Role of carnitine in hepatic ketogenesis

Primary ketosis in the high-producing dairy cow: clinical and subclinical disorders, treatment, prevention, and outlook

Bovine ketosis typically occurs in early lactation. Clinical signs include diminished appetite, decreased milk production, loss of weight, hypoglycemia, and hyperketonemia. Susceptibility to ketosis is probably due to the combination of appetite limitation and a high degree of precedence given to the demand of the mammary gland for nutrients, in particular glucose. The precipitating cause is likely to be development of a marked imbalance between glucose supply and glucose requirement. This imbalance then leads to decreased carbohydrate status, decreased insulin secretion, increased fat mobilization, and increased hepatic ketogenesis. Hepatic ketogenesis may be augmented by the diminished carbohydrate status. The role of hormones other than insulin in the etiology of ketosis, although probably important, has not yet been elucidated satisfactorily. Treatment of ketosis involves increasing glucose supply relative to glucose demand. Incidence of clinical ketosis can be minimized by correct nutrition and management as outlined in recommended guidelines. Besides decreasing milk field, clinical ketosis may affect productivity adversely in other ways, for example, by impairing fertility. Subclinical ketosis is important because it may remain undetected and yet have effects on productivity which parallel those elicited by clinical ketosis. Future research should be directed toward understanding mechanisms conferring priority on milk production and regulating appetite.

Concurrent occurrence of elevated ketone body and depressed carnitine level in underfed guinea-pigs

Food ingestion of male guinea-pigs was restricted to 10 g/day. Total carnitine content of liver of the underfed animals fell to 3.75 mumoles per liver (46.1% of control). Serum level of total carnitine was 35.0 nmoles/ml (64.4%) in the underfed animals while they developed high ketonemia, 384.2 nmoles/ml (555.2%). In "restricted' animals the carnitine levels also decreased in the muscles.

Regulation of hepatic fatty acid metabolism. The activities of mitochondrial and microsomal acyl-CoA:sn-glycerol 3-phosphate O-acyltransferase and the concentrations of malonyl-CoA, non-esterified and esterified carnitine, glycerol 3-phosphate, ketone bodies and long-chain acyl-CoA esters in livers of fed or starved pregnant, lactating and weaned rats

1. The concentrations of malonyl-CoA, glycerol 3-phosphate, non-esterified carnitine, acid-soluble and acid-insoluble acylcarnitines, acetoacetate, 3-hydroxybutyrate and acid-insoluble acyl-CoA were measured in rapidly-frozen liver samples from fed or starved (24h) virgin, pregnant (19-20 days), lactating (2, 10-12 and 18-20 days) and weaned (for 24h, on 10th day of lactation) rats. The activities of total and N-ethylmaleimide-sensitive and -insensitive glycerophosphate acyltransferase (acyl-CoA:sn-glycerol 3-phosphate O-acyltransferase; EC 2.3.1.15) were also measured. 2. The concentration of malonyl-CoA was significantly higher in liver of fed pregnant, mid- and late-lactating rats than in liver of fed virgin rats. After starvation for 24h hepatic malonyl-CoA concentrations were higher in mid-lactating rats and lower in pregnant and weaned rats than in virgin animals. 3. After starvation for 24h the hepatic concentrations of glycerol 3-phosphate, ketone bodies, acid-soluble acylcarnitines and the value for the [3-hydroxybutyrate]/[acetoacetate] ratio were all highest in pregnant rats, intermediate in virgin, 2-day lactating and weaned animals and lowest in mid- and late-lactating rats. The concentrations of acid-insoluble acylcarnitines also increased most in pregnant rats, after starvation. The concentration of acid-insoluble acyl-CoA increased equally after starvation in virgin and pregnant animals but did not increase significantly in all other animals studied. 4. The total concentration of carnitine was similar in livers of fed virgin, pregnant and 2-day lactating animals but fell markedly by the 10th day of lactation and remained low in late-lactating animals. The concentration of non-esterified carnitine followed the same pattern. After starvation for 24h the hepatic concentration of non-esterified carnitine decreased significantly in virgin, pregnant and 2-day lactating animals, but remained unchanged in mid- and late-lactating or weaned animals. 5. The activities of N-ethylmaleimide-sensitive and -insensitive glycerophosphate acyltransferase both increased significantly in livers of mid-lactating animals. After starvation for 24h the activity of the N-ethylmaleimide-insensitive O-acyltransferase decreased in livers of virgin, pregnant and mid-lactating animals, whereas the activity of the N-ethylmaleimide-sensitive O-acyltransferase was unchanged in virgin animals but decreased markedly in livers of pregnant and lactating rats. 6. The results are discussed in relation to the importance of different metabolic parameters in the regulation of long-chain acyl-CoA metabolism in the liver.

