the insulinotropic factor was not confirmed. The insulin level after feeding seems to be determined by the carbohydrate status of the animal before feeding. No significant changes in plasma glucagon were observed after feeding, and no consistent differences in plasma levels of this hormone were found when non-ketonemic, ketonemic, and clinically ketotic cows were compared. The plasma level of enteroglucagon (GLI) was positively correlated to the relative amount of concentrates consumed, but no relation to plasma glucose was found.

Interaction of carnitine with insulin-stimulated glucose metabolism in humans

To characterize the interactions of carnitine with glucose metabolism, we administered L-carnitine as a primed (3 mmol) constant (17 mumol/min) intravenous infusion to healthy young volunteers during short-term (2 h) euglycemic hyperinsulinemia. In comparison with a control (saline) infusion, exogenous carnitine administration resulted in a stable, fourfold increase in basal serum carnitine levels (160 +/- 14 vs. 36 +/- 2 microM, P less than 0.001). At similar steady-state plasma insulin levels (75 microU/ml), carnitine infusion was associated with a 17 +/- 3% stimulation of whole body glucose utilization (6.56 +/- 0.60 vs. 5.57 +/- 0.44 mg.min-1.kg-1, P less than 0.001). This effect was more pronounced in the subjects with higher rates of glucose disposal (r = 0.65, P less than 0.05). Net rates of insulin-induced glucose oxidation (measured by continuous, computerized indirect calorimetry) were similar with or without carnitine (1.67 +/- 0.23 vs. 1.65 +/- 0.10 mg.min-1.kg-1, respectively). As a consequence, the carnitine-induced enhancement of total glucose metabolism was quantitatively accounted for by a 50% increase in nonoxidative glucose disposal (2.89 +/- 0.81 vs. 1.92 +/- 0.51 mg.min-1.kg-1, P less than 0.05). The inhibitory effect of insulin on net lipid oxidation was not altered by carnitine (-0.67 +/- 0.09 vs. -0.62 +/- 0.06 mg.min-1.kg-1). Circulating levels of free fatty acids (FFA), glycerol, and beta-hydroxybutyrate fell in parallel during insulin infusion in the test and control study, and blood lactate concentrations rose by similar amounts (approximately 0.35 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Carnitine metabolism in the vitamin B-12-deficient rat

In vitamin B-12 (cobalamin) deficiency the metabolism of propionyl-CoA and methylmalonyl-CoA are inhibited secondarily to decreased L-methylmalonyl-CoA mutase activity. Production of acylcarnitines provides a mechanism for removing acyl groups and liberating CoA under conditions of impaired acyl-CoA utilization. Carnitine metabolism was studied in the vitamin B-12-deficient rat to define the relationship between alterations in acylcarnitine generation and the development of methylmalonic aciduria. Urinary excretion of methylmalonic acid was increased 200-fold in vitamin B-12-deficient rats as compared with controls. Urinary acylcarnitine excretion was increased in the vitamin B-12-deficient animals by 70%. This increase in urinary acylcarnitine excretion correlated with the degree of metabolic impairment as measured by the urinary methylmalonic acid elimination. Urinary propionylcarnitine excretion averaged 11 nmol/day in control rats and 120 nmol/day in the vitamin B-12-deficient group. The fraction of total carnitine present as short-chain acylcarnitines in the plasma and liver of vitamin B-12-deficient rats was increased as compared with controls. When the rats were fasted for 48 h, relative or absolute increases were seen in the urine, plasma, liver and skeletal-muscle acylcarnitine content of the vitamin B-12-deficient rats as compared with controls. Thus vitamin B-12 deficiency was associated with a redistribution of carnitine towards acylcarnitines. Propionylcarnitine was a significant constituent of the acylcarnitine pool in the vitamin B-12-deficient animals. The changes in carnitine metabolism were consistent with the changes in CoA metabolism known to occur with vitamin B-12 deficiency. The vitamin B-12-deficient rat provides a model system for studying carnitine metabolism in the methylmalonic acidurias.

