Sites and regulation of carnitine biosynthesis in mammals

Although the pathway of carnitine biosynthesis in mammals is known, the location of active synthesis of carnitine and regulation of the pathway have not been clearly defined. Studies in several laboratories have shown that the enzymes that collectively convert epsilon-N-trimethyllysine (epsilon-N-TML) to gamma-butyrobetaine are found in all tissues studied in rats and humans, but distribution of the final enzyme of the pathway, gamma-butyrobetaine, 2-oxoglutarate dioxygenase (gamma-butyrobetaine hydroxylase) is variable from one species to another. Evidence from studies in rats and humans indicates that uptake and metabolism of epsilon-N-TML by the kidney is necessary for carnitine biosynthesis from circulating epsilon-N-TML. Limited data now available suggest that some of the intracellularly derived epsilon-N-TML is metabolized to gamma-butyrobetaine and carnitine in the tissue of origin, and some is released into the circulation. epsilon-N-TML in mammals is apparently derived from lysine residues in proteins, which are methylated and later released by protein hydrolysis. This source probably provides sufficient substrate for carnitine biosynthesis. Carnitine biosynthesis from epsilon-N-TML is not regulated by end-product feedback mechanisms. Hepatic gamma-butyrobetaine hydroxylase activity in rats and humans is developmentally regulated, and is increased by dietary L-thyroxine in adult rats. No other mechanisms for regulation of carnitine biosynthesis have been identified.     

Carnitine biosynthesis in hepatic peroxisomes. Demonstration of gamma-butyrobetaine hydroxylase activity.

We have investigated whether hepatic peroxisomes are capable of synthesizing carnitine. When purified peroxisomes were incubated with gamma-butyrobetaine, a precursor of carnitine, formation of carnitine was observed. These results indicate that peroxisomes contain gamma-butyrobetaine hydroxylase, the enzyme which catalyzes the final step in the biosynthesis of carnitine. This enzyme was previously believed to be present only in the cytosol. gamma-Butyrobetaine hydroxylase activity in peroxisomes was not due to cytosolic contamination as evaluated by marker enzyme analysis. When proliferation of peroxisomes was induced by clofibrate treatment, gamma-butyrobetaine hydroxylase/mass liver increased by 7.6-fold and the specific activity by 2.5-fold. We conclude that hepatic peroxisomes synthesize carnitine and this synthesis becomes substantial under conditions of peroxisomal proliferation.     

PPAR alpha-activation results in enhanced carnitine biosynthesis and OCTN2-mediated hepatic carnitine accumulation

In fasted rodents hepatic carnitine concentration increases considerably which is not observed in PPAR alpha-/- mice, indicating that PPAR alpha is involved in carnitine homeostasis. To investigate the mechanisms underlying the PPAR alpha-dependent hepatic carnitine accumulation we measured carnitine biosynthesis enzyme activities, levels of carnitine biosynthesis intermediates, acyl-carnitines and OCTN2 mRNA levels in tissues of untreated, fasted or Wy-14643-treated wild type and PPAR alpha-/- mice. Here we show that both enhancement of carnitine biosynthesis (due to increased gamma-butyrobetaine dioxygenase activity), extra-hepatic gamma-butyrobetaine synthesis and increased hepatic carnitine import (OCTN2 expression) contributes to the increased hepatic carnitine levels after fasting and that these processes are PPAR alpha-dependent.     

Hydroxylation of gamma-butyrobetaine by rat and ovine tissues.

Ovine tissues were assayed for the capacity to synthesize carnitine from γ-butyrobetaine. Activity in liver, kidney and muscle was 0.25, 0.10 and 0.08 nmoles per mg protein per min, respectively. Heart was devoid of the enzyme. Of the rat tissues that were assayed only liver contained the hydroxylase (0.39 nmoles per mg per min). Although the specific activity of the enzyme was approximately three fold higher in sheep liver than in sheep skeletal muscle, on the basis of total activity, muscle would constitute the major portion of the total hydroxylase activity present in the body. The synthesis of carnitine in ovine skeletal muscle may in part explain the high level of carnitine found in that tissue and emphasizes the existence of species differences in the localization of carnitine synthesis.     

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.     

Molecular cloning and characterization of the cDNA encoding the rat liver gamma-butyrobetaine hydroxylase.

