Deficiency of carnitine palmitoyltransferase I

Fatty acid metabolism by the osteoblast

The emergence of bone as an endocrine organ able to influence whole body metabolism, together with comorbid epidemics of obesity, diabetes, and osteoporosis, have prompted a renewed interest in the intermediary metabolism of the osteoblast. To date, most studies have focused on the utilization of glucose by this specialized cell, but the oxidation of fatty acids results in a larger energy yield. Osteoblasts express the requisite receptors and catabolic enzymes to take up and then metabolize fatty acids, which appears to be required during later stages of differentiation when the osteoblast is dedicated to matrix production and mineralization. In this article, we provide an overview of fatty acid β-oxidation and highlight studies demonstrating that the skeleton plays a significant role in the clearance of circulating lipoproteins and non-esterified fatty acids. Additionally, we review the requirement for long-chain fatty acid metabolism during post-natal bone development and the effects of anabolic stimuli on fatty acid utilization by osteoblasts. These recent findings may help to explain the skeletal manifestations of human diseases associated with impaired lipid metabolism while also providing additional insights into the metabolic requirements of skeletal homeostasis.

The use of nutritional supplements to induce ketosis and reduce symptoms associated with keto-induction: a narrative review

Background

Adaptation to a ketogenic diet (keto-induction) can cause unpleasant symptoms, and this can reduce tolerability of the diet. Several methods have been suggested as useful for encouraging entry into nutritional ketosis (NK) and reducing symptoms of keto-induction. This paper reviews the scientific literature on the effects of these methods on time-to-NK and on symptoms during the keto-induction phase.

Methods

PubMed, Science Direct, CINAHL, MEDLINE, Alt Health Watch, Food Science Source and EBSCO Psychology and Behavioural Sciences Collection electronic databases were searched online. Various purported ketogenic supplements were searched along with the terms “ketogenic diet”, “ketogenic”, “ketosis” and ketonaemia (/ ketonemia). Additionally, author names and reference lists were used for further search of the selected papers for related references.

Results

Evidence, from one mouse study, suggests that leucine doesn’t significantly increase beta-hydroxybutyrate (BOHB) but the addition of leucine to a ketogenic diet in humans, while increasing the protein-to-fat ratio of the diet, doesn’t reduce ketosis. Animal studies indicate that the short chain fatty acids acetic acid and butyric acid, increase ketone body concentrations. However, only one study has been performed in humans. This demonstrated that butyric acid is more ketogenic than either leucine or an 8-chain monoglyceride. Medium-chain triglycerides (MCTs) increase BOHB in a linear, dose-dependent manner, and promote both ketonaemia and ketogenesis. Exogenous ketones promote ketonaemia but may inhibit ketogenesis.

Conclusions

There is a clear ketogenic effect of supplemental MCTs; however, it is unclear whether they independently improve time to NK and reduce symptoms of keto-induction. There is limited research on the potential for other supplements to improve time to NK and reduce symptoms of keto-induction. Few studies have specifically evaluated symptoms and adverse effects of a ketogenic diet during the induction phase. Those that have typically were not designed to evaluate these variables as primary outcomes, and thus, more research is required to elucidate the role that supplementation might play in encouraging ketogenesis, improve time to NK, and reduce symptoms associated with keto-induction.

Novel metabolic and molecular findings in hepatic carnitine palmitoyltransferase I deficiency

Detection of hepatic carnitine palmitoyltransferase I (CPT IA) deficiency by metabolite screening may be problematic. The urine organic acid profile is generally said to be normal and no abnormal or increased acylcarnitine species are evident on bloodspot tandem MS examination. We diagnosed CPT IA deficiency presenting with acute encephalopathy +/- hypoglycemia and hepatomegaly in one Bukharan Jewish and two Palestinian Arab infants from consanguineous families. CPT1A mutation analysis identified two novel nonsense mutations, c.1737C>A (Y579X) and c.1600delC (L534fsX), extending the known genetic heterogeneity in this disorder. A distinctive organic aciduria was observed in all three patients, even several days after initiation of treatment and resolution of symptoms. Abnormal findings included a hypoketotic dicarboxylic aciduria with prominence of the C12 dicarboxylic (dodecanedioic) acid. This C12 dicarboxylic aciduria suggests that CPT I may play a role in uptake of long-chain dicarboxylic acids by mitochondria after their initial shortening by beta-oxidation in peroxisomes. In addition, increased excretion of 3-hydroxyglutaric acid was detected in all three patients, a finding previously observed only in glutaric aciduria type 1, ketosis, and short-chain hydroxyacyl-CoA dehydrogenase deficiency. Examination of urine organic acids with awareness of these metabolic findings may lead to improved diagnosis of this seemingly rare disorder.

Diagnosis and management of defects of mitochondrial beta-oxidation

PURPOSE OF REVIEW

At least 22 different inborn errors of metabolism affecting beta-oxidation in skeletal muscle and other tissues have been identified in the past 30 years. Early diagnosis and therapeutic diets offer the best chance for normal growth and development in most patients.