Carnitine stimulation of pyruvate dehydrogenase complex (PDHC) in isolated human skeletal muscle mitochondria

L-carnitine stimulated CO2 production from 1-14C pyruvate in mitochondria from human skeletal muscle nearly twofold. A comparable increase in the pyruvate dehydrogenase complex (PDHC) activity was seen. Moreover, in the presence of L-carnitine and at pyruvate concentration greater than 0.25 mM, this effect was associated with a marked increase of acetylcarnitine synthesis. Deoxycarnitine, an inhibitor of carnitine acetyltransferase (CAT), partially reversed the effect of carnitine on PDHC activity. The stimulatory effect of carnitine on PDHC activity in human mitochondria is mediated by the modulation of the intramitochondrial acetyl-CoA/CoASH ratio.

Plasma carnitine alterations in premature infants receiving various nutritional regimes

Plasma carnitine, carnitine esters, and triglyceride concentrations were determined in 36 appropriate-forgestational-age (AGA) infants at various stages of prematurity throughout hospitalization to determine the effect of a carnitine-free and carnitine-containing diet on plasma carnitine and triglyceride concentrations. The infants were entered into one of three experimental groups based on birth weight: group I less than 1.0 kg; group II 1.0-1.51 kg; and group III 1.52-2.5 kg. Throughout the study subjects were placed on appropriate nutritional regimes which included hyperalimentation (HA), intravenous (iv) fat emulsion (Intralipid), Portagen, Enfamil-24 Premature Formula, Enfamil-20, and breastmilk. Blood samples were drawn from each infant at birth, days 1-5,7 then weekly, also before and after each nutritional intervention to determine carnitine and triglyceride concentrations. Results showed that plasma total carnitine and nonesterified carnitine decreased in all groups when the infants were maintained on a carnitine-free diet (HA, Intralipid, Portagen). In general, the carnitine levels continued to decrease until a carnitine-containing diet was initiated. Once a carnitine-containing diet was begun, plasma total carnitine (TC) and nonesterified carnitine (NEC) levels increased at fairly similar rates in all groups. However, an inverse relationship between carnitine and triglyceride (TG) concentrations were not seen in these infants. This would indicate that most premature infants require exogenous carnitine to maintain the plasma concentration of carnitine. However, a decreased concentration of plasma carnitine was not correlated with an elevated TG level under the conditions of this study.

L-carnitine effect on plasma lipoproteins of hyperlipidemic fat-loaded rats

The effect of oral L-carnitine administration to rats fed olive oil has been studied. Carnitine significantly decreased triglyceride, cholesterol and phospholipid levels. Particularly, the levels of chylomicron and very low density lipoproteins in the blood were lowered. Low density lipoprotein levels were not affected, and high density lipoproteins were found to be decreased by 20%. Because carnitine did not change the composition of chylomicron and very low density lipoproteins fraction or affect the gastrointestinal triglyceride residue (about 1/3 of the original load), an effect of carnitine on hepatic fatty acid handling is most likely. The lowering of plasma free fatty acid levels by carnitine administration is in favor of an effect of carnitine on fatty acid handling. The effect on the liver is illustrated by the study of acetoacetate formation in in vitro perfused livers from previously olive oil loaded +/- carnitine-treated rats. Carnitine pretreatment stimulated ketogenesis. It is speculated that carnitine administration, by promoting beta-oxidation, lowers the production of very low density lipoproteins. This may be accomplished partly by an increase in the hepatic level of fatty acid binding protein, which also has been observed.