Plasma and urine free L-carnitine in human diabetes mellitus

L-carnitine is essential for the transport of long-chain fatty acids into mitochondria and their oxidation. Recently, a relationship between plasma free fatty acids (FFA) and L-carnitine metabolism has been observed. Plasma free L-carnitine (FC), FFA, triglycerides, cholesterol, blood glucose concentration and daily excretion of FC were determined in 20 diabetic patients as well as in 18 control subjects. Both in male diabetics and in male controls, plasma FC was significantly higher than in females. Mean plasma FC was found to be significantly reduced in diabetics (21 +/- 2 vs 35 +/- 2 mumol/1 in non-diabetic subjects; p less than 0.005). Daily urinary excretion of FC was clearly lower in diabetic patients than in controls (172 +/- 34 vs 403 +/- 38 mumol/24 h; p less than 0.001). The reduced plasma FC in diabetes mellitus may be due to redistribution between circulating free and esterified carnitine and to increased utilization of FC for synthesis of acylcarnitine in tissues.

Evaluation of malonyl-CoA in the regulation of long-chain fatty acid oxidation in the liver. Evidence for an unidentified regulatory component of the system

Palmitate oxidation by liver mitochondria from fed and starved rats exhibited markedly different sensitivities to inhibition by malonyl-CoA. In the mitochondrial system from fed rats, 50% inhibition required 19 muM-malonyl-CoA, whereas the mitochondria from starved rats were by comparison refractory to malonyl-CoA. Inhibition by malonyl-CoA was completely reversed by increasing the molar ratio of fatty acid to albumin. Results indicate that the potential effectiveness of malonyl-CoA as an inhibitor of fatty acid oxidation in the liver is dependent on an unidentified regulatory component of the system. The functional activity of this component is modified by the nutritional state, and its site of action is at the mitochondrial level.

Systemic carnitine deficiency--a treatable inherited lipid-storage disease presenting as Reye's syndrome

A 3 1/2-year-old boy presented at three months of age with an acute episode of lethargy, somnolence, hypoglycemia, hepatomegaly, and cardiomegaly, which responded poorly to restoration of the blood sugar level to normal. The absence of ketonuria during subsequent episodes of severe hypoglycemia prompted a search for a defect in fatty acid oxidation. Plasma carnitine (2.0 to 5.0 mumol per liter), muscle carnitine (0.01 to 0.02 mumol per gram, wet weight) and liver carnitine (0.021 to 0.065 mumol per gram, wet weight) were all less than 5 per cent of the normal mean. During a 36-hour fast, ketones were barely detectable. Prolonged treatment with oral carnitine over a six-month period resulted in increased muscle strength, a dramatic reduction in cardiac size, relief of cardiomyopathy, partial repletion of carnitine levels in plasma and muscle, and complete repletion in the liver. Systemic carnitine deficiency is an easily treatable cause of recurrent Reye's-like syndrome. Its diagnosis requires measurement of carnitine levels.

Carnitine improves lipid anomalies in haemodialysis patients

51 chronic haemodialysis patients with hypertriglyceridaemia were given a daily oral dose of 2.4 g D,L-carnitine for 30 days to investigate a possible hypolipaemic effect. After 30 days' D,L-carnitine treatment the mean (+/- SEM) serum triglyceride concentration had decreased significantly from 3.50 +/- 0.39 to 2.87 +/- 0.27 mmol/l. Serum total cholesterol did not change. However, HDL cholesterol increased significantly from 0.89 +/- 0.05 to 1.35 +/- 0.07 mmol/l. This decrease in serum triglycerides and return of HDL cholesterol to normal levels in haemodialysis patients may be the result of correction of carnitine deficiency. Such treatment could reduce the risk factors for atherosclerosis and coronary-artery disease in uraemic patients.

