The development of carnitine synthesis from gamma-butyrobetaine in the rat

Maternal Plasma l-Carnitine Reduction During Pregnancy Is Mainly Attributed to OCTN2-Mediated Placental Uptake and Does Not Result in Maternal Hepatic Fatty Acid β-Oxidation Decline

l-Carnitine (l-Car) plays a crucial role in fatty acid β-oxidation. However, the plasma l-Car concentration in women markedly declines during pregnancy, but the underlying mechanism and its consequences on maternal hepatic β-oxidation have not yet been clarified. Our results showed that the plasma l-Car level in mice at gestation day (GD) 18 was significantly lower than that in nonpregnant mice, and the mean fetal-to-maternal plasma l-Car ratio in GD 18 mice was 3.0. Carnitine/organic cation transporter 2 (OCTN2) was highly expressed in mouse and human placenta and upregulated as gestation proceeds in human placenta, whereas expressions of carnitine transporter (CT) 1, CT2, and amino acid transporter B0,+ were extremely low. Further study revealed that renal peroxisome proliferator-activated receptor α (PPARα) and OCTN2 were downregulated and the renal l-Car level was reduced, whereas the urinary excretion of l-Car was lower in late pregnant mice than in nonpregnant mice. Meanwhile, progesterone (pregnancy-related hormone) downregulated the expression of renal OCTN2 via PPARα-mediated pathway, and inhibited the activity of OCTN2, but estradiol, corticosterone, and cortisol did not. Unexpectedly, the maternal hepatic level of l-Car and β-hydroxybutyrate (an indicator of mitochondrial β-oxidation), and mRNA levels of several enzymes involved in fatty acid β-oxidation in GD 18 mice were higher than that in nonpregnant mice. In conclusion, OCTN2 mediated l-Car transfer across the placenta played a major role in maternal plasma l-Car reduction during pregnancy, which did not subsequently result in maternal hepatic fatty acid β-oxidation decrease.

Dioxygenases of Carnitine Biosynthesis: 6-N-Trimethyllysine and γ-Butyrobetaine Hydroxylases

This chapter describes the state of knowledge of the two 2-oxoglutarate-dependent dioxygenases of carnitine biosynthesis: 6-N-trimethyllysine hydroxylase and γ-butyrobetaine hydroxylase. Both enzymes have been extensively investigated as carnitine plays an important role in fatty acid metabolism in animals and some other life forms. Carnitine metabolism is introduced followed by a comprehensive review of the properties of the two carnitine biosynthesis dioxygenases including their purification, kinetic and biophysical characterization, regulation and roles in metabolism.

Systematic Evaluation of Key L-Carnitine Homeostasis Mechanisms during Postnatal Development in Rat

Background

The conditionally essential nutrient, L-carnitine, plays a critical role in a number of physiological processes vital to normal neonatal growth and development. We conducted a systematic evaluation of the developmental changes in key L-carnitine homeostasis mechanisms in the postnatal rat to better understand the interrelationship between these pathways and their correlation to ontogenic changes in L-carnitine levels during postnatal development.

Methods

mRNA expression of heart, kidney and intestinal L-carnitine transporters, liver γ-butyrobetaine hydroxylase (Bbh) and trimethyllysine hydroxylase (Tmlh), and heart carnitine palmitoyltransferase (Cpt) were measured using quantitative RT-PCR. L-Carnitine levels were determined by HPLC-UV. Cpt and Bbh activity were measured by a spectrophotometric method and HPLC, respectively.

Results

Serum and heart L-carnitine levels increased with postnatal development. Increases in serum L-carnitine correlated significantly with postnatal increases in renal organic cation/carnitine transporter 2 (Octn2) expression, and was further matched by postnatal increases in intestinal Octn1 expression and hepatic γ-Bbh activity. Postnatal increases in heart L-carnitine levels were significantly correlated to postnatal increases in heart Octn2 expression. Although cardiac high energy phosphate substrate levels remained constant through postnatal development, creatine showed developmental increases with advancing neonatal age. mRNA levels of Cpt1b and Cpt2 significantly increased at postnatal day 20, which was not accompanied by a similar increase in activity.

Conclusions

Several L-carnitine homeostasis pathways underwent significant ontogenesis during postnatal development in the rat. This information will facilitate future studies on factors affecting the developmental maturation of L-carnitine homeostasis mechanisms and how such factors might affect growth and development.