Effect of starvation and diabetes on the sensitivity of carnitine palmitoyltransferase I to inhibition by 4-hydroxyphenylglyoxylate

The sensitivity of carnitine palmitoyltransferase I to inhibition by 4-hydroxyphenylglyoxylate was decreased markedly in liver mitochondria isolated from either 48 h-starved or streptozotocin-diabetic rats. These treatments of the rat also decreased the sensitivity of fatty acid oxidation by isolated hepatocytes to inhibition by this compound. Furthermore, incubation of hepatocytes prepared from fed rats with N6O2'-dibutyryl cyclic AMP also decreased the sensitivity, whereas incubation of hepatocytes prepared from starved rats with lactate plus pyruvate had the opposite effect on 4-hydroxyphenylglyoxylate inhibition of fatty acid oxidation. The sensitivity of carnitine palmitoyltransferase I of mitochondria to 4-hydroxyphenylglyoxylate increased in a time-dependent manner, as previously reported for malonyl-CoA. Likewise, oleoyl-CoA activated carnitine palmitoyltransferase I in a time-dependent manner and prevented the sensitization by 4-hydroxyphenylglyoxylate. Increased exogenous carnitine caused a moderate increase in fatty acid oxidation by hepatocytes under some conditions and a decreased 4-hydroxyphenylglyoxylate inhibition of fatty acid oxidation at low oleate concentration, without decreasing the difference in 4-hydroxyphenylglyoxylate inhibition between fed- and starved-rat hepatocytes. Time-dependent changes in the conformation of carnitine palmitoyltransferase I or the membrane environment may be involved in differences among nutritional states in 4-hydroxyphenylglyoxylate-sensitivity of carnitine palmitoyltransferase I.

Potential role of carnitine in patients with renal insufficiency

Carnitine metabolism is altered in renal insufficiency and influenced by the treatment modalities. Chronically uremic patients with end-stage renal disease under conservative therapy, hemodialysis, or peritoneal dialysis show low, normal, or elevated serum levels of TC and a distorted pattern of FC, SCAC, and LCAC. HD induces a marked depletion of FC, while predialytic elevated SCAC and LCAC are in the normal range at the end of dialysis treatment. All carnitine fractions rapidly return to predialysis levels 6 h after HD due to a transport of carnitine from muscle stores to plasma pool. Muscle carnitine content is elevated in chronic uremic patients under conservative therapy. Normal or decreased levels are observed in patients on long-term HD treatment. In addition, weekly losses of carnitine in patients undergoing HD or peritoneal dialysis do not exceed urinary carnitine excretion of CO. Supplementation with currently recommended doses (1-2 g L-carnitine i.v. at the end of each HD) is followed by a marked rise in plasma carnitine levels, suggesting limited carnitine utilization in uremia. Therefore, lower carnitine doses and modified application regimens should be considered to avoid exaggerated plasma levels of carnitine and carnitine esters. Furthermore, carnitine application has been reported to show beneficial, worsening, or no effect on the deranged lipid metabolism of the uremic patients. In patients undergoing CAPD or IPD predominantly normal serum carnitine levels have been reported. On the other hand, SCAC and LCAC esters are markedly elevated in these patients. After kidney transplantation the pattern of carnitine fractions is fully normalized in patients with plasma creatinine less than or equal to 120 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of oxidative metabolism by propionic acid and its reversal by carnitine in isolated rat hepatocytes