Carnitine biosynthesis from lysine and methionine involves five enzymatic reactions. gamma-butyrobetaine hydroxylase (BBH; EC 1.14. 11.1) is the last enzyme of this pathway. It catalyzes the reaction of hydroxylation of gamma-butyrobetaine to carnitine. The cDNA encoding this enzyme has been isolated and characterized. The cDNA contained an open reading frame of 1161 bp encoding a protein of 387 amino acids with a deduced molecular weight of 44.5 kDa. The sequence of the cDNA showed an important homology with the human cDNA recently isolated. Northern analysis showed gamma-butyrobetaine hydroxylase expression in the liver and in some extend in the testis and the epididymis. During this study, it also appeared that BBH mRNA expression was undetectable by Northern analysis during the perinatal period. During the development of the rat, the amount of BBH mRNA appeared after the weaning of the young rat and reached a maximal expression at the adult stage.     

Significance of renal gamma-butyrobetaine hydroxylase for carnitine biosynthesis in man

Carnitine biosynthesis was studied in man and rat. Three healthy adult men were given intravenous injections of 1 mCi of [methyl-3H]epsilon-N-trimethyl-L-lysine, a precursor of carnitine. Labeled metabolites of this compound were monitored in serum and urine at 2, 6, 12, 24, and 48 h. At least nine radioactive metabolites were detected. For each collecton period, the specific activity of urinary carnitine exceeded the average serum specific activity. In man, the amount of labeled carnitine in urine was 2 to 8 times greater than labeled gamma-butyrobetaine (the immediate precursor of carnitine). In similar experiments in rats (intravenous injection of 0.1 mCi of [methyl-3H]epsilon-N-trimethyl-L-lysine), the specific activity of carnitine in urine was always lower than the corresponding average specific activity in serum. Between 0 and 2 h after administration of labeled precursor, the animals excreted large amounts of labeled gamma-butyrobetaine but little labeled carnitine. Significant gamma-butyrobetaine, 2-oxoglutarate dioxygenase (EC 1.14.11.1) activity was found in human kidney but this activity was absent in rat kidney. The results indicate that in man and rat the kidney accumulates intravenously administered [methyl-3H]epsilon-N-trimethyl-L-lysine. This compound is metabolized predominantly to gamma-butyrobetaine in rat kidney and to carnitine in human kidney. In both species, the synthesized products are at least partially leaked (either by secretion or by passive diffusion down a concentration gradient) into the renal tubular lumen from which they are either reabsorbed into the circulation for distribution to other tissues or excreted.     

Inducible γ-Butyrobetaine-Degrading Enzymes in Pseudomonas Species AK 1

Activities of gamma-butyrobetaine hydroxylase and carnitine dehydrogenase were low in cells of Pseudomonas sp. AK 1 grown in the absence of their respective substrates.     

gamma-Butyrobetaine hydroxylase and the protective role of glutathione peroxidase

The selenoenzyme glutathione peroxidase in the presence of GSH effectively replaced catalase in the in vitro assay for gamma-butyrobetaine hydroxylase. Quantitatively, glutathione peroxidase was an order of magnitude more efficient than catalase, with maximal activity at less than 0.1 microM glutathione peroxidase in a standard reaction. Glutathione peroxidase prevented the loss of gamma-butyrobetaine hydroxylase during preliminary incubation with ferrous ions but without other substrates as well as in the course of the reaction. Regardless of whether glutathione peroxidase or catalase was present in the assay, the ascorbate concentrations needed to achieve half-maximal rates were similar (about 1 mM). Phosphate stimulated the rate of L-carnitine synthesis. Ferrous ion saturation indicated a pronounced effect of phosphate on the maximal velocity of the enzyme-catalyzed reaction, but its mechanism of action remains to be elucidated. Based on the subcellular distribution of gamma-butyrobetaine hydroxylase, catalase, and glutathione peroxidase, the role of glutathione peroxidase assumes importance. However, initial studies indicated that the assayable activity of liver gamma-butyrobetaine hydroxylase and L-carnitine concentrations in liver, blood plasma, and muscle were not significantly altered in selenium-deficient rats.     

Rat liver gamma-butyrobetaine hydroxylase catalyzed reaction: influence of potassium, substrates, and substrate analogues on hydroxylation and decarboxylation

