Renal adaptation to dietary carnitine in humans

Carnitine homeostasis in humans is maintained by dietary carnitine intake, a modest rate of endogenous carnitine synthesis, and efficient conservation of carnitine by the kidney. To assess the effect of dietary carnitine on the efficiency of carnitine reabsorption in humans, rates of carnitine excretion and reabsorption, indexed to the glomerular filtration rate, were determined over a range of plasma free and total carnitine concentrations in 12 strict vegetarians before and after dietary carnitine supplementation (0.248 mmol/d). This amount of dietary carnitine supplementation did not significantly increase plasma carnitine concentration and did not alter the glomerular filtration rate. At normal physiological plasma carnitine concentrations, the rate of carnitine excretion was increased and the rate of carnitine reabsorption was decreased by carnitine supplementation. We conclude that the kidney adapts to carnitine intake by reducing the efficiency of carnitine reabsorption.

Preferential elimination of pivalate with supplemental carnitine via formation of pivaloylcarnitine in man

1. To evaluate the effectiveness of carnitine administration in aiding the elimination of pivalate liberated from pivampicillin, studies were undertaken on seven paediatric patients treated for 7 days with combined pivampicillin and molar excess of carnitine. 2. A 22-fold increase occurred in urinary carnitine ester excretion on the last day of treatment (2967 +/- 604 versus 134 +/- 50 mumol/day, p < 0.05); the pivaloylcarnitine was identified with 13C-n.m.r. Only pivalate was detected in the urinary carnitine ester g.l.c. profile, the amount of this ester was equal to 92% of the daily pivalate intake. 3. The renal clearance rate of carnitine esters significantly exceeded that of creatinine indicating that the carnitine ester was eliminated by active transport. 4. The plasma concentration and urinary output of free carnitine were not changed significantly by the treatment, and the free and esterified carnitine concentrations in red cells remained unchanged indicating that carnitine deficiency was prevented.

Pharmacokinetics and organ distribution of liposome-encapsulated L-carnitine in rats

The pharmacokinetics and organ distribution of 3H-L-carnitine (CAS 541-15-1) were investigated in rats following direct intravenous administration of the drug substance and administration of the drug encapsulated in liposomes, respectively. The retention in the blood system of carnitine in liposomes, was significantly higher, namely up to 300% as compared to the standard administration. The half-life of distribution t1/2 alpha to 0.68 h (+154%), the terminal half-life t1/2 beta to 7.94 h (+140%), whereas the total clearance decreased by 400% as compared with the standard carnitine administration. Carnitine, in the novel dosage form, accumulated to a higher extent in the liver (156%) and spleen (336%), while the concentration in lung (52%), heart (55%) and muscle tissue (54%) decreased markedly relative to the standard. The novel dosage form is stable in vitro (t1/2 4 degree C = 187 days) as well as in vivo and, thus, may be successfully used in the therapy of carnitine deficiency, for instance in patients with renal failure or liver disease.

Some pharmacokinetic considerations about homeostatic equilibrium of endogenous substances

Endogenous substances in the body are controlled through simple, very effective mechanisms, that preserve an optimum homeostatic equilibrium of baseline concentration and restore it when impaired. When planning a pharmacokinetic investigation of an endogenous substance exogenously administered, it is imperative to carefully ascertain the above mechanisms as well as the baseline value and their possible variations associated with daily rhythm, food, age, sex, menstrual cycle. Often the control mechanisms operate through non-linear processes, therefore a non-compartmental analysis or a tailored model may be more appropriate than the compartmental models used in standard pharmacokinetic analysis. Some specific examples of endogenous substances are discussed here on the basis of the data from the literature and personal experience.

Effect of acetyl-L-carnitine treatment on the levels of levocarnitine and its derivatives in streptozotocin-diabetic rats