The present study was designed to study the interaction of propionic acid and carnitine on oxidative metabolism by isolated rat hepatocytes. Propionic acid (10 mM) inhibited hepatocyte oxidation of [1-14C]-pyruvate (10 mM) by 60%. This inhibition was not the result of substrate competition, as butyric acid had minimal effects on pyruvate oxidation. Carnitine had a small inhibitory effect on pyruvate oxidation in the hepatocyte system (210 +/- 19 and 184 +/- 18 nmol of pyruvate/60 min per mg of protein in the absence and presence of 10 mM-carnitine respectively; means +/- S.E.M., n = 10). However, in the presence of propionic acid (10 mM), carnitine (10 mM) increased the rate of pyruvate oxidation by 19%. Under conditions where carnitine partially reversed the inhibitory effect of propionic acid on pyruvate oxidation, formation of propionylcarnitine was documented by using fast-atom-bombardment mass spectroscopy. Propionic acid also inhibited oxidation of [1-14C]palmitic acid (0.8 mM) by hepatocytes isolated from fed rats. The degree of inhibition caused by propionic acid was decreased in the presence of 10 mM-carnitine (41% inhibition in the absence of carnitine, 22% inhibition in the presence of carnitine). Propionic acid did not inhibit [1-14C]palmitic acid oxidation by hepatocytes isolated from 48 h-starved rats. These results demonstrate that propionic acid interferes with oxidative metabolism in intact hepatocytes. Carnitine partially reverses the inhibition of pyruvate and palmitic acid oxidation by propionic acid, and this reversal is associated with increased propionylcarnitine formation. The present study provides a metabolic basis for the efficacy of carnitine in patients with abnormal organic acid accumulation, and the observation that such patients appear to have increased carnitine requirements ('carnitine insufficiency').

Interacting effects of L-carnitine and malonyl-CoA on rat liver carnitine palmitoyltransferase

Malonyl-CoA significantly increased the Km for L-carnitine of overt carnitine palmitoyltransferase in liver mitochondria from fed rats. This effect was observed when the molar palmitoyl-CoA/albumin concentration ratio was low (0.125-1.0), but not when it was higher (2.0). In the absence of malonyl-CoA, the Km for L-carnitine increased with increasing palmitoyl-CoA/albumin ratios. Malonyl-CoA did not increase the Km for L-carnitine in liver mitochondria from 24h-starved rats or in heart mitochondria from fed animals. The Km for L-carnitine of the latent form of carnitine palmitoyltransferase was 3-4 times that for the overt form of the enzyme. At low ratios of palmitoyl-CoA/albumin (0.5), the concentration of malonyl-CoA causing a 50% inhibition of overt carnitine palmitoyltransferase activity was decreased by 30% when assays with liver mitochondria from fed rats were performed at 100 microM-instead of 400 microM-carnitine. Such a decrease was not observed with liver mitochondria from starved animals. L-Carnitine displaced [14C]malonyl-CoA from liver mitochondrial binding sites. D-Carnitine was without effect. L-Carnitine did not displace [14C]malonyl-CoA from heart mitochondria. It is concluded that, under appropriate conditions, malonyl-CoA may decrease the effectiveness of L-carnitine as a substrate for the enzyme and that L-carnitine may decrease the effectiveness of malonyl-CoA to regulate the enzyme.

Altered carnitine metabolism in spontaneously hypertensive rats

Carnitine metabolism was examined in spontaneously hypertensive rats (SHR). Carnitine levels were elevated by 25% in hypertrophied hearts of 10- and 15-wk-old SHR when compared with Wistar-Kyoto (WKy) controls. This elevation was associated with a greater than 25% increase in total serum carnitine. The elevated serum carnitine does not appear to be due to increased mobilization from skeletal muscle because carnitine levels were elevated by 25% in gastrocnemius and diaphragm of SHR. Elevated serum carnitine is also not a result of reduced urinary excretion because daily urinary carnitine output was increased by 150% in SHR. These findings suggest that the most likely mechanism for increased serum carnitine is increased carnitine synthesis by the liver. The changes in carnitine metabolism in SHR appear to occur between 5 and 10 wk of age, because the carnitine levels in serum and organs were comparable in 5-wk-old WKy and SHR. The observed alterations in tissue and serum carnitine levels may result in altered fatty acid utilization in SHR.