RECENT FINDINGS

Clinical heterogeneity has become the hallmark of defects in beta-oxidation. In many cases a correct diagnosis will only be made if these disorders are specifically considered and appropriate studies are obtained, since screening tests which detect other inborn errors of metabolism are often normal in patients with beta-oxidation defects. Dietary management provides the only opportunity for therapy in many cases, including carbohydrate supplements intended to provide more extended delivery of glucose to the bloodstream. Use of a novel odd chain fat supplement as an alternative fuel source in long chain fat metabolism defects offers promise of alleviating muscular symptoms not well controlled by diet. The introduction of expanded newborn screening will lead to the recognition of an increasing number of individuals with these disorders, placing greater demand for services on practitioners knowledgeable in their therapy. Study of the clinical outcome in these patients will provide a better understanding of defects of beta-oxidation.

SUMMARY

Clinical symptoms, diagnostic testing, and issues of newborn screening for this important group of disorders are discussed.

Acute fatty liver of pregnancy associated with short-chain acyl-coenzyme A dehydrogenase deficiency

There is a correlation between pregnancy complications such as acute fatty liver of pregnancy and long-chain 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD) deficiency. We diagnosed another fatty acid beta-oxidation defect, short-chain acyl-coenzyme A dehydrogenase deficiency, in an infant when evaluating him because his mother had acute fatty liver of pregnancy. Other beta-oxidation defects, in addition to LCHAD deficiency, should be considered in children born after pregnancies complicated by liver disease.

Hypoglycemia in association with various organic and amino acid disorders

A number of organic and amino acidemias, particularly those that involve the oxidation of fatty acids, cause hypoglycemia intermittently. This may be associated with distrubances of acid base equilibrium and accumulation of lactic acid and/or ketone bodies. When such diseases are not diagnosed rapidly, they might lead to neurological crippling and, at times, death. As a group, these disorders involve more than 1 organ and their phenotypic expression may include all or a single system. The symptoms may appear soon after birth or as late as 1 year of age. Their early recognition and rapid intervention provide rewarding clinical outcome. With the recent advances in diagnostic techniques, such as the introduction of tandem mass spectrometry (MS), screening for these diseases now can be performed because rapid identification on a large scale is possible. The phenotypes, mutations involved, pathognomonic laboratory findings, prognosis, and treatment procedures available have been reviewed for major diseases.

Carnitine palmitoyltransferase deficiencies

Carnitine palmitoyltransferase (CPT) deficiencies are common disorders of mitochondrial fatty acid oxidation. The CPT system is made up of two separate proteins located in the outer- (CPT1) and inner- (CPT2) mitochondrial membranes. While CPT2 is a ubiquitous protein, two tissue-specific CPT1 isoforms-the so-called "liver" (L) and "muscle" (M) CPT1s-have been shown to exist. Amino acid and cDNA nucleotide sequences have been identified for all of these proteins. L-CPT1 deficiency (13 families reported) presents as recurrent attacks of fasting hypoketotic hypoglycemia. Two L-CPT1 mutations have been reported to date. M-CPT1 deficiency has not been hitherto identified. CPT2 deficiency has several clinical presentations. The "benign" adult form (more than 150 families reported) is characterized by episodes of rhabdomyolysis triggered by prolonged exercise. The prevalent S113L mutation is found in about 50% of mutant alleles. The infantile-type CPT2 deficiency (10 families reported) presents as severe attacks of hypoketotic hypoglycemia, occasionally associated with cardiac damage commonly responsible for sudden death before 1 year of age. In addition to these symptoms, features of brain and kidney dysorganogenesis are frequently seen in the neonatal-onset CPT2 deficiency (13 families reported), almost always lethal during the first month of life. More than 25 CPT2 mutations (private missense or truncating mutations) have hitherto been detected. Treatment is based upon avoidance of fasting and/or exercise, a low-fat diet enriched with medium chain triglycerides and carnitine ("severe" CPT2 deficiency). Prenatal diagnosis may be offered for pregnancies at a 1/4 risk of infantile/severe-type CPT2 deficiency.

Genetic disorders of carnitine metabolism and their nutritional management

Carnitine functions as a substrate for a family of enzymes, carnitine acyltransferases, involved in acyl-coenzyme A metabolism and as a carrier for long-chain fatty acids into mitochondria. Carnitine biosynthesis and/or dietary carnitine fulfill the body's requirement for carnitine. To date, a genetic disorder of carnitine biosynthesis has not been described. A genetic defect in the high-affinity plasma membrane carnitine-carrier(in) leads to renal carnitine wasting and primary carnitine deficiency. Myopathic carnitine deficiency could be due to an increase in efflux moderated by the carnitine-carrier(out). Defects in the carnitine transport system for fatty acids in mitochondria have been described and are being examined at the molecular and pathophysiological levels. the nutritional management of these disorders includes a high-carbohydrate, low-fat diet and avoidance of those events that promote fatty acid oxidation, such as fasting, prolonged exercise, and cold. Large-dose carnitine treatment is effective in systemic carnitine deficiency.