Timing and Duration of Drug Exposure Affects Outcomes of a Drug-Nutrient Interaction During Ontogeny

Significant drug-nutrient interactions are possible when drugs and nutrients share the same absorption and disposition mechanisms. During postnatal development, the outcomes of drug-nutrient interactions may change with postnatal age since these processes undergo ontogenesis through the postnatal period. Our study investigated the dependence of a significant drug-nutrient interaction (cefepime-carnitine) on the timing and duration of drug exposure relative to postnatal age. Rat pups were administered cefepime (5 mg/kg) twice daily subcutaneously according to different dosing schedules (postnatal day 1-4, 1-8, 8-11, 8-20, or 1-20). Cefepime significantly reduced serum and heart L-carnitine levels in postnatal day 1-4, 1-8 and 8-11 groups and caused severe degenerative changes in ventricular myocardium in these groups. Cefepime also altered the ontogeny of several key L-carnitine homeostasis pathways. The qualitative and quantitative changes in levels of hepatic γ-butyrobetaine hydroxylase mRNA and activity, hepatic trimethyllysine hydroxlase mRNA, intestinal organic cation/carnitine transporter (Octn) mRNA, and renal Octn2 mRNA depended on when during postnatal development the cefepime exposure occurred and duration of exposure. Despite lower levels of heart L-carnitine in earlier postnatal groups, levels of carnitine palmitoyltransferase mRNA and activity, heart Octn2 mRNA and ATP levels in all treatment groups remained unchanged with cefepime exposure. However, changes in other high energy phosphate substrates were noted and reductions in the phosphocreatine/ATP ratio were found in rat pups with normal serum L-carnitine levels. In summary, our data suggest a significant drug-nutrient transport interaction in developing neonates, the nature of which depends on the timing and duration of exposure relative to postnatal age.

The Effect of Dietary l-Carnitine on Semen Traits of White Leghorns

A previous study conducted in our laboratory showed that feeding 500 ppm of dietary L-carnitine to young and aging White Leghorns for 5 wk improved sperm concentration and reduced sperm lipid peroxidation during the last half of supplementation. The current study examined the effect of feeding dosimetric as well as lower levels of L-carnitine for longer durations on semen traits of White Leghorns. In experiments 1 and 2, White Leghorns consumed diets supplemented with 0, 125, 250, or 500 mg of L-carnitine/kg of feed. For experiment 1, an 8-wk trial was conducted with 48 White Leghorns from 46 to 54 wk of age. For experiment 2, a 17-wk trial was conducted with 96 White Leghorn roosters from 46 to 63 wk of age. For experiment 3, 84 roosters were provided for ad libitum consumption a diet formulated to contain 0 or 125 ppm of L-carnitine beginning at hatch until 37 wk of age. Long-term consumption of 125 ppm of L-carnitine beginning at hatch was the only dietary treatment that sustained a persistent increase in sperm concentration. These results suggest that L-carnitine's antioxidant influence on sperm production begins before the onset of sexual maturity.

Developmental maturation and segmental distribution of rat small intestinal L-carnitine uptake

Oral L-carnitine supplementation is commonly used in sports nutrition and in medicine; however, there is controversy regarding the mechanisms that mediate intestinal L-carnitine transport. We have previously reported that the Na(+)/L-carnitine transporter OCTN2 is present in the small intestinal apical membrane. Herein we aimed to find out if this step of intestinal L-carnitine absorption is ontogenically regulated, and if so, to determine the molecular mechanism(s) involved. L-[(3)H]-Carnitine uptake was measured in the jejunum and ileum of fetuses (E17 and E21), newborn (1 day-old), suckling (15 day-old), weaning (1 month-old) and adult (2 and 6 month-old) Wistar rats. Both, Na(+) -dependent and Na(+) -independent L-carnitine uptake rates, normalized to intestinal weight, significantly increased during the late gestation period, and then declined during the suckling period. After weaning, the rate of Na(+) -dependent L-carnitine uptake is no longer measurable. In E21- fetuses and newborn rats, L-carnitine uptake was higher in the ileum than in the jejunum. The decline in Na(+) -dependent L-carnitine uptake with maturation was mediated via a decrease in the V(max) of the uptake process with no change in its apparent K(m). Semi-quantitative RT-PCR assays showed that OCTN2 mRNA levels were significantly higher in E21-fetuses and newborn rats compared to suckling rats, which were in turn significantly higher than that in adult rats. Neither retardation of weaning nor L-carnitine supplementation prevented the down-regulation of Na(+)/L-carnitine transport activity. The results demonstrate for the first time that intestinal Na(+) -dependent L-carnitine uptake activity is under genetic regulation at the transcriptional level.

Carnitine depletion in rat pups from mothers given mildronate: a model of carnitine deficiency in late fetal and neonatal life

Mildronate (3-(2,2,2,-trimethylhydrazinium)propionate), is a butyrobetaine analogue that is known to inhibit gamma-butyrobetaine hydroxylase, the enzyme catalyzing the last step of carnitine biosynthesis. When administered to adult rats it determines a systemic carnitine deficiency and may therefore serve as an animal model for human carnitine depletion. The aim of this study was to evaluate the effect of mildronate administration to pregnant and lactating rats on tissue carnitine concentrations in 4- and 13-day-old rat pups. At 14 days of gestation female rats began to receive mildronate in the diet (200 mg/kg/d) and this continued for entire lactation period. Mildronate treatment determined a large reduction of carnitine levels in the milk of lactating dams. Because organ carnitine concentrations in neonatal rats are directly related to dietary supply, pups from mildronate group had significantly depleted levels of total carnitine in serum, heart, liver, muscle, brain and pancreas relative to controls, at 4 and 13 days of age. Correspondingly, an increase in triglyceride levels was observed in liver, heart and muscle of mildronate pups. This is in agreement with a reduction of basal rate of oxidation of [U-(14)C]-palmitate to (14)CO(2) and (14)C-acid-soluble products observed in liver homogenates from carnitine-deficient pups. All functional and biochemical modifications were compatible with a carnitine deficiency status. In conclusion our results describe a model of carnitine depletion in pups, suitable for the investigation of carnitine deficiency in fetal-neonatal nutrition, without any concomitant mildronate-mediated metabolic alterations.