Glucagon physiology and pathophysiology in the light of new advances

Recent advances in the understanding of glucagon-insulin relationships at the level of the islets of Langerhans and of hepatic fuel metabolism are reviewed and their impact on our understanding of glucagon physiology and pathophysiology is considered. It now appears that alpha cells can respond directly to hyperglycaemia in the absence of insulin and beta cells, but that antecedent hyperglycaemia masks or attenuates this response. Insulin appears to exert ongoing release inhibition upon glucagon secretion, probably via the intra-islet microvascular system that connects beta cells to alpha cells. Diabetic hyperglucagonemia in insulin deficient states appears to be secondary to lack of the restraining influence of insulin. The alpha cell response to glucopenia, by contrast, may be in large part mediated by release of noradrenaline from nerve endings in contact with alpha cells. Glucagon's action on glucose and ketone production by hepatocytes is mediated by increase in cyclic-AMP-dependent protein kinase. The opposing action of insulin upon glucagon-mediated events probably occurs largely at this level. Consequently, when glucagon secretion or action is blocked, cyclic-AMP-dependent protein kinase activity is low even in the absence of insulin, explaining why marked glucose and ketone production is absent in bihormonal deficiency states.

Coenzyme A metabolism

The metabolism of coenzyme A and control of its synthesis are reviewed. Pantothenate kinase is an important rate-controlling enzyme in the synthetic pathway of all tissues studied and appears to catalyze the flux-generating reaction of the pathway in cardiac muscle. This enzyme is strongly inhibited by coenzyme A and all of its acyl esters. The cytosolic concentrations of coenzyme A and acetyl coenzyme A in both liver and heart are high enough to totally inhibit pantothenate kinase under all conditions. Free carnitine, but not acetyl carnitine, deinhibits the coenzyme A-inhibited enzyme. Carnitine alone does not increase enzyme activity. Thus changes in the acetyl carnitine-to-carnitine ratio that occur with nutritional states provides a mechanism for regulation of coenzyme A synthetic rates. Changes in the rate of coenzyme A synthesis in liver and heart occurs with fasting, refeeding, and diabetes and in heart muscle with hypertrophy. The pathway and regulation of coenzyme A degradation are not understood.

The hepatotoxicity of valproic acid and its metabolites in rats. I. Toxicologic, biochemical and histopathologic studies

Valproic acid (VPA), its unsaturated metabolites and pent-4-enoate (4-PA) were studied for potential hepatotoxicity in rats. 4-PA, 4-en-VPA and 2,4-dien-VPA were potent inducers of microvesicular steatosis in young rats. Microvesicular steatosis induced by the 4-en-VPA was accompanied by ultrastructural changes characterized by myeloid bodies, lipid vacuoles and mitochondrial abnormalities. Myeloid bodies and lipid vacuoles were seen to a lesser extent in 2,4-dien-VPA and 4-PA-treated rats. VPA failed to induce discernible liver lesions in young rats even at near lethal doses of 700 mg per kg per day. The drug did, however, induce hepatic lipid accumulation in mature rats and in young rats dosed concomitantly with phenobarbital. beta-oxidation inhibition and several other biochemical alterations were observed in rats dosed with VPA, its unsaturated metabolites and 4-PA. It was suggested that beta-oxidation inhibition observed in both VPA and en-metabolite-treated rats occurred by different mechanisms. VPA inhibits by a transient sequestering of CoA while the CoA esters of some en-VPA-metabolites, particularly 4-en-VPA, inhibit specific enzyme(s) in the beta-oxidation sequences.