The effect of diabetes induced by streptozotocin and that of acetyl-L-carnitine (ALC) hydrochloride (CAS 5080-50-2) treatment on the homeostasis of the levocarnitine (L-carnitine) moiety was investigated in Sprague-Dawley rats. The diabetic status was ascertained by measuring blood glucose. L-carnitine (LC), total acid soluble L-carnitine (TC) and ALC were measured in serum, tissues and urine by radioenzymatic methods. Short-chain L-carnitine esters (SCLCE) were obtained by subtracting LC from TC. Serum concentration of L-carnitine moiety was decreased in diabetic when compared to normal rats; whereas ALC oral treatment (50 and 150 mg/kg p.o. for 4 weeks) in diabetic rats increased, dose-dependently, all the components of L-carnitine moiety, SCLCE and ALC being completely restored. In the liver of diabetic rats all the analytes proved to be higher than in normal rats, mainly LC and TC. A similar trend was observed in skeletal muscle, at least with LC and TC, whereas SCLCE and ALC were not affected. The treatment with ALC increased the liver concentration of all the analytes in a dose-related way whereas in skeletal muscle only LC and TC showed an increase with the highest dose of ALC. Myocardium and kidneys showed a decrease of all the analytes in diabetes; the treatment with ALC normalized the situation in kidneys, in a dose-related way, but not in the myocardium. Urinary excretion and renal clearance of L-carnitine moiety increased in diabetes; an additional dose-related increase was observed with the ALC treatment.

Carnitine deficiency associated with long-term pivampicillin treatment: the effect of a replacement therapy regime

A 51 year old female developed a skeletal muscle myopathy after 3 months of pivampicillin therapy. Pivampicillin can cause carnitine deficiency due to the pivalic acid side group. Plasma carnitine content and the patients symptoms failed to improve significantly on discontinuing the drug. Oral carnitine replacement therapy was administered for 6 weeks but the patient's plasma carnitine levels responded only slowly to this treatment. It is suggested that pivampicillin inhibits uptake of carnitine from the gut and may either directly or indirectly depress endogenous carnitine synthesis. In such cases a more aggressive carnitine replacement regime is indicated and pivampicillin should be avoided in patients requiring long-term antibiotic administration.

Influence of long-chain acylcarnitines on voltage-dependent calcium current in adult ventricular myocytes

Long-chain acylcarnitines (LCAC) increase 3.5-fold within 2 min in ischemic myocardium in vivo, and previous studies have suggested, through indirect evidence, that LCAC can stimulate the voltage-dependent L-type Ca2+ current [ICa(L)] in both cardiac and smooth muscle cells. In the present study, whole cell voltage-clamp procedures were performed in isolated adult guinea pig ventricular myocytes to assess the direct effect of LCAC on ICa(L). The intracellular solution contained (in mM) 80 CsCl, 40 K-aspartic acid, and 5 ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). Maximal current density of ICa(L) at 0 mV was 10.1 +/- 0.5 pA/pF (n = 22) at extracellular Ca2+ concentration ([Ca2+]o) = 2.7 mM. LCAC induced a concentration (1-25 microM, n = 23)- and time-dependent, reversible decrease in ICa(L). When delivered extracellularly for 10 min, LCAC (5 microM) inhibited the maximal current of ICa(L) by 48.1 +/- 1.3% (n = 9, P less than 0.01) and shifted the half-maximal voltage of steady-state activation and inactivation from -13.1 +/- 0.5 to -6.8 +/- 0.4 mV (n = 4; P less than 0.05) and from -21.8 +/- 0.2 to -16.5 +/- 0.6 mV (n = 4; P less than 0.01), respectively. Intracellular delivery of LCAC (5 microM) also suppressed ICa(L) to a similar degree (47.5 +/- 1.5%, n = 4; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Maternal and fetal tissue distribution of L-carnitine in pregnant mice: low accumulation in the brain

The distribution of L-carnitine was studied by whole-body autoradiography in pregnant CD-1 mice at 1, 2, 3, and 6 hr after receiving L-[14C]carnitine. Highest concentrations of carnitine were found in maternal tissues including liver, placenta, kidney, myocardium, and choroid plexus. High retention of tissue carnitine in excess of blood levels suggests the existence of concentrative uptake mechanism. Labeled carnitine was not detectable in either maternal or fetal brain. This suggests that the brain barrier systems limit the access of L-carnitine to the brain. In fetus, the level of carnitine was less than that seen in the maternal tissues, however, the tissue distribution was similar. The fetal tissue carnitine concentration increased with time. These findings suggest that relief of encephalopathy due to toxic organic anions in metabolic disorders following L-carnitine supplementation appears to be peripheral metabolic effects rather than direct access to the central nervous system. However, the physiological role for the concentrative uptake of L-carnitine by the choroid plexus remains to be determined. Transport of carnitine into fetal tissues via placenta further suggests the possibility of prenatal therapy in pregnancies at risk for certain inherited metabolic disorders.

