Lysine deficiency in the rat: concomitant impairement in carnitine biosynthesis

Environmental Enteric Dysfunction is Associated with Carnitine Deficiency and Altered Fatty Acid Oxidation

BACKGROUND

Environmental enteric dysfunction (EED), a condition characterized by small intestine inflammation and abnormal gut permeability, is widespread in children in developing countries and a major cause of growth failure. The pathophysiology of EED remains poorly understood.

METHODS

We measured serum metabolites using liquid chromatography-tandem mass spectrometry in 400 children, aged 12-59months, from rural Malawi. Gut permeability was assessed by the dual-sugar absorption test.

FINDINGS

80.7% of children had EED. Of 677 serum metabolites measured, 21 were negatively associated and 56 were positively associated with gut permeability, using a false discovery rate approach (q<0.05, p<0.0095). Increased gut permeability was associated with elevated acylcarnitines, deoxycarnitine, fatty acid β-oxidation intermediates, fatty acid ω-oxidation products, odd-chain fatty acids, trimethylamine-N-oxide, cystathionine, and homocitrulline, and with lower citrulline, ornithine, polyphenol metabolites, hippurate, tryptophan, and indolelactate.

INTERPRETATION

EED is a syndrome characterized by secondary carnitine deficiency, abnormal fatty acid oxidation, alterations in polyphenol and amino acid metabolites, and metabolic dysregulation of sulfur amino acids, tryptophan, and the urea cycle. Future studies are needed to corroborate the presence of secondary carnitine deficiency among children with EED and to understand how these metabolic derangements may negatively affect the growth and development of young children.

Neonatal nutritional carnitine deficiency: a piglet model

The clinical significance of nutritional carnitine deficiency remains controversial. To investigate this condition under controlled conditions, an animal model was developed using the parenterally alimented, carnitine-deprived newborn piglet. Forty-five piglets received total parenteral nutrition for 2-3 wk that was either carnitine-free or supplemented with 100-400 mg/L L-carnitine. Blood and a muscle biopsy were taken at the initial surgery. Carnitine balance studies were performed at 11-14 d of age. Blood, liver, heart, and skeletal muscle were taken at sacrifice for analysis of carnitine, electron microscopy, and oxidation studies. Carnitine-deprived piglets were in negative carnitine balance and had lower blood, urine, and tissue levels of carnitine than carnitine-supplemented animals. There was a positive correlation between excretion and plasma concentrations of free carnitine with an apparent renal threshold between 15 and 35 micromol/L. Plasma levels were correlated with liver and heart, but not muscle, concentrations of total acid-soluble carnitine. Carnitine-deprived piglets had evidence of lipid deposition in liver and skeletal muscle and tended to have a higher incidence of muscle weakness and cardiac failure. Basal rates of oxidation of [14C]palmitate to 14CO2 and 14C-acid-soluble products were lower in liver homogenates from carnitine-deprived piglets than in those from carnitine-supplemented animals and increased in a dose-dependent fashion with the addition of L-carnitine (0, 50, and 500 micromol/L) in vitro. In summary, carnitine deprivation in the neonatal piglet resulted in low carnitine status and morphologic/functional disturbances compatible with carnitine deficiency. The described animal model appears to be suitable for the investigation of neonatal nutritional carnitine deficiency.

Memories of microbes and metabolism

As Jackie Gleason was wont to say: "How sweet it really is!" And--reflecting on the 1940s-1980s, when studies of microbial nutrition revealed exciting structure-function relationships of the B-complex vitamins with relevance to metabolism in humans--it really is. A chemistry degree from Beloit College and a doctorate in biochemistry from the University of Wisconsin set the stage for my life's work at Lederle Laboratories, the University of Illinois, and Vanderbilt University. At Lederle my research contributed to folic acid chemistry: coenzyme forms and function; antimetabolites and cancer chemotherapy. My subsequent university studies centered on lysine biosynthesis and metabolism, e.g. its precursor role in carnitine and in indolizidine alkaloids of physiological interest. There were also many opportunities to reach out and give something back to the system via teaching and diverse service activities, all of which has led to a happy, fulfilling career, one for which I am ever thankful.