Relationship between acid-soluble carnitine and coenzyme A pools in vivo

The relationship between the acid-soluble carnitine and coenzyme A pools was studied in fed and 24-h-starved rats after carnitine administration. Carnitine given by intravenous injection at a dose of 60mumol/100g body wt. was integrated into the animal's endogenous carnitine pool. Large amounts of acylcarnitines appeared in the plasma and liver within 5min of carnitine injection. Differences in acid-soluble acylcarnitine concentrations were observed between fed and starved rats after injection and reflected the acylcarnitine/carnitine relationship seen in the endogenous carnitine pool of the two metabolic states. Thus, a larger acylcarnitine production was seen in starved animals and indicated a greater source of accessible acyl-CoA molecules. In addition to changes in the amount of acylcarnitines present, the specific acyl groups present also varied between groups of animals. Acetylcarnitine made up 37 and 53% of liver acid-soluble acylcarnitines in uninjected fed and starved animals respectively. At 5min after carnitine injection hepatic acid-soluble acylcarnitines were 41 and 73% in the form of acetylcarnitine in fed and starved rats respectively. Despite these large changes in carnitine and acylcarnitines, no changes were observed in plasma non-esterified fatty acid or beta-hydroxybutyrate concentrations in either fed or starved rats. Additionally, measurement of acetyl-CoA, coenzyme A, total acid-soluble CoA and acid-insoluble CoA demonstrated that the hepatic CoA pool was resistant to carnitine-induced changes. This lack of change in the hepatic CoA pool or ketone-body production while acyl groups are shunted from acyl-CoA molecules to acylcarnitines suggests a low flux through the carnitine pool compared with the CoA pool. These results support the concept that the carnitine/acid-soluble acylcarnitine pool reflects changes in, rather than inducing changes in, the hepatic CoA/acyl-CoA pool.

Role of the liver in regulation of ketone body production during sepsis

During caloric deprivation, the septic host may fail to develop ketonemia as an adaptation to starvation. Because the plasma ketone body concentration is a function of the ratio of hepatic production and peripheral usage, a pneumococcal sepsis model was used in rats to measure the complex metabolic events that could account for this failure, including the effects of infection on lipolysis and esterification in adipose tissue, fatty acid transport in plasma and the rates of hepatic ketogenesis and whole body oxidation of ketones. Some of the studies were repeated with tularemia as the model infection. From these studies, it was concluded that during pneumococcal sepsis, the failure of rats to become ketonemic during caloric deprivation was the result of reduced ketogenic capacity of the liver and a possibly decreased hepatic supply of fatty acids. The latter appeared to be a secondary consequence of a severe reduction in circulating plasma albumin, the major transport protein for fatty acids, with no effect on the degree of saturation of the albumin with free fatty acids. Also, the infection had no significant effect on the rate of lipolysis or release of fatty acids from adipose tissue. Ketone body usage (oxidation) was either unaffected or reduced during pneumococcal sepsis in rats. Thus, a reduced rate of ketone production in the infected host was primarily responsible for the failure to develop starvation ketonemia under these conditions. The liver of the infected rat host appears to shuttle the fatty acids away from beta-oxidation and ketogenesis and toward triglyceride production, with resulting hepatocellular fatty metamorphosis.

