Transport and function of carnitine: relevance to carnitine-deficient diseases

A comparison of L-carnitine and several cardiovascular-related biomarkers between healthy vegetarians and omnivores

OBJECTIVE

A plant-based diet has been associated with a reduced risk of cardiovascular (CV) diseases. This study aimed to determine the levels and correlations of CV-related biomarkers and the beneficial role of dietary habits.

METHODS

A total of 63 healthy vegetarians (n = 32) and omnivores (n = 31) were recruited. The baseline characteristics were recorded and measured (including lipid profiles, blood glucose, etc.). Liquid chromatography-mass spectrometry method was developed for the simultaneous determination of seven circulating CV-related biomarkers.

RESULTS

L-carnitine (L-Car), L-methionine, and ascorbic acid (AA) were significantly higher in vegetarians than in omnivores. In the vegetarians, L-Car had a negative correlation with triacylglycerols (P = 0.042) and blood glucose (P = 0.048) and a positive correlation with high-density lipoprotein cholesterol (P = 0.049). L-Car was also positively correlated with L-lysine (P = 0.009), L-methionine (P = 0.006), and AA (P = 0.035). The vegetarians' AA also had a negative correlation with L-homocysteine (P = 0.028). In the omnivores, L-Car was negatively correlated with total cholesterol (P = 0.008), low-density lipoprotein cholesterol (P = 0.004), and high-density lipoprotein cholesterol (P = 0.038). Omnivores' body mass index was positively correlated with L-homocysteine (P = 0.033), and age was positively correlated with trimethylamine N-oxide (P < 0.001) and blood glucose (P = 0.007), but not in vegetarians.

CONCLUSIONS

Our results suggest that vegetarians have an elevated level of L-Car, which might be associated with endogenous biosynthesis and diet composition. Circulating L-Car might play an important role in CV protection, especially in vegetarians.

Serum carnitine, triglyceride and cholesterol profiles in Korean neonates

This study evaluated carnitine and lipid status of fifty Korean newborns. Each subject was assigned to two groups: one according to body weight at birth and the other according to gestational age. Serum total, HDL- and LDL-cholesterol were significantly lower and triacylglycerols were significantly higher, by 14 %, in the low birth weight infant (LBWI, 1310–2490 g) group compared with the normal birth weight infant (NBWI, 2570–4420 g) group. Neither birth weight nor gestational age affected serum total carnitine concentrations. However, serum ASAC (acid-soluble acylcarnitine) concentrations were 43 % higher (P < 0·001) in the LBWI group compared with the NBWI group, and approximately twice as high (P < 0·05) in the 28–32 gestational age group compared with the other gestational age groups. NEC (non-esterified acyl carnitine) fractions were significantly higher in the NBWI and 28–32 week groups (P < 0·001 andP < 0·05); consequently serum acyl/NEC carnitine ratios were four times higher in the LBWI group compared with the NBWI group and 2–3 times higher in the 25–32 week age group compared with the more advanced gestational age groups. Urinary carnitine excretion, including the NEC fraction and total carnitine, was significantly higher (P < 0·001) for LBWI than for NBWI. By gestational age, NEC excretion of the 28–32 week group was significantly (P < 0·05) higher than that of the other two groups, but total carnitine excretion was not different among the groups. This study demonstrated that Korean immature and preterm newborns have higher serum triacylglycerol concentrations but lower carnitine status than NBWI. Therefore, the lower carnitine status and moderately higher triacylglycerols may suggest that LBWI in Korea might be at risk for poor carnitine status and decreased capacity to utilise fatty acids for energy.

Treatment of carnitine deficiency

Carnitine deficiency is a secondary complication of many inborn errors of metabolism. Pharmacological treatment with carnitine not only corrects the deficiency, it facilitates removal of accumulating toxic acyl intermediates and the generation of mitochondrial free coenzyme A (CoA). The United States Food and Drug Administration (US FDA) approved the use of carnitine for the treatment of inborn errors of metabolism in 1992. This approval was based on retrospective chart analysis of 90 patients, with 18 in the untreated cohort and 72 in the treated cohort. Efficacy was evaluated on the basis of clinical and biochemical findings. Compelling data included increased excretion of disease-specific acylcarnitine derivatives in a dose-response relationship, decreased levels of metabolites in the blood, and improved clinical status with decreased hospitalization frequency, improved growth and significantly lower mortality rates as compared to historical controls. Complications of carnitine treatment were few, with gastrointestinal disturbances and odour being the most frequent. No laboratory or clinical safety issues were identified. Intravenous carnitine preparations were also approved for treatment of secondary carnitine deficiency. Since only 25% of enteral carnitine is absorbed and gastrointestinal tolerance of high doses is poor, parenteral carnitine treatment is an appealing alternative therapeutic approach. In 7 patients treated long term with high-dose weekly to daily venous boluses of parenteral carnitine through a subcutaneous venous port, benefits included decreased frequency of decompensations, improved growth, improved muscle strength and decreased reliance on medical foods with liberalization of protein intake. Port infections were the most troubling complication. Theoretical concerns continue to be voiced that carnitine might result in fatal arrhythmias in patients with long-chain fat metabolism defects. No published clinical studies substantiate these concerns. Carnitine treatment of inborn errors of metabolism is a safe and integral part of the treatment regime for these disorders.