Factors influencing peroxisome proliferation in cultured rat hepatocytes

A primary rat hepatocyte culture system has been developed for the study of peroxisome proliferation. Maximal induction of peroxisomal activity requires supplementation of the culture medium with hydrocortisone. The addition of clofibric acid (0.01-1 mM), mono-(2-ethylhexyl)phthalate (0.01-0.5 mM) and trichloroacetic acid (0.1-5 mM) to cultured rat hepatocytes resulted in a time- and dose-related increase in CN- insensitive palmitoyl CoA oxidation (maximal increases: 27-, 15.5-, and 5-fold respectively) and mitochondrial alpha-glycerophosphate dehydrogenase activity (maximal increases: 7.3-, 5.8-, and 1.6-fold respectively). Electron microscopic examination revealed smooth endoplasmic reticulum proliferation and morphometric analysis indicated an increase in fractional peroxisomal volume of X 8 and X 4 for clofibric acid (1 mM) and trichloroacetic acid (2.5 mM), respectively. SDS-PAGE of cell homogenates revealed an intensified protein band of mol. wt. 76-78,000. The induction of peroxisomal beta-oxidation by clofibric acid was elevated from 9- to 12-fold by supplementation of the medium with L-carnitine (2 mM).

Absorption and metabolism of the volatile fatty acids in the hind-gut of the rabbit

Volatile fatty acids (VFA) absorption in the large intestine of the anaesthetized rabbit was evaluated by measuring variations in the concentration of VFA in intestinal loops and plasma arteriovenous differences. Metabolic conversions were studied using [1-14C]acetate, [1-14C]propionate and [3,4-14C]butyrate. The hind-gut tissues metabolized the three VFA, although this metabolism varied with the segment studied. Butyrate was the best respiratory fuel for the colonic wall, followed by propionate; acetate participated also, but it was mainly converted to glutamate. The liver was the main organ metabolizing absorbed propionate and butyrate; acetate was available for extrahepatic tissue metabolism. For the rabbit, VFA represented about 40% of the maintenance energy requirement.

Effect of carnitine on lipid metabolism in the neonate. II. Carnitine addition to lipid infusion during prolonged total parenteral nutrition

The effect of carnitine administration on lipid metabolism and carnitine and acylcarnitine plasma values of newborn infants, given total parenteral nutrition for the first 7 days of life, was studied during a 4-hour infusion of Intralipid. An increase in plasma concentrations of total carnitine, free carnitine, and short-chain and long-chain acylcarnitine was found, but no significant change in triglycerides, free fatty acids, glycerol, or beta-hydroxybutyrate plasma values was noted, as compared with values obtained without carnitine administration. Moreover, the low free carnitine and short-chain and long-chain acylcarnitine plasma levels found in newborn infants after 7 days of total parenteral nutrition did not seem to impair the utilization of infused lipids. The results support the concept that the relation between the carnitine pool and lipid metabolism can be influenced by intravenous glucose infusion. Low carnitine plasma concentrations do not necessarily signify a depletion of body carnitine, and sufficient tissue carnitine concentrations can probably maintain good lipid utilization for an extended period.

Improved N-retention during L-carnitine-supplemented total parenteral nutrition

The influence of intravenously administered L-carnitine on lipid- and nitrogen-metabolism was studied during total parenteral nutrition of piglets (mean weight 4077 g; n = 9). The infusion protocol was divided into three isocaloric and isonitrogenous 48-hr periods. Amino acids (3 g/kg day) were administered throughout all three periods: 140 cal/kg/day were given as nonprotein calories, consisting only of glucose during period 1; during periods 2 and 3, an amount of glucose calorically equivalent to 4 g fat/kg/day was substituted with a lipid emulsion, and L-carnitine (1.5 mg/kg/day) was added in period 3. Key parameters of fat- and nitrogen-metabolism were determined during the entire regime. Indirect calorimetry was performed and the respiratory quotient calculated during all three periods. The results demonstrate a more effective lipolysis and oxidation of fatty acids during L-carnitine supplementation. These changes produce an increased energy gain from exogenously administered fat and a distinct improvement in nitrogen balance.