The effect of enteral carnitine administration in humans

We previously determined that the L-carnitine uptake by human duodenal tissue occurs by both active (KT 558 mumol/L) and passive mechanisms. The effects of enteral carnitine was studied in humans. A hamburger meal (345 mumol total carnitine) induced peak jejunal fluid free (unesterified) and short-chain acylcarnitine concentrations (SCAC) of 209 and 130 mumol/L, respectively. Plasma carnitine concentrations and the percent renal reabsorption remained unchanged. By contrast, a pharmacologic dose of free carnitine (25,298 mumol) raised peak intraluminal free and SCAC to 20,660 and 4204 mumol/L. Plasma total carnitine concentrations doubled to 93 mumol/L, and the percent renal reabsorption of free and SCAC declined to 76% and 52%, respectively. In triple-lumen perfusions, 200 mumol carnitine/L was absorbed at 484 nmol.min-1.30 cm-1 jejunum, a rate sufficient for prandial but not pharmacologic assimilation. Our findings indicate that absorption of physiologic and pharmacologic amounts of carnitine occurs predominantly by active transport and passive diffusion, respectively.

Phase I clinical studies of S-1108: safety and pharmacokinetics in a multiple-administration study with special emphasis on the influence on carnitine body stores

S-1108, the prodrug of S-1006, was given to healthy volunteers three times a day (TID) for 8 days in a dose of 200 mg in a crossover placebo-controlled study. The safety of S-1108 and the pharmacokinetics of S-1006 and pivalic acid liberated from pivaloyloxymethyl ester of S-1108 were investigated. There were no abnormal symptoms or signs, as observed by physical and laboratory tests. The half-life and area under the concentration-time curve of S-1006 was reduced from 1.11 +/- 0.17 h at the first dose to 0.87 +/- 0.18 h at the last dose and from 7.30 +/- 1.10 to 5.20 +/- 0.85 micrograms.h/ml, respectively. However, there was no significant difference in the peak concentration between the two doses. Pivalic acid was found to be completely detoxified by conjugation with carnitine. The total urinary recovery of pivalic acid as pivaloylcarnitine was 98.7 +/- 3.6%, resulting in an increase of daily carnitine urinary excretion two- to threefold the predose value. During the multiple administration of S-1108, the plasma carnitine concentration was reduced to and maintained at 50 to 70% of the control value, suggesting that there might be enough carnitine store in the body to detoxify the pivalic acid in a dose of 200 mg TID. Moreover, the reduced plasma carnitine was rapidly returned to the control value within a few days after the cessation of the administration of 200 mg TID.

L-carnitine treatment for congestive heart failure--experimental and clinical study

To evaluate the therapeutic efficacy of l-carnitine in heart failure, the myocardial carnitine levels and the therapeutic efficacy of l-carnitine were studied in cardiomyopathic BIO 14.6 hamsters and in patients with chronic congestive heart failure and ischemic heart disease. BIO 14.6 hamsters and patients with heart failure were found to have reduced myocardial free carnitine levels (BIO 14.6 vs FI, 287 +/- 26.0 vs 384.8 +/- 83.8 nmol/g wet weight, p less than 0.05; patients with heart failure vs without heart failure, 412 +/- 142 vs 769 +/- 267 nmol/g p less than 0.01). On the other hand, long-chain acylcarnitine level was significantly higher in the patients with heart failure (532 +/- 169 vs 317 +/- 72 nmol/g, p less than 0.01). Significant myocardial damage in BIO 14.6 hamsters was prevented by the intraperitoneal administration of l-carnitine in the early stage of cardiomyopathy. Similarly, oral administration of l-carnitine for 12 weeks significantly improved the exercise tolerance of patients with effort angina. In 9 patients with chronic congestive heart failure, 5 patients (55%) moved to a lower NYHA class and the overall condition was improved in 6 patients (66%) after treatment with l-carnitine. L-carnitine is capable of reversing the inhibition of adenine nucleotide translocase and thus can restore the fatty acid oxidation mechanism which constitutes the main energy source for the myocardium. Therefore, these results indicate that l-carnitine is a useful therapeutic agent for the treatment of congestive heart failure in combination with traditional pharmacological therapy.