Effect of short- and long-term treatments by a low level of dietary L-carnitine on parameters related to fatty acid oxidation in Wistar rat

This study was designed to examine whether short- and long-term treatments by a low level of dietary L-carnitine are capable of altering enzyme activities related to fatty acid oxidation in normal Wistar rats. Under controlled feeding, ten days of treatment changed neither body weights nor liver and gastrocnemius weights, but succeeded in reducing the weight of peri-epididymal adipose tissues. Triacylglycerol contents were lowered in liver and ketone body concentrations were found slightly more elevated in blood. In the liver, mitochondrial carnitine palmitoyltransferase I (CPT I) exhibited a slightly higher specific activity and a lower sensitivity to malonyl-CoA inhibition, while peroxisomal fatty acid oxidizing system (PFAOS) was found to be less active. Carnitine supplied for one month reduced the mass of the periepididymal fat tissue, but not those of the other studied organs, and produced a slight but non-significant gain in body weight after ten days of treatment. In the liver, CPTI characteristics were comparable in control and treated groups, while PFAOS activity was less in rats receiving carnitine. Data show that L-carnitine at a low level in the diet exerted two paradoxical effects before and after ten days of treatment. Results are discussed in regard to fatty acid oxidation in mitochondria and peroxisomes, and to the possible altered acyl-CoA/acylcarnitine ratio with increased concentrations of L-carnitine in the liver.

Enhancement of activities relative to fatty acid oxidation in the liver of rats depleted of L-carnitine by D-carnitine and a gamma-butyrobetaine hydroxylase inhibitor

This study was designed to examine whether the depletion of L-carnitine may induce compensatory mechanisms allowing higher fatty acid oxidative activities in liver, particularly with regard to mitochondrial carnitine palmitoyltransferase I activity and peroxisomal fatty acid oxidation. Wistar rats received D-carnitine for 2 days and 3-(2,2,2,-trimethylhydrazinium)propionate (mildronate), a noncompetitive inhibitor of gamma-butyrobetaine hydroxylase, for 10 days. They were starved for 20 hr before being sacrificed. A dramatic reduction in carnitine concentration was observed in heart, skeletal muscles and kidneys, and to a lesser extent, in liver. Triacylglycerol content was found to be significantly more elevated on a gram liver and whole liver basis as well as per mL of blood (but to a lesser extent), while similar concentrations of ketone bodies were found in the blood of D-carnitine/mildronate-treated and control rats. In liver mitochondria, the specific activities of acyl-CoA synthetase and carnitine palmitoyltransferase I were enhanced by the treatment, while peroxisomal fatty acid oxidation was higher per gram of tissue. It is suggested that there may be an enhancement of cellular acyl-CoA concentration, a signal leading to increased liver fatty acid oxidation in acute carnitine deficiency.

Wheat gluten-based diet retarded ethanol metabolism by altering alcohol dehydrogenase and not carnitine status in adult rats

The objective of this study was to determine the effect of a lysine-deficient diet on carnitine status in adult rats and subsequently on ethanol metabolism. Adult male rats were fed either the AIN-76 diet (NS), the AIN-76 diet with wheat gluten (WG) replacing casein, the WG diet plus 0.8% L-lysine (LS), or the LS diet plus 0.5% L-carnitine (CS) for 30 days. On the 31st day the rats were given an oral dose of ethanol and blood-ethanol concentrations (BEC) were monitored for the next 8 hours. One week later the rats were given a second dose of ethanol and urine was collected until killed, 3 hours post-ethanol administration (PEA). Besides growth retardation and hypoproteinemia, BEC were significantly elevated in the WG group compared to the other group at hours 3-8 PEA. There were no significant differences in BEC between the LS and CS groups; however, their BEC were significantly higher than that of the NS group. The BEC were inversely related to liver alcohol dehydrogenase (ADH) activities which were significantly lower in WG, LS and CS groups than in the NS group. Plasma, liver and urine carnitine values were significantly higher in the CS group than in the NS, WG and LS groups, wherein the values were similar. It is concluded that the WG diet reduced ADH activity and attenuated ethanol metabolism without significantly altering blood, liver and urinary carnitines in the adult rat.

Effect of food restriction on tissue carnitine concentration in rats

The effect of feeding different amounts of a standard laboratory pellet diet on tissue carnitine concentration was studied in four groups of rats. Group I was fed ad libitum, whereas food intake was restricted to 25, 20, and 15g protein/kg body weight/day in group II, III, and IV, respectively. The intake of food, protein, energy and carnitine was constant and adjusted to actual body weight in groups 2-4. Six weeks food restriction had no effect on muscle carnitine. Restricted diet caused lowered concentrations of carnitine in serum (group I, fed ad libitum, total 95.0 +/- 13.8, free 80.2 +/- 2.7; group II total 78.4 +/- 8.4, free 56.9 +/- 4.7; group III total 81.7 +/- 8.8, free 66.0 +/- 8.8; and group IV total 73.8 +/- 8.7, free 59.5 +/- 7.6 micromol/l) and urinary carnitine excretion (group I, total 7.1 +/- 3.3, free 6.3 +/- 3.1; group II, total 2.5 +/- 0.7, free 2.2 +/- 0.7; group III, total 1.9 +/- 0.8, free 1.6 +/- 0.8; and group IV, total 1.3 +/- 0.4 free 1.1 +/- 0.3 micromol/day). In contrast, the liver carnitine tended to increase when dietary intake was reduced (group I total 1.1 +/- 0.1, free 1.0 +/- 0.1; group II total 1.5 +/- 0.2, free 1.4 +/- 0.2; group III total 1.3 +/- 0.1, free 1.1 +/- 0.1; and group IV total 1.5 +/- 0.2, free 1.4 +/- 0.2 micromol/g dry wt). The highest liver carnitine concentrations were observed during the lowest dietary intake when also the serum and urine carnitine were lowest. We conclude that the amount of food intake has a direct impact on carnitine concentrations in the liver, serum, and urine while muscle carnitine concentration remains relatively stable despite wide variations in food intake.