Regulation of hepatic free fatty acid metabolism by glucagon and insulin

The roles of glucagon and insulin in the direct short-term regulation of hepatic free fatty acid (FFA) metabolism were studied in hepatocytes isolated from fed, fasted, and streptozotocin-induced diabetic rats. In fed animals, the principal metabolic product of palmitate metabolism was triglyceride, whereas ketones were the major product in fasted and diabetic animals. Glucagon at physiological concentrations increased ketogenesis and decreased triglyceride synthesis from palmitate in hepatocytes from fed rats at FFA concentrations 1.0 mM or less. Insulin had no effect on FFA metabolism when present as the sole hormone, but could antagonize the actions of submaximal concentrations of glucagon. The metabolism of palmitate in fasted or diabetic hepatocytes was unaffected by either hormone. Ketogenesis from octanoate was also unaffected by hormone addition in all cell types. These data are consistent with a locus of hormonal regulation at a step prior to beta-oxidation of fatty acid. Glucagon and insulin may modulate FFA metabolism by both intrahepatic and extrahepatic mechanisms.

Roles for insulin and glucagon in the development of ruminant ketosis -- a review

Ketonemia can be a physiological response to a reduction in dietary intake. It also may occur when energy demands exceed the energy intake. Normally, alimentary ketogenesis is the major source of ketone bodies in ruminants. During ketonemia there is increased hepatic ketone body production. During physiological ketosis, the mobilization of free fatty acids is inadequate to support a high rate of hepatic ketogenesis. However, during clinical ketosis, the hormonal status (low insulin, high glucagon/insulin ratio) in combination with hypoglycemia promotes excessive lipid mobilization and a greater hepatic removal of fatty acids and switches the liver to a higher rate of ketogenesis. The low insulin, furthermore, can impair maximal ketone body utilization, thus exacerbating the hyperketonemia.

Hypoglycemia in children

The factors that sustain postabsorptive glucose concentrations have been analyzed and the adverse effects of various hypoglycemic disorders on these factors examined. The role of alanine has been reviewed and the importance of glycerol as a precursor of glucose and of ketones as a fuel substitute for glucose emphasized. Finally, we have suggested that fasting functional hypoglycemia replace ketotic hypoglycemia as a descriptive term and that we relinquish the concept of leucine-sensitive hypoglycemia as a specific entity.

Variation in tissue carnitine concentrations with age and sex in the rat

Diabetes, starvation and various hormonal treatments are known to alter drastically carnitine concentrations in the body. Before the mechanisms controlling carnitine metabolism could be determined, it was necessary to establish normal carnitine concentrations in both sexes at different ages. Carnitine was assayed in plasma, liver, heart and skeletal muscle of rats from birth to weaning. The plasma carnitine increased rapidly during the first 2 days after birth. Carnitine in both heart and skeletal muscle increased, whereas liver concentrations declined during the first week of life. A carnitine-free diet containing sufficient precursors for carnitine biosynthesis was fed to weanling rats. Groups of ten male and ten female rats were killed each week for 10 consecutive weeks. Carnitine was determined in plasma, liver, heart, skeletal muscle, urine and epididymis in the male. There was no difference in carnitine concentrations between the sexes at weaning. Plasma, heart and muscle concentrations were higher in adult male rats than in adult females. However, liver carnitine and urinary carnitine concentrations were higher in adult female than in adult male rats. The epididymal carnitine concentration increased very rapidly during 50 to 70 days of age and the differences in carnitine concentrations between the sexes also became apparent during this time. Thus both the age and the sex of the human subject or experimental animal must be considered when investigating carnitine metabolism.

The development of ketogenesis at birth in the rat

In the suckling newborn rat, blood ketone bodies begin to increase slowly 4h after birth and then rise sharply between 12 and 16h, whereas the major increase in plasma non-esterified fatty acids and liver carnitine occurs during the first 2h of life, parallel with the onset of suckling. In the starved newborn rat, which shows no increase in liver carnitine unless it is fed with a carnitine solution, the developmental pattern of the ketogenic capacity (tested by feeding a triacylglycerol emulsion, which increases plasma non-esterified fatty acids by 3-fold) is the same as in the suckling animal. This suggests that the increases in plasma non-esterified fatty acids and liver carnitine seen 2h after birth in the suckling animal are not the predominant factors inducing the switch-on of ketogenesis. Injection of butyrate to starved newborn pups resulted in a pattern of blood ketone bodies which was similar to that found after administration of triacylglycerols, but, at all time points studied, the hyperketonaemia was more pronounced with butyrate. It is suggested that, even if the entry of long-chain fatty acids into the mitochondria is a rate-limiting step, it is not the only factor controlling ketogenesis after birth in the rat. As in the adult rat, there is a reciprocal correlation between the liver glycogen content and the concentration of ketone bodies in the blood.