Interaction of rat liver gamma-butyrobetaine hydroxylase (EC 1.14.11.1) with various ligands was studied by following the decarboxylation of alpha-ketoglutarate, formation of L-carnitine, or both. Potassium ion stimulates rat liver gamma-butyrobetaine hydroxylase catalyzed L-carnitine synthesis and alpha-ketoglutarate decarboxylation by 630% and 240%, respectively, and optimizes the coupling efficiency of these two activities. Affinities for alpha-ketoglutarate and gamma-butyrobetaine are increased in the presence of potassium. gamma-Butyrobetaine hydroxylase catalyzed decarboxylation of alpha-ketoglutarate was dependent on the presence of gamma-butyrobetaine, L-carnitine, or D-carnitine in the reaction and exhibited Km(app) values of 29, 52, and 470 microM, respectively. gamma-Butyrobetaine saturation of the enzyme indicated a substrate inhibition pattern in both the assays. Omission of potassium decreased the apparent maximum velocity of decarboxylation supported by all three compounds by a similar percent. beta-Bromo-alpha-ketoglutarate supported gamma-butyrobetaine hydroxylation, although less effectively than alpha-ketoglutarate. The rat liver enzyme was rapidly inactivated by 1 mM beta-bromo-alpha-ketoglutarate at pH 7.0. This inactivation reaction did not show a rate saturation with increasing concentrations of beta-bromo-alpha-ketoglutarate. None of the substrates or cofactors, including alpha-ketoglutarate, protected the enzyme against this inactivation. Unlike beta-bromo-alpha-ketoglutarate, beta-mercapto-alpha-ketoglutarate did not replace alpha-ketoglutarate as a cosubstrate. Both beta-mercapto-alpha-ketoglutarate and beta-glutathione-alpha-ketoglutarate were noncompetitive inhibitors with respect to alpha-ketoglutarate.     

Tissue distribution of carnitine biosynthetic enzymes in man

The distribution in human tissues of enzymes which convert epsilon-N-trimethyl-L-lysine to L-carnitine was studied. Existing methodology was modified and new procedures were developed to measure enzyme activities. Epsilon-N-Trimethyl-L-lysine was converted to gamma-butyrobetaine in three enzymatic steps (hydroxylation at carbon 3, aldol cleavage between carbons 2 and 3 to yield glycine and gamma-trimethylaminobutyraldehyde, and subsequent oxidation of the aldehyde) in all tissues studied (liver, brain, kidney, heart and skeletal muscle), but gamma-butyrobetaine was hydroxylated to form L-carnitine only in liver, kidney and brain. Gamma-Butyrobetaine hydroxylase (4-trimethylaminobutyrate, 2-oxoglutarate: oxygen oxidoreductase (3-hydroxylating), EC 1.14.11.1) activity in liver was dependent on the age of the subject. The activity rose from 12% in infants to 100% of the adult mean by age 15 years. No age dependence could be demonstrated for the other three enzymes studied.     

Biosynthesis of carnitine in Neurospora crassa.

In growing cultures of Neurospora crassa lysine auxotroph 33933, (a) beta-hydroxy-epsilon-N-trimethyllysine and gamma-N-trimethylaminobutyraldehyde, postulated precursors of carnitine in the rat, effectively blocked synthesis of labeles carnitine from epsilon-N-[CH3-3H]trimethyllysine; and (b) beta-hydroxy-epsilon-N[CH3-3H[trimethyllysine and gamma-N-[CH3-3H]trimethylaminobutyraldehyde were effectively utilized for carnitine formation. From these isotopic experiments, the latter steps of carnitine synthesis in Neurospora are postulated to be epsilon-N-trimethyllysine leads to beta-hydroxy-epsilon-N-trimethyllysine leads to gamma-N-trimethylaminobutyraldehyde leads to gamma-butyrobetaine leads to carnitine.     

Development and characterization of an animal model of carnitine deficiency.

Mammals cover their carnitine needs by diet and biosynthesis. The last step of carnitine biosynthesis is the conversion of butyrobetaine to carnitine by butyrobetaine hydroxylase. We investigated the effect of N-trimethyl-hydrazine-3-propionate (THP), a butyrobetaine analogue, on butyrobetaine hydroxylase kinetics, and carnitine biosynthesis and body homeostasis in rats fed a casein-based or a vegetarian diet. The K(m )of butyrobetaine hydroxylase purified from rat liver was 41 +/- 9 micromol x L(-1) for butyrobetaine and 37 +/- 5 micromol x L(-1) for THP, and THP was a competitive inhibitor of butyrobetaine hydroxylase (K(i) 16 +/- 2 micromol x L(-1)). In rats fed a vegetarian diet, renal excretion of total carnitine was increased by THP (20 mg.100 g(-1) x day(-1) for three weeks), averaging 96 +/- 36 and 5.3 +/- 1.2 micromol x day(-1) in THP-treated and control rats, respectively. After three weeks of treatment, the total carnitine plasma concentration (8.8 +/- 2.1 versus 52.8 +/- 11.4 micromol x L(-1)) and tissue levels were decreased in THP-treated rats (liver 0.19 +/- 0.03 versus 0.59 +/- 0.08 and muscle 0.24 +/- 0.04 versus 1.07 +/- 0.13 micromol x g(-1)). Carnitine biosynthesis was blocked in THP-treated rats (-0.22 +/- 0.13 versus 0.57 +/- 0.21 micromol x 100 g(-1) x day(-1)). Similar results were obtained in rats treated with the casein-based diet. THP inhibited carnitine transport by rat renal brush-border membrane vesicles competitively (K(i) 41 +/- 3 micromol x L(-1)). Palmitate metabolism in vivo was impaired in THP-treated rats and the livers showed mixed steatosis. Steady-state mRNA levels of the carnitine transporter rat OCTN2 were increased in THP-treated rats in skeletal muscle and small intestine. In conclusion, THP inhibits butyrobetaine hydroxylase competitively, blocks carnitine biosynthesis in vivo and interacts competitively with renal carnitine reabsorption. THP-treated rats develop systemic carnitine deficiency over three weeks and can therefore serve as an animal model for human carnitine deficiency.     