Experimental carnitine depletion in rats

We studied tissue carnitine concentrations after long-term peroral feeding with carnitine-free parenteral nutrient solutions in rats. Group I (n = 22) was fed perorally for 6 weeks with the carnitine free experimental diet. The control group (group II, n = 22) was pair-fed a standard laboratory pellet diet containing carnitine 60 nmol/g. The carnitine free experimental diet caused approximately 50% depletion of carnitine in serum, muscle, and liver while the concentrations in the pair-fed rats were normal. The free and total carnitine concentrations in serum were 25.5 +/- 7.8 and 32.9 +/- 9.3 micromol/l (group I), and 69.3 +/- 13.7 and 84.1 +/- 16.5 micromol/l (group II, p < 0.001), in muscle 2.1 +/- 0.3 and 2.3 +/- 0.4 micromol/g dry weight (group I), and 3.8 +/- 0.6 and 4.3 +/- 0.8 micromol/g dry weight (group II, p < 0.001), and in liver 0.5 +/- 0.1 and 0.6 +/- 0.1 micromol/g dry weight (group I), and 1.2 +/- 0.1 and 1.3 +/- 0.1 micromol/g dry weight (group II p < 0.001). Daily supplementation of the experimental liquid diet with I-carnitine caused normal tissue carnitine concentrations, indicating the exclusion of dietary carnitine as the cause of carnitine depletion. We conclude that in rats carnitine depletion in serum, muscle, and liver can be induced by prolonged peroral feeding with carnitine free diet.

Free carnitine and acylcarnitine levels in sera of alcoholics

We report the free, acyl-, and total carnitine contents of 49 clinically healthy volunteers and 167 chronic alcoholics with various clinically and/or anatomopathologically identified degrees of hepatic affection. There was a gradual upward trend in carnitine levels as the degree of hepatic affection increased. In cirrhotic patients, both free and acylcarnitine levels were significantly higher than normal, but there was no systematic hypercarnitinemia in other stages of alcoholism; on the contrary, noncirrhotic alcoholic patients accounted for 82.6% of all hypocarnitinemia cases. Hypercarnitinemia among cirrhotic alcoholics was due chiefly to increased free carnitine concentrations. Acylcarnitine levels in patients with hepatic steatosis were significantly higher than those in normal subjects (P less than 0.001), but there were no other statistically significant differences in either acyl- or free carnitine levels between normals on the one hand and, on the other, patients with hepatic steatosis, alcoholic hepatitis, slight hepatopathy, or chronic hepatopathy without portal hypertension.

Large variations in skeletal muscle carnitine level fail to modify energy metabolism in exercising rats

1. The importance of carnitine status in energy metabolism during exercise was studied in experimentally carnitine-depleted or supplemented rats. 2. Muscle carnitine concentration can be decreased by 40% with D-carnitine and increased by 40% with L-carnitine supplementation. 3. In spite of large variation of carnitine content, neither the exercising capacity nor the rate of muscle or liver glycogenolysis were modified during submaximal exercise. 4. The increased lipid metabolism induced by exercise can be adequately supported by endogenous levels of tissue carnitine. 5. Before any impairment in energy metabolism during exercise can be demonstrated, carnitine concentration has to be reduced to a level close to that measured with primary carnitine deficiency, i.e. less than 20 mumol/l of plasma.

Carnitine supplementation vs. medium-chain triglycerides in postburn nutritional support

The effect of dietary supplementation of carnitine on protein metabolism was studied in a burned guinea-pig model. Animals bearing a 30 per cent total body surface area burn were enterally infused with three isocaloric and isonitrogenous diets via gastrostomy feeding tubes for 14 days. Two diets contained safflower oil (long-chain triglycerides, LCT) and another diet contained medium-chain triglycerides (MCT) as their lipid sources (30 per cent of total calories as lipid). L-Carnitine was added to one of the two diets containing safflower oil. There were no significant differences in nitrogen balance, urinary excretion, serum albumin or transferrin among the three groups. However, the use of MCT in place of LCT appeared to increase liver weight and liver nitrogen. In this model, carnitine supplementation did not enhance the nitrogensparing effect of fat following burn injury.