Long and medium chain triglycerides increase plasma concentrations of ketone bodies in suckling rats

The potential of medium chain triglyceride (MCT) and long chain triglyceride (LCT) as sources of plasma ketones was investigated in suckling rats. Initially high concentrations of plasma ketones in 6-, 10, and 17-day-old rats increased 2- to 3-fold after acute feeding of MCT. This feeding had the same effect in fed or fasted adult rats. Corn oil (as a source of LCT) induced a large increase in the plasma ketone concentration of suckling rats and a relatively small but significant increase in fasted adult rats. The LCT treatment did not affect plasma ketone levels in fed adult rats. The results show clearly that feeding either LCT or MCT will enhance hyperketonemia in suckling rats. In the livers of all animals, regardless of age, the capacity for incorporation of [1(-14C)]octanoate into CO2 and acetoacetate far exceeded that for [1(-14C)]palmitate. The hyperketonemic action of LCT in suckling rats was accompanied by an increased activity of carnitine palmityltransferase and increased level of carnitine.

Glucagon and diabetes

We have considered the evidence, first, that the presence of glucagon is essential in the pathogenesis of the full syndrome that results from complete insulin deficiency; second, that in the diabetic in whom insulin levels are relatively fixed, a rise in glucagon concentration contributes to endogenous hyperglycemia; and, third, that conventional methods of treatment of diabetes do not fully correct either the abnormal glucagon levels or the hyperglycemia, but when insulin therapy is supplemented with somatostatin, an agent which suppresses both glucagon and growth hormone, both hyperglycemia and hyperglucagonemia are corrected. These facts may one day provide a rationale for therapeutic efforts to suppress excess glucagon secretion in the management of diabetes in man.

Deficiency of carnitine in cachectic cirrhotic patients

Carnitine is synthesized from lysine and methionine. In the rat, inadequate intake of either of these essential amino acids causes carnitine depletion. Inasmuch as protein deficiency is common in the hospital population, we have investigated the possible occurrence of nosocomial carnitine deficiency. Fasting serum carnitine concentration was measured in 16 normal and 247 patients in 16 disease groups. Normal range of carnitine was 55-103 muM. Only the cirrhotic group showed significant (P < 0.05) hypocarnitinemia. 14 of 36 hospitalized cirrhotics had subnormal values for serum carnitine. The creatinine/height index, midarm muscle circumference, and triceps skin-fold thickness indicated protein-calorie starvation in the 14 hypocarnitinemic liver patients. In six of the hypocarnitinemic cirrhotics (average serum level 50% of normal), spontaneous dietary intakes of carnitine, lysine, and methionine were measured and found to be only 5-15% as great as in six normocarnitinemic, healthy controls. When these six cirrhotic and six normal subjects were given the same lysine-rich, methionine-rich, and carnitine-free nutritional intake, the normals maintained normal serum carnitine levels and excreted 100 mumol/day, whereas the cirrhotics' serum level fell to 25% of normal, and urinary excretion declined to 15 mumol/day. Seven hypocarnitinemic cirrhotics died. Postmortem concentrations of carnitine in liver, muscle, heart, kidney, and brain averaged only one-fourth to one-third those in corresponding tissues of eight normally nourished nonhepatic patients who died after an acute illness of a 1-3-day duration. THESE DATA SHOW THAT CARNITINE DEPLETION IS COMMON IN PATIENTS HOSPITALIZED FOR ADVANCED CIRRHOSIS, AND THAT IT RESULTS FROM THREE FACTORS: substandard intake of dietary carnitine; substandard intake of lysine and methionine, the precursors for endogenous carnitine synthesis; and loss of capacity to synthesize carnitine from lysine and methionine.