Effect of carnitine loading on long-chain fatty acid oxidation, maximal exercise capacity, and nitrogen balance

Carnitine has a potential effect on exercise capacity due to its role in the transport of long-chain fatty acids into the mitochondria for beta-oxidation, the export of acyl-coenzyme A compounds from mitochondria and the activation of branched-chain amino acid oxidation in the muscle. We studied the effect of carnitine supplementation on palmitate oxidation, maximal exercise capacity and nitrogen balance in rats. Daily carnitine supplementation (500 mg.kg-1 body mass for 6 weeks) was given to 30 rats, 15 of which were on an otherwise carnitine-free diet (group I) and 15 pair-fed with a conventional pellet diet (group II). A control group (group III, n = 6) was fed ad libitum the pellet diet. Palmitate oxidation was measured by collecting 14CO2 after an intraperitoneal injection of [1-14C]palmitate and exercise capacity by swimming to exhaustion. After carnitine supplementation carnitine concentrations in serum were supranormal [group I, total 150.8 (SD 48.5), free 78.9 (SD 18.4); group II, total 170.9 (SD 27.9), free 115.8 (SD 24.6) mumol.l-1] and liver carnitine concentrations were normal in both groups [group I, total 1.6 (SD 0.3), free 1.2 (SD 0.2); group II, total 1.3 (SD 0.3), free 0.9 (SD 0.2) mumol.g-1 dry mass]. In muscle carnitine concentrations were normal in group I [total 3.8 (SD 1.2), free 3.2 (SD 1.0) mumol.g-1 dry mass] and increased in group II [total 6.6 (SD 0.5), free 4.9 (SD 0.9) mumol.g-1 dry mass].(ABSTRACT TRUNCATED AT 250 WORDS)

Enzymes in stereoselective pharmacokinetics of endogenous substances

The use of enzymes to assay individual components of the L-carnitine family in pharmaceuticals, foodstuffs, and biological fluids with various forms of detection is reviewed. The most useful enzyme in the assay of compounds of the L-carnitine family is carnitine acetyl transferase (CAT), which catalyses the reversible interconversion of L-carnitine and its short-chain acyl esters. CAT can be used in one or more coupled reactions combined with U.V., or radiolabelled detection, or combined with HPLC, allowing, enantioselective, structurally specific, and, in the case of radiolabelled tracing, highly sensitive assays to be carried out. When compared with chromatographic separation of enantiomers or diastereoisomers, enantioselective enzyme mediated assays may be cheaper, more sensitive, and simpler, but they do not allow the nonpreferred isomer to be assayed. Consequently, they are appropriate for the specific assay of endogenous enantiomeric substrates of the enzyme concerned, in biological samples. The analysis of the other enantiomer in raw materials or in pharmaceuticals must be more properly approached by enantioselective chromatographic methods.

Quantitative estimation of absorption and degradation of a carnitine supplement by human adults

Results of kinetic and pharmacokinetic studies have suggested that dietary carnitine supplements are not totally absorbed, and are in part degraded in the gastrointestinal tract of humans. To determine the metabolic fate of dietary carnitine supplements in humans, we administered orally a tracer dose of [methyl-3H]L-carnitine with a meal to five normal adult males, who had been adapted to a high-carnitine diet plus carnitine supplement (2 g/d) for 14 days. Appearance of [methyl-3H]L-carnitine and metabolites in serum, and urinary and fecal excretion of radiolabeled carnitine and metabolites was monitored for 5 to 11 days following administration of the test dose. Maximum concentration of [methyl-3H]L-carnitine in serum occurred at 2.0 to 4.5 hours after administration of the tracer, indicating relatively slow absorption from the intestinal lumen. Total radioactive metabolites excreted in urine and feces ranged from 47% to 55% of the ingested tracer. Major metabolites found were [3H]trimethylamine N-oxide (8% to 49% of the administered dose; excreted primarily in urine) and [3H]gamma-butyrobetaine (0.44% to 45% of the administered dose; excreted primarily in feces). Urinary excretion of total carnitine was 16% to 23% of intake. Fecal excretion of total carnitine was negligible (less than 2% of total carnitine excretion).

Chronic cardiomyopathy and weakness or acute coma in children with a defect in carnitine uptake

A defect in intracellular uptake of carnitine has been identified in patients with severe carnitine deficiency. To define the clinical manifestations of this disorder, the presenting features of 15 affected infants and children were examined. Progressive cardiomyopathy, with or without chronic muscle weakness, was the most common presentation (median age of onset, 3 years). Other patients presented with episodes of fasting hypoglycemia during the first 2 years of life before cardiomyopathy had become apparent. A defect in carnitine uptake was demonstrable in fibroblasts and leukocytes from patients. The defect also appears to be expressed in muscle and kidney. Concentrations of plasma carnitine and rates of carnitine uptake in parents were intermediate between affected patients and normal control subjects, consistent with recessive inheritance. Early recognition and treatment with high doses of oral carnitine may be life-saving in this disorder of fatty acid oxidation.