Serum carnitine in children with kwashiorkor

Concentrations of free and acylcarnitine were measured in serum of children with kwashiorkor and compared with those obtained for well nourished children of similar age. The mean values (S.E.) for both free and acylcarnitine were significantly lower in the kwashiorkor group [32.6 (6.2) and 8.1 (2.2), respectively] than in the controls [53.2 (2.9) and 13.8 (3.1), respectively]. Serum albumin was also low in kwashiorkor patients, but there was no significant correlation with carnitine values.

Effect of insulin and glucagon on the uptake of carnitine by perfused rat liver

The possible direct effects of insulin and glucagon on carnitine uptake by perfused rat liver were studied with L-[3H]carnitine of an initial concentration of 50 microM in the perfusate. Insulin (10 nM) did not significantly affect the uptake by livers from fed animals. However, insulin could reverse the stimulated transport by livers from 24-h fasted animals, reducing the uptake rate from 852 +/- 54.1 to 480 +/- 39.9 (mean +/- S.E.), P less than 0.01 (rates are expressed as nmol per h per 100 g body wt). Glucagon (50 nM) stimulated the uptake rate when livers were either from fed (551 +/- 40.1 vs. 915 +/- 55.3, P less than 0.01) or from fasted animals (852 +/- 54.1 vs. 1142 +/- 88.1, P less than 0.02). Based on these and earlier observations, we propose that the carnitine concentration in rat liver is controlled by insulin and glucagon via cellular transport processes.

The effect of dietary lysine levels on growth and metabolism of rainbow trout (Salmo gairdneri)

Groups of rainbow trout (Salmo gairdneri; mean weight 5 g) were given diets containing 10, 12, 14, 17, 21, 24 and 26 g lysine/kg diet for 12 weeks. By analysis of the growth values the dietary requirement of lysine in this experiment was found to be 19 g/kg diet. A similar requirement value was obtained from a dose-response curve of expired 14CO2 (following an intraperitoneal injection of L-[U-14C]lysine) v. dietary lysine concentration. Liver concentrations of total lipid and carnitine and activities of lysine-alpha-ketoglutarate reductase (saccharopine dehydrogenase (NADP+, lysine-forming), EC 1.5.1.8) in the liver were not significantly different in fish from the different dietary treatments. Hepatosomatic index, however, was higher in those fish given low levels of dietary lysine.

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.

Urinary carnitine excretion in surgical patients on total parenteral nutrition

Urinary free and total carnitine excretions were measured in 41 normal adults and seven surgical patients on fat-free total parenteral nutrition for 8 to 45 days. The means (+/-SEM) of urinary free and total carnitine excretion in normal adults were 162 +/- 19 and 328 +/- 28 micrometers/days, respectively. All of the patients exhibited protein-calorie malnutrition with a mean carnitine intake of 11.6 +/- 1.5 micrometers/day. Under this stringent carnitine economy with the adequate supply of lysine and methionine, urinary total carnitine excretion significantly reduced to 127 to 162 micrometers/day. This probably reflects the carnitine biosynthetic rate. However, during the periods of operation and/or infection, urinary total carnitine excretion significantly increased 2- to 7-fold that of normal levels. Significant positive correlation was found between the two forms of urinary carnitine and total nitrogen excretions. Serum free and total carnitine levels in patients were significantly higher than normal adults. Such findings can be explained by the endocrine responses to the stress phenomenon and indicate a catabolic response of skeletal muscle in which most of the body carnitine resides. This can impair their carnitine status.

Plasma carnitine levels in children with protein-calorie malnutrition before and after rehabilitation

Plasma carnitine and albumin levels were measured in children suffering from protein-calorie malnutrition, before and after rehabilitation, and in apparently healthy controls. Both the constituents were lower in malnourished children and improved after treatment. Plasma carnitine showed significant positive correlation with plasma albumin.

The relationship between serum carnitine levels and the nutritional status of patients with schistosomiasis

Serum carnitine levels were investigated in a group of normal adults and two groups of patients with active schistosomiasis who also showed signs of malnutrition and vitamin deficiency. The first group consisted of 16 patients with Schistosoma mansoni and/or Schistosoma haematobium infection. They received an adequate diet supplemented with vitamin and iron therapy and received no treatment for their parasitic infection till their hemoglobin levels were restored to normal. The second group consisted of 12 patients with schistosomiasis as well as intestinal polyposis. They received the same diet as the first group but because of their poor condition were immediately treated for parasitic infection. Results showed that both groups of patients had subnormal levels of serum carnitine with the polyps patients (Group II) having a significantly lower level than patients with simple schistosomiasis (Group I). After nutritional repletion a significant increase was observed in the carnitine levels of most patients in group I indicating a relationship between the nutritional status of the patients and their serum carnitine levels. The patients with polyps also showed considerably increased carnitine levels after treatment and dietary repletion. The usefulness of serum carnitine measurement as an index of protein malnutrition in man is discussed.