Gamma-butyrobetaine hydroxylase: stereochemical course of the hydroxylation reaction

The stereochemical course of the aliphatic hydroxylation of gamma-butyrobetaine by calf liver and by Pseudomonas sp AK1 gamma-butyrobetaine hydroxylases has been determined. With [3(RS)-3-3H]-gamma-butyrobetaine or [3(R)-3-3H]-gamma-butyrobetaine as substrate, a rapid and significant loss of tritium to the medium occurred. On the other hand, with [3(S)-3-3H]-gamma-butyrobetaine, only a negligible release of tritium to the aqueous medium was observed. Indeed, on hydroxylation of [3(S)-3-2H]-gamma-butyrobetaine by either the calf liver or bacterial hydroxylase, the isolated product L-carnitine was found to have retained all of the deuterium initially present in the 3(S) position. Since the absolute configuration of the product L-carnitine has been determined to be R, such results are only compatible with a hydroxylation reaction that proceeded with retention of configuration. With [methyl-14C,3(R)-3-3H]-gamma-butyrobetaine as substrate for the calf liver hydroxylase, the percentage of tritium retained in the [methyl-14C]-L-carnitine product was determined as a function of percent reaction. The results of these studies indicated that pro-R hydrogen atom abstraction exceeded 99.9%. Experiments using racemic [methyl-14C,3(RS)-3-3H]-gamma-butyrobetaine as substrate yielded similar results and additionally allowed us to estimate alpha-secondary tritium kinetic isotope effects of 1.10 and 1.31 for the bacterial and calf liver enzymes, respectively. These results are discussed within the context of the radical mechanism for gamma-butyrobetaine hydroxylase previously proposed [Blanchard, J. S., & Englard, S. (1983) Biochemistry 22, 5922], and the required topographical arrangement of enzymic oxidant and substrate is illustrated.     

Distribution of 4a-hydroxytetrahydropterin dehydratase in rat tissues. Comparison with the aromatic amino acid hydroxylases.

A 4a-carbinolamine intermediate is generated stoichiometrically during the tetrahydrobiopterin-dependent phenylalanine hydroxylation reaction catalyzed by phenylalanine hydroxylase. The dehydration of the carbinolamine is catalyzed by the enzyme, 4a-hydroxytetrahydropterin dehydratase. We have now examined the distribution of the dehydratase activity in various rat tissues by activity measurements and by immunoblot analysis to explore the possibility that the dehydratase may also play a role in tyrosine and tryptophan hydroxylation. The only two tissues that express relatively high dehydratase activity are liver and kidney, which are also the only two tissues that express phenylalanine hydroxylase activity. The dehydratase activity was generally very low in those tissues which contain high levels of tyrosine and tryptophan hydroxylase activity, except for the pineal gland. These results suggest that the dehydratase may not play an important role in the regulation of the synthesis of those neurotransmitters which are derived from the hydroxylated aromatic amino acids.     

Carnitine synthesis and uptake into cells are stimulated by fasting in pigs as a model of nonproliferating species

In rodents, fasting increases the carnitine concentration in the liver by an up-regulation of enzymes of hepatic carnitine synthesis and novel organic cation transporter (OCTN) 2, mediated by activation of peroxisome proliferator-activated receptor (PPAR) α. This study was performed to investigate whether such effects occur also in pigs which like humans, as nonproliferating species, have a lower expression of PPARα and are less responsive to treatment with PPARα agonists than rodents. An experiment with 20 pigs was performed, which were either fed a diet ad-libitum or fasted for 24 h. Fasted pigs had higher relative mRNA concentrations of the PPARα target genes carnitine palmitoyltransferase 1 and acyl-CoA oxidase in liver, heart, kidney, and small intestinal mucosa than control pigs, indicative of PPARα activation in these tissues (P<.05). Fasted pigs had a higher activity of γ-butyrobetaine dioxygenase (BBD), enzyme that catalyses the last step of carnitine biosynthesis in liver and kidney, and higher relative mRNA concentrations of OCTN2, the most important carnitine transporter, in liver, kidney, skeletal muscle, and small intestinal mucosa than control pigs (P<.05). Fasted pigs moreover had higher concentrations of free and total carnitine in liver and kidney than control pigs (P<.05). This study shows for the first time that fasting increases the activity of BBD in liver and kidney and up-regulates the expression of OCTN2 in various tissues of pigs, probably mediated by PPARα activation. It is concluded that nonproliferating species are also able to cover their increased demand for carnitine during fasting by an increased carnitine synthesis and uptake into cells.     