Plasma carnitine in women. Effects of the menstrual cycle and of oral contraceptives

The plasma concentrations of carnitine were determined in a group of 35 women and 35 men admitted to a clinic, and in another group of 18 women during their menstrual cycle. The values found for the women (45.1 +/- 2.6 nmol/ml of free carnitine and 59.1 +/- 2.8 nmol/ml of total carnitine) were not significantly different from the values obtained in men (respectively 42.4 +/- 1.7 and 55.5 +/- 1.9 nmol/ml). No direct relationship between the free or total carnitine concentrations and the concentrations of circulating lipids could be demonstrated. During the menstrual cycle the plasma concentrations of free and total carnitine remained unchanged. Intake of oral contraceptives caused an elevation in blood triacylglycerols and decreases in the levels of luteinizing hormone, follicle-stimulating hormone, and free and total carnitine.

The control of fatty acid metabolism in liver cells from fed and starved sheep

Isolated liver cells prepared from starved sheep converted palmitate into ketone bodies at twice the rate seen with cells from fed animals. Carnitine stimulated palmitate oxidation only in liver cells from fed sheep, and completely abolished the difference between fed and starved animals in palmitate oxidation. The rates of palmitate oxidation to CO2 and of octanoate oxidation to ketone bodies and CO2 were not affected by starvation or carnitine. Neither starvation nor carnitine altered the ratio of 3-hydroxybutyrate to acetoacetate or the rate of esterification of [1-14C]palmitate. Propionate, lactate, pyruvate and fructose inhibited ketogenesis from palmitate in cells from fed sheep. Starvation or the addition of carnitine decreased the antiketogenic effectiveness of gluconeogenic precursors. Propionate was the most potent inhibitor of ketogenesis, 0.8 mM producing 50% inhibition. Propionate, lactate, fructose and glycerol increased palmitate esterification under all conditions examined. Lactate, pyruvate and fructose stimulated oxidation of palmitate and octanoate to CO2. Starvation and the addition of gluconeogenic precursors stimulated apparent palmitate utilization by cells. Propionate, lactate and pyruvate decreased cellular long-chain acylcarnitine concentrations. Propionate decreased cell contents of CoA and acyl-CoA. It is suggested that propionate may control hepatic ketogenesis by acting at some point in the beta-oxidation sequence. The results are discussed in relation to the differences in the regulation of hepatic fatty acid metabolism between sheep and rats.

The relationship between carnitine and ketone body levels in diabetic children

Free carnitine was significantly (p less than 0.001) reduced both in the ketotic (29.7 +/- 3.4 nmol/ml) and in the ketoacidotic (24.6 +/- 1.4 nmol/ml) groups when compared to controls (50.0 +/- 2.4 nmol/ml). At the same time, acylcarnitine values in the ketotic (21.2 +/- 2.4 nmol/ml) and ketoacidotic (25.4 +/- 2.3 nmol/ml) groups were significantly above the control value (4.71 +/- 0.6 nmol/ml). There was no significant difference between the two ketotic groups in carnitine derivatives. The abnormal distribution of plasma free and acylcarnitines could be reversed by insulin treatment. There was an inverse correlation between ketone body levels and free carnitine in the ketotic (r = -0.71, p less than 0.02) and ketoacidotic group (r = -0.71, p less than 0.05). However, there was no correlation between ketone bodies and acylcarnitine and between free carnitine and acylcarnitines. We concluded that the increased acylation was only partly responsible for the reduction of free carnitine in diabetic ketosis.

Carnitine in human nutrition

The oxidation of long-chain fatty acids is carnitine-dependent. Indeed, only when they are bound to carnitine, in the form of acyl-carnitines, do fatty acids penetrate into the mitochondria to be oxidized. To meet the need for carnitine, animals depend on both endogenous synthesis and an exogenous supply. A diet rich in meat supplies a lot of carnitine, while vegetables, fruits, and grains furnish relatively little. Although it has a low molecular weight and acts at low doses in a vital metabolic pathway, carnitine should not be considered a vitamin, but rather a nutritive substance. Indeed, it seems that the diet of the adult human need not necessarily furnish carnitine: the healthy organism, given a balanced nutrition (sufficiently rich in lysine and methionine), may well be able to meet all its needs. Furthermore, it seems that a reduction of the exogenous supply of carnitine results in a lowering of its elimination in the urine. However, dietary carnitine is more important during the neonatal period. The transition from fetal to extrauterine life is accompanied by an increased role of lipids in meeting energy needs. This change is accompanied by a rise in the body of the levels of carnitine, which is mainly supplied in the maternal milk. Finally, this review briefly surveys the illnesses in which a dietary carnitine supplement proves useful.