l-Carnitine transport in human placental brush-border membranes is mediated by the sodium-dependent organic cation transporter OCTN2

Maternofetal transport of l-carnitine, a molecule that shuttles long-chain fatty acids to the mitochondria for oxidation, is thought to be important in preparing the fetus for its lipid-rich postnatal milk diet. Using brush-border membrane (BBM) vesicles from human term placentas, we showed that l-carnitine uptake was sodium and temperature dependent, showed high affinity for carnitine (apparent Km= 11.09 ± 1.32 μM; Vmax= 41.75 ± 0.94 pmol·mg protein−1·min−1), and was unchanged over the pH range from 5.5 to 8.5. l-Carnitine uptake was inhibited in BBM vesicles by valproate, verapamil, tetraethylammonium, and pyrilamine and by structural analogs of l-carnitine, including d-carnitine, acetyl-d,l-carnitine, and propionyl-, butyryl-, octanoyl-, isovaleryl-, and palmitoyl-l-carnitine. Western blot analysis revealed that OCTN2, a high-affinity, Na+-dependent carnitine transporter, was present in placental BBM but not in isolated basal plasma membrane vesicles. The reported properties of OCTN2 resemble those observed for l-carnitine uptake in placental BBM vesicles, suggesting that OCTN2 may mediate most maternofetal carnitine transport in humans.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Sodium-dependent carnitine transport in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells were found to transport carnitine into the intracellular space by a Na(+)-dependent process. The transport showed no requirement for anions. The Na+-dependent process was saturable and the apparent Michaelis-Menten constant for carnitine was 12.3 +/- 0.5 microM. Na+ activated the transport by increasing the affinity of the transport system for carnitine. The transport system specifically interacted with L-carnitine, D-carnitine, acetyl-DL-carnitine and betaine. 6-N-Trimethyllysine and choline had little or no effect on carnitine transport. Of the total transport measured, transport into the intracellular space represented 90%. Plasma membrane vesicles prepared from JAR cells were found to bind carnitine in a Na(+)-dependent manner. The binding was saturable with an apparent dissociation constant of 0.66 +/- 0.08 microM. The binding process was specific for L-carnitine, D-carnitine, acetyl-DL-carnitine, and betaine. 6-N-Trimethyllysine and choline showed little or no affinity. It is concluded that the JAR cells express a Na(+)-dependent high-affinity system for carnitine transport and that the Na(+)-dependent high-affinity carnitine binding detected in purified JAR cell plasma membrane vesicles is possibly related to the transmembrane transport process.

Sodium-dependent high-affinity binding of carnitine to human placental brush border membranes

The interaction of carnitine with human placental brush-border membrane vesicles was investigated. Carnitine was found to associate with the membrane vesicles in a Na(+)-dependent manner. The time course of this association did not exhibit an overshoot, which is typical of a Na+ gradient-driven transport process. The absolute requirement for Na+ was noticeable whether the association of carnitine with the vesicles was measured with a short time incubation or under equilibrium conditions, indicating Na(+)-dependent binding of carnitine to the human placental brush-border membranes. The binding was saturable and was of a high-affinity type with a dissociation constant of 1.37 +/- 0.03 microM. Anions had little or no influence on the binding process. The binding process was specific for carnitine and its acyl derivatives. Betaine also competed for the binding process, but other structurally related compounds did not. Kinetic analyses revealed that Na+ increased the affinity of the binding process for carnitine and the Na+/carnitine coupling ratio for the binding process was 1. The dissociation constant for the interaction of Na+ with the binding of carnitine was 24 +/- 4 mM. This constitutes the first report on the identification of Na(+)-dependent high-affinity carnitine binding in the plasma membrane of a mammalian cell. Studies with purified rat renal brush-border membrane vesicles demonstrated the presence of Na+ gradient-driven carnitine transport but no Na(+)-dependent carnitine binding in these membrane vesicles. In contrast, purified intestinal brush-border membrane vesicles posses neither Na+ gradient-driven carnitine transport nor Na(+)-dependent carnitine binding.

Effect of inhibition of fatty acid oxidation on neonatal liver carnitine content

The hypoglycaemic agent 2-tetradecylglycidic acid (compound McN-3802) caused an increase in total liver carnitine content, this being due primarily to an increase in the free carnitine pool. In the neonatal animal, this may represent a mechanism to overcome the inhibitory effect of fatty acid oxidation by the drug.