gamma-Butyrobetaine hydroxylation to carnitine in mammalian kidney.

Activity of γ-butyrobetaine hydroxylase (γ-butyrobetaine, 2-oxoglutarate dioxygenase; EC 1.14.11.1) in liver and kidney of several mammalian species was assayed by measurement of tritium release from γ-[2,3-³H]butyrobetaine. Crude extracts from cat, hamster, rabbit, and Rhesus monkey kidneys effectively converted γ-butyrobetaine to carnitine. In these species, the levels of hydroxylating activity in kidney exceeded or nearly equaled the level of γ-butyrobetaine hydroxylase activity in the corresponding liver. In contrast, dog, guinea pig, mouse, and rat kidney exhibited no or insignificant capacity to hydroxylate γ-butyrobetaine. The notion that the liver is the exclusive or primary site of carnitine synthesis must be reconsidered at least for some mammalian species.     

Catabolic pathways for high-dosed L(-)- or D(+)-carnitine in germ-free rats?

Gnotobiotic rats received up to 3 mmol L-carnitine/day with the drinking water during 9 days. They excreted about a quarter of the administered dose with the urine, partially in form of acetyl-L-carnitine, but trimethylamine, trimethylamine N-oxide or gamma-butyrobetaine were not detectable in urine or faeces in contrast to conventional animals. After oral loading with D-carnitine the unphysiological isomer was absorbed and either excreted unchanged in urine or metabolized to acetonyltrimethylammonium. With regard to the development of carnitine deficiency syndromes and the degradation of nutritional carnitine the conclusion has to be drawn, that the bacteria of the gastro-intestinal tract, but not the tissues of the mammals, are responsible for the metabolization of L-carnitine to gamma-butyrobetaine or trimethylamine.     

Acceleration of gluconeogenesis from propionate by dl-carnitine in the rat kidney cortex

1. The rate of gluconeogenesis from propionate in rat kidney-cortex slices was stimulated up to 3.5-fold by dl-carnitine and by bicarbonate, and was inhibited by inorganic phosphate or high concentrations of propionate (above 3mm). 2. The stimulatory effect of carnitine was dependent on the bicarbonate concentration and could be replaced at low propionate concentration by addition of 25mm-bicarbonate-carbon dioxide buffer. At low bicarbonate concentration the carnitine concentration can be rate-limiting. 3. All observations are in accordance with the view that the action of carnitine is in principle the same as that established for other fatty acids in other tissues, namely that carnitine promotes the appearance of propionyl-CoA within the mitochondrion by acting as a carrier. 4. The accelerating effects of carnitine and bicarbonate and the inhibitory effect of phosphate can be explained on the basis of the known properties of key enzymes of propionate metabolism, i.e. the reversibility of the reactions leading to the formation of methylmalonyl-CoA from propionyl-CoA. 5. 5mm-Propionate caused a five- to ten-fold fall in the free CoA content of the tissue. This fall can account for the inhibition of respiration and gluconeogenesis caused by high propionate concentration. 6. Relatively large quantities of propionyl-l-carnitine (15% of the propionate removed) were formed when dl-carnitine was present; thus the ;activation' of propionate proceeded at a faster rate than the carboxylation of propionyl-CoA. The metabolism of added propionyl-l-carnitine was accompanied by glucose synthesis. 7. The appearance of radioactivity from [2-(14)C]propionate in both glucose and carbon dioxide was as expected on account of the randomization of C-2 and C-3 of propionate, i.e. the formation of succinate as an intermediate. 8. The maximum rate of glucose synthesis from propionate (93.3+/-3.3mumoles/g. dry wt./hr.) was not affected by dietary changes aimed at varying the rate of caecal volatile fatty acid formation in the rat. 9. Inhibition of gluconeogenesis by high propionate concentration was not found in those species where the rate of caecal or ruminal propionate production is high under normal conditions (rabbit, sheep and cow).