Metabolic fate of dietary carnitine in human adults: identification and quantification of urinary and fecal metabolites

Results of kinetic and pharmacokinetic studies have suggested that dietary carnitine is not totally absorbed and is in part degraded in the gastrointestinal tract of humans. To determine the metabolic fate of dietary carnitine in humans, we administered orally a tracer dose of [methyl-3H]L-carnitine with a meal to subjects who had been adapted to a low-carnitine diet or a high-carnitine diet. Urinary and fecal excretion of radiolabeled carnitine and metabolites was monitored for 5 to 11 d following administration of the test dose. Total radioactive metabolites excreted ranged from 13 to 34% (low carnitine diet) and 27 to 46% (high carnitine diet) of the ingested tracer. Major metabolites found were [3H]trimethylamine N-oxide (8 to 39% of the administered dose; excreted primarily in urine) and [3H]gamma-butyrobetaine (0.09 to 8% of the administered dose; excreted primarily in feces). Urinary excretion of total carnitine was 42 to 95% (high carnitine diet) and 190 to 364% (low carnitine diet) of intake. These results indicate that oral carnitine is 54 to 87% bioavailable from normal Western diets; the percentage of intake absorbed is related to the quantity ingested.

L-carnitine and carnitine ester transport in the rat small intestine

L-carnitine and its esters (acetyl-L-carnitine and propionyl-L-carnitine) at pharmacological doses (1, 5 and 10 mM) are absorbed by the rat jejunum by simple diffusion. Partition coefficients of carnitine esters determined in lipophilic media (diethyl ether/water and olive oil/water) are greater than that of L-carnitine. It would therefore seem that esters diffuse more easily through the lipid component of the intestinal barrier. The transport of acetyl- and propionyl-L-carnitine at pharmacological doses seems to be linearly and positively correlated with K+ transport but not with Na+ transport.

The therapeutic potential of carnitine in cardiovascular disorders

The naturally occurring compound L-carnitine plays an essential role in fatty acid metabolism. It is only by combining with carnitine that the activated long-chain fatty acyl coenzyme A esters in the cytosol are able to be transported to the mitochondrial matrix where beta-oxidation occurs. Carnitine also functions in the removal of compounds that are toxic to metabolic pathways. Clinical evidence indicates that carnitine may have a role in the management of a number of cardiovascular disorders. Supplemental administration of carnitine has been shown to reverse cardiomyopathy in patients with systemic carnitine deficiency. Experimental evidence obtained in laboratory animals and the initial clinical experience in man indicate that carnitine may also have potential in the management of both chronic and acute ischemic syndromes. Peripheral vascular disease, congestive heart failure, cardiac arrhythmias, and anthracycline-induced cardiotoxicity are other cardiovascular conditions that may benefit from carnitine administration, although at this time data on the use of carnitine for these indications are very preliminary.

Pulmonary surfactant in bronchoalveolar lavage after canine lung transplantation: effect of L-carnitine application

Pulmonary surfactant during lung transplantation was investigated in the control group of a canine single lung transplantation model by measuring dipalmitoyl-phosphatidylcholine, the main phosphocholine fraction of surfactant in bronchoalveolar lavage. In a second group of dogs, L-carnitine, an essential cofactor for transfer of long-chain fatty acids into the mitochondria, was applied. Organ function after pulmonary artery flushing with modified Euro-Collins solution and hypothermic storage for 4 hours was adequate in both groups, with significantly higher arterial oxygen pressure levels in the L-carnitine group after 12 (p less than 0.05) and 24 (p less than 0.025) hours, respectively. In the control group, a reduction of the dipalmitoyl-phosphotidylcholine portion on total phosphotidylcholines was observed 4 and 12 hours after transplantation of the left lung (p less than 0.005 and p less than 0.01, respectively, both specified by Student's t test for dependent data, not significant by Bonferroni correction). In the simultaneously stored right lungs, a constant fall of the dipalmitoyl-phosphotidylcholine portion was demonstrated. In the L-carnitine group, significantly higher dipalmitoyl-phosphotidylcholine levels were observed in the transplanted left lungs after 4 hours (p less than 0.01) and in the continuously stored right lungs after 24 hours (p less than 0.005), when compared with the control group. These results suggest that dipalmitoyl-phosphotidylcholine portion on total phosphotidylcholine decreases parallel to the extent of the ischemic damage. Furthermore, the application of L-carnitine improved pulmonary function after transplantation, possibly by reducing the impairing effect of ischemia on alveolar type II cell metabolism and thereby on pulmonary surfactant system.