A possible role for malonyl-CoA in the regulation of hepatic fatty acid oxidation and ketogenesis

Studied on the oxidation of oleic and octanoic acids to ketone bodies were carried out in homogenates and in mitochondrial fractions of livers taken from fed and fasted rats. Malonyl-CoA inhibited ketogenesis from the former but not from the latter substrate. The site of inhibition appeared to be the carnitine acyltransferase I reaction. The effect was specific and easily reversible. Inhibitory concentrations were in the range of values obtained in livers from fed rats by others. It is proposed that malonyl-CoA functions as both precursor for fatty acid synthesis and suppressor of fatty acid oxidation. As such, it might be an important element in the carbohydrate-induced sparing of fatty acid oxidation.

Isolation and identification of aliphatic short-chain acylcarnitines from beef heart: possible role for carnitine in branched-chain amino acid metabolism

Aliphatic acylearnitines isolated from a water-soluble fraction of beef heart have been characterized by gas chromatography and mass spectrometry. The following acyl residues derived from the acylcarnitine fraction were unequivocally identified: acetyl, propionyl, isobutyryl, butyryl, alpha-methylbutyryl, valeryl, isovaleryl, tiglyl, and caproyl. beta-methylcrotonyl and methacrylyl were tentatively identified. This occurrence of considerable quantities of branched-chain acylcarnitines indicates a role for carnitine in branched-chain amino acid metabolism.

Effects of lactation of ketogenesis from oleate or butyrate in rat hepatocytes

1. Rates of ketogenesis from endogenous butyrate or oleate were measured in isolated hepatocytes prepared from fed rats during different reproductive states [virgin, pregnant, early-lactating (2-4 days) and peak-lactating (10-17 days)]. In the peak-lactation group there was a decrease (25%) in the rate of ketogenesis from butyrate, but there were no differences in the rates between the other groups. Wth oleate, the rate of ketogenesis was increased in the pregnant and in the early-lactation groups compared with the virgin group, whereas the rate was 50% lower in the peak-lactation group. 2. Experiments with [1-(14)C]oleate indicated that these differences in rates of ketogenesis were not due to alterations in the rate of oleate utilization, but to changes in the amount of oleoyl-CoA converted into ketone bodies. 3. Although the addition of carnitine increased the rates of ketogenesis from oleate in all groups of rats, it did not abolish the differences between the groups. 4. Measurements of the accumulation of glucose and lactate showed that hepatocytes from rats at peak lactation had a higher rate of glycolytic flux than did hepatocytes from the other groups. After starvation, the rate of ketogenesis from oleate was still lower in the peak-lactation group compared with the control group. This suggests that the alteration in ketogenic capacity in the former group is not merely due to a higher glycolytic flux. 5. It is concluded that livers from rats at peak lactation have a lower capacity to produce ketone bodies from long-chain fatty acids which is due to an alteration in the partitioning of long-chain acyl-CoA esters between the pathways of triacylglycerol synthesis and beta-oxidation. The physiological relevance of this finding is discussed.

[Utilization of trimethylammonium-compounds by Acinetobacter calcoaceticus (author's transl)]

The utilization of carnitine and carnitine derivatives (O-acylcarnitines, carnitine carboxylderivatives) and structure-related trimethylammonium-compounds (betaines and nitrogen-bases) by Acinetobacter calcoaceticus was studied by means of the control of growth and the quantitative detection of metabolites. The strain grew only on L-carnitine, L-O-acylcarnitines, and gamma-butyrobetaine as the sole carbon sources. The utilization of these compounds and the growth correlated with the cleavage of the C-N bond and thereby with the formation of trimethylamin. D-Carnitine was metabolized, if an additional carbon source, like L-carnitine, was present in the incubation mixture, or if the bacteria were preincubated with L- or DL-carnitine, but no growth was observed on D-carnitine as the sole carbon source. The bacteria oxidized choline to glycinebetaine in the presence of additional carbon sources, glycinebetaine itself was not assimilated. With regard to the catabolism of quaternary nitrogen compounds Acinetobacter calcoaceticus shows a different pathway in comparison with other bacterial species metabolizing carnitine.