Carnitine deficiency presenting as familial cardiomyopathy: a treatable defect in carnitine transport

We studied a boy who presented at age 3 1/2 years with cardiomegaly, a distinctive electrocardiogram, and a history of a brother dying with cardiomyopathy. From age 3 1/2 to 5 years, cardiac disease progressed, resulting in intractable congestive heart failure. Skeletal muscle weakness developed and a muscle biopsy showed lipid myopathy. Muscle and plasma carnitine were reduced to 2 and 10% of the normal mean values, respectively. Therapy with L-carnitine (174 mg/kg/da) was begun at age 5 1/2 years and continued to the present (age 6 1/2 years). The cardiac disease has resolved and the muscle strength has returned to normal. Plasma carnitine concentrations have risen to the low-normal range, while urinary carnitine excretion has increased to values which are 30 times normal. The renal clearance of carnitine exceeds normal at all plasma concentrations and plasma carnitine values do not change acutely after an oral carnitine load. These results suggest that there is a distinct form of carnitine deficiency which presents as cardiomyopathy and may be successfully treated with L-carnitine. A defect in renal and possibly gastrointestinal transport of carnitine is a likely cause of this patient's disorder.

Urinary excretion of acetylcarnitine during human diabetic and fasting ketosis

The urinary excretion of acetylcarnitine was studied in patients with diabetic ketosis before and during insulin therapy and in normal-weight and obese subjects during fasting. In the diabetic ketotic patients, acetylcarnitine represented 61% of the total acylcarnitine excretion. During the first 24 h of insulin treatment, acetylcarnitine excretion decreased and on the 5th day of treatment was 18% of the acylcarnitines excreted. The urinary excretion of the other acylcarnitines fell slowly. In normal-weight subjects fasted for 3 days, the urinary excretion of acetylcarnitine increased on the 2nd day of fasting, and on the 3rd day acetylcarnitine accounted for 78% of the excreted acylcarnitine. In obese subjects there was a progressive increase in urinary acetylcarnitine excretion, but on day 6 it represented only 55% of the total acylcarnitine excreted. The urinary excretion of acetylcarnitine correlated with blood beta-hydroxybutyrate concentration in the normal-weight subjects during fasting and in the diabetic ketotic patients. Acetylcarnitine accounts for a major fraction of the acylcarnitines excreted in the three ketotic conditions studied. The contribution of acetylcarnitine to the change in acylcarnitines as ketosis appears or disappears is significantly less in the obese subjects than in the normal-weight subjects or in the diabetic patients. This difference may reflect an alteration in the production or disposition of acetyl-CoA and acetylcarnitine in obesity.

Fat-derived fuels during a 24-hour fast in children

We examined the availability of fat-derived fuels in 23 normal children aged 1.9 to 16.7 years who fasted for 24 h. We found a rapid and progressive rise in the blood concentrations of free fatty acids (FFA) and ketones. There was a highly significant negative correlation between the concentrations of beta-hydroxybutyrate (beta OHB) and glucose and also between beta OHB and age. With time, the ratio of beta OHB to acetoacetate (AcAc) progressively increased. We briefly review the vital role of ketones in the adaptation to fasting and point out that qualitative tests of ketones can be misleading. Our results indicate that quantitative determinations are essential in the evaluation of suspected disorders of fuel metabolism and that the results must be interpreted according to the age of the child, the duration of fasting, and the concomitant concentrations of glucose.