Carnitine improves peripheral glucose disposal in non-insulin-dependent diabetic patients

To investigate the effects of carnitine on insulin sensitivity in non-insulin-dependent diabetes, insulin-mediated glucose disposal was measured in nine diabetic patients (age 54 +/- 3 years, BMI 27 +/- 1 kg/mq) during a primed (3 mmol) constant (1.7 mumol/min) intravenous infusion of carnitine. In control experiments, the same patients received saline instead of carnitine. Plasma glucose concentration was maintained constant at the level of 100 mg/dl during both studies while plasma insulin was raised to a plateau of 60 microU/ml. Despite similar insulin levels, whole-body glucose utilization was higher with carnitine (4.05 +/- 0.37 mg/kg/min) than saline infusion (3.52 +/- 0.36). Blood lactate concentrations were similar in the basal state and decreased significantly during carnitine infusion (P less than 0.05-0.005), whereas it remained substantially unchanged during saline infusion. Plasma FFA decreased to a similar level (0.1 mmol/l) in both studies. We conclude that an acute carnitine administration is able to improve insulin sensitivity in NIDDM patients. The lactate data suggest that this effect may at least in part be mediated by carnitine activation of pyruvate dehydrogenase.

Delineation of the influence of propionylcarnitine on the accumulation of long-chain acylcarnitines and electrophysiologic derangements evoked by hypoxia in canine myocardium

To investigate the potential influence on one analogue of carnitine on the electrophysiologic derangements elicited by myocardial ischemia and subsequent reperfusion, we evaluated whether increasing concentrations of propionylcarnitine would interact with carnitine acyltransferase I and thereby decrease the accumulation of long-chain acylcarnitines during hypoxia in isolated adult canine myocytes. Propionylcarnitine (1-100 microM) did not alter the sixfold reversible increase in long-chain acylcarnitines elicited by 10 minutes of hypoxia. Likewise, propionylcarnitine did not alter the reversal of the accumulation of long-chain acylcarnitines associated with reoxygenation of hypoxic myocytes. To assess whether analogues of carnitine could influence the development or reversal of the electrophysiologic derangements induced by hypoxia in adult canine epicardial tissue, selected concentrations of propionylcarnitine (1 microM to 10 mM) were administered prior to and during 15 minutes of hypoxic perfusion at 35 degrees C followed by 5-20 minutes of reoxygenation. Continuous intracellular transmembrane action potentials were recorded with glass microelectrodes. Administration of propionylcarnitine prior to and during hypoxia did not alter the electrophysiologic derangements elicited by hypoxia or subsequent reoxygenation. Therefore, propionylcarnitine does not influence the activity of carnitine acyltransferase I and does not alter the accumulation of long-chain acylcarnitines during hypoxia. Although propionylcarnitine may protect ischemic myocardium by enhancing the recovery of contractile function during reperfusion, propionylcarnitine does not attenuate any of the electrophysiologic alterations observed during hypoxia or subsequent reoxygenation in isolated tissue.

L-carnitine delays the killing of cultured hepatocytes by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The role of fatty acid metabolism in chemical-dependent cell injury is poorly understood. Addition of L-carnitine to the incubation medium of cultured hepatocytes delayed cell killing initiated by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Protection by L-carnitine was stereospecific and observed as late as 1 h following addition of MPTP. D-Carnitine, but not iodoacetate, reversed the L-carnitine effect. Monoamine oxidase A and B activities, MPTP/N-methyl-4-phenyl-pyridinium levels, and MPTP-dependent loss of mitochondrial membrane potential measured by release of [3H]triphenylmethylphosphonium were not altered by addition of L-carnitine. Significant changes in MPTP-induced depletion of total cellular ATP did not occur with excess L-carnitine. Although the mechanism of cytoprotection exerted by L-carnitine remains unresolved, the data suggest that L-carnitine does not significantly alter: (i) mitochondrial-dependent bioactivation of MPTP; (ii) MPTP-dependent loss of mitochondrial membrane potential; or (iii) MPTP-mediated depletion of total cellular ATP content. We conclude that alterations of fatty acid metabolism may contribute to the toxic consequences of exposure to MPTP. Moreover, the lack of L-carnitine-mediated cytoprotection of monolayers incubated with 4-phenylpyridine or potassium cyanide suggests: (i) a link between fatty acid metabolism and mitochondrial membrane-mediated, bioactivation-dependent cell killing; and (ii) that inhibition of NADH dehydrogenase may not totally explain the mechanism of MPTP cytotoxicity.