Uptake of L-carnitine by rat jejunal brush border microvillous membrane vesicles. Evidence of passive diffusion

We have previously described apparent active transport of carnitine into rat intestinal mucosa with intracellular accumulation against a concentration gradient in a process dependent upon the presence of sodium ions, oxygen, and energy. In the work described here, we sought to define the interaction between carnitine and the brush border membrane, which we presumed contained the transport mechanism. Using isolated rat jejunal brush border microvillous membrane vesicles, we found evidence of passive diffusion alone. We found no evidence of carrier-mediated transport--in particular no saturation over a concentration range, inhibition by structural analogs, transstimulation phenomenon, and no influence of sodium ions, potential difference or proton gradients. We conclude that a carnitine transporter does not exist in the brush border membrane of enterocytes and that other cellular mechanisms are responsible for the apparent active transport observed.

L-carnitine supplementation in humans. The effects on physical performance

The use of supplementary L-carnitine by athletes has become rather widespread over the recent years even in the absence of unequivocal results from human experimental studies that might support this practice. To justify the above procedure, the most commonly purported reasons are that L-carnitine administration could hypothetically: 1. increase lipid turnover in working muscles leading to glycogen saving and, as a consequence, allow longer performances for given heavy work loads; 2. contribute to the homeostasis of free and esterified L-carnitine in plasma and muscle, the allegation being that the levels of one or more of these compounds may decrease in the course of heavy repetitive exercise. A critical survey of the literature on carnitine metabolism in healthy humans at exercise does not appear to be available. The authors are of the opinion that this paper, besides shedding light into some relevant aspects of energy turnover in muscle, could also be of practical use not only for the physiologists but particularly for the Sport Medicine practitioners.

Kinetics of intravenously administered carnitine in haemodialysed children

The pharmacokinetics of low-dose bolus L-carnitine (5 mg kg-1 body wt) in five haemodialysed children were investigated. Kinetic variables were obtained by applying a two-compartment open model. The elimination half-life was very short, 2.43 +/- 0.35 h, despite the reduced plasma clearance of 41.2 +/- 5.7 ml min-1, compared with healthy adults. The apparent volume of distribution, 0.27 +/- 0.07 1 kg-1 body wt, corresponds well to the size of the extracellular space. The kinetic behaviour of intravenously supplied carnitine may assist in future evaluations of the therapeutic application of this drug in uraemic children.

Utilization of dietary precursors for carnitine synthesis in human adults

Endogenous synthetic pathways are presumed to be sufficient to provide adequate amounts of carnitine to meet the needs of the body. However, circulating carnitine levels of strict vegetarian adults and children, and particularly of infants fed carnitine-free formulas, are significantly lower than normal. Therefore, we investigated loci at which rates of carnitine synthesis may be restricted in human adults. Excess amounts of the carnitine precursors lysine plus methionine, epsilon-N-trimethyllysine or gamma-butyrobetaine were fed as supplements to a low carnitine diet for 10 d. Rate of carnitine synthesis was estimated by changes in carnitine excretion and changes in serum and muscle carnitine levels. Dietary gamma-butyrobetaine dramatically increased carnitine production, epsilon-N-trimethyllysine had a somewhat smaller effect, and lysine plus methionine had even less effect on carnitine synthesis. We conclude that carnitine synthesis is not limited by the activity of gamma-butyrobetaine hydroxylase. Carnitine synthesis from exogenous epsilon-N-trimethyllysine is limited either by enzymatic processes that lead to the final intermediate, gamma-butyrobetaine, or by the ability of this substrate to enter tissues capable of carrying out these transformations.

Carnitine and deoxycarnitine concentration in rat tissues and urine after their administration

Administration of L-carnitine to rats was followed by an increase of deoxycarnitine in urine. Conversely, administration of deoxycarnitine caused an increase of carnitine. The latter treatment also produced a transient but significant diminution of L-carnitine in heart, skeletal muscle and kidney, but not in liver and plasma. Administration of D-carnitine to rats previously loaded with deoxycarnitine significantly depleted the elevated deoxycarnitine concentration in skeletal muscle and kidney while increasing it in plasma. These results suggest that the tissue exchange between L-carnitine and deoxycarnitine, already demonstrated in vitro, occurs also in vivo.