Cloning and characterization of murine carnitine acetyltransferase: evidence for a requirement during cell cycle progression

We have employed a newly developed differential screening technique (reverse strand priming) to identify murine carnitine acetyltransferase (CARAT) as a growth- and cell cycle-regulated gene in S3T3 mouse fibroblasts. Sequence analysis of the full-length cDNA clone and homology comparisons have revealed 87% homology to the human CARAT gene. On Northern blots we were able to measure a 2-3-fold induction 18 h after a mitogenic stimulus following serum deprivation as well as after release from a sodium butyrate block. The cell cycle induction pattern of the CARAT gene was analysed in mouse fibroblasts at different stages of the unperturbed cell cycle. Fractions obtained by elutriation of an exponentially growing culture showed a biphasic maximum of transcript abundance in the G1 and G2 phases of the cell cycle. CARAT expression was investigated in several organs of the adult mouse. Among those measured, CARAT expression was highest, relative to liver, in heart muscle (56-fold) and testis (21-fold). Using both conventional antisense oligodeoxynucleotides and novel single-stranded antisense phagemid DNA, we obtained evidence that the CARAT enzyme function is necessary for progression through G1 and into the S-phase of the cell cycle.

Over-expression and characterization of active recombinant rat liver carnitine palmitoyltransferase II using baculovirus

The cDNA encoding rat liver carnitine palmitoyltransferase II (CPT-II) was heterologously expressed using a recombinant baculovirus/insect cell system. Unlike Escherichia coli, the baculovirus-infected insect cells expressed mostly soluble active recombinant CPT-II (rCPT-II). CPT activity from crude lysates of recombinant baculovirus-infected insect cells was maximal between 50 and 72 h post-infection, with a peak specific activity of 100-200 times that found in the mock- or wild-type-infected control lysates. Milligram quantities (up to 1.8 mg/l of culture) of active rCPT-II were chromatographically purified from large-scale cultures of insect cells infected with the recombinant baculovirus. The rCPT-II was found to be: (1) similar in size to the native rat liver enzyme (approximately 70 kDa) as judged by SDS/PAGE; (2) immunoreactive with a polyclonal serum raised against rat liver CPT-II; and (3) not glycosylated. Kinetic analysis of soluble rCPT-II revealed Km values for carnitine and palmitoyl-CoA of 950 +/- 27 microM and 34 +/- 5.6 microM respectively.

Cloning, sequencing and heterologous expression of a cDNA encoding pigeon liver carnitine acetyltransferase

Two overlapping cDNA clones encoding pigeon liver carnitine acetyltransferase (EC 2.3.1.7) (CAT) were isolated from a pigeon liver lambda gt11 cDNA library by gene amplification using oligonucleotide primers based on the N-terminal amino acid sequence of the enzyme. The two clones, which represent the 5' and 3' ends of the gene, were spliced together to form a single cDNA construct containing the entire coding sequence for CAT, with an in-frame TGA stop codon 42 bases before the first ATG start site and a 3'-untranslated segment of 1057 bases. The largest open reading frame of 1942 nucleotides predicted a polypeptide of 627 amino acids and a molecular mass of 71.1 kDa. The N-terminus and four internal peptides from the amino acid sequence of pigeon breast muscle CAT were identified in the predicted sequence of the liver cDNA clone. The identity of the CAT cDNA was confirmed by heterologous expression of active recombinant CAT (rCAT) in insect cells using the baculovirus expression system. Western blots of rCAT from infected insect cell lysates and immunodetection with a rabbit anti-CAT polyclonal serum showed an immunoreactive protein band similar in size to native CAT from pigeon breast muscle. Like the native enzyme, rCAT was capable of acylating carnitine with a preference for small-chain acyl-CoAs of carbon chain lengths C2-C4.

Atypical features of the hepatic form of carnitine palmitoyltransferase deficiency in a Hutterite family

We describe hepatic carnitine palmitoyltransferase (CPT I) deficiency in three children (a brother and sister and their second cousin) from an extended inbred Hutterite kindred. The patients were first seen between 8 and 18 months of age with recurrent episodes of hypoketotic hypoglycemia accompanied by a decreased level of consciousness and hepatomegaly. One patient had two Reye syndrome-like episodes. Abnormal organic acids were rarely detected in urine. Serum total and free carnitine levels were elevated in all three patients. Fibroblast acyl-coenzyme A dehydrogenase activities were normal in all, but palmitic acid oxidation, performed in fibroblasts from one patient, was less than 10% of control values. Activity of CPT I in cultured skin fibroblasts from the three patients was 10% to 15% of control levels; CPT II activity was normal. Activity of CPT I and CPT II in muscle from one patient was normal. Atypical features in two of these patients were greatly elevated levels of liver enzymes and creatine kinase during acute episodes. The patients have recently been successfully treated with medium-chain triglycerides and avoidance of fasting. Early identification and treatment of this disorder may avert potentially fatal episodes of hypoglycemia.

Elevated plasma carnitine in the hepatic form of carnitine palmitoyltransferase-1 deficiency

In a boy with a defect in fatty acid oxidation due to the hepatic form of carnitine palmitoyltransferase-1 deficiency, plasma carnitine concentrations were found to be twice normal. The elevation in plasma carnitine levels was accompanied by an unusually high renal threshold for free carnitine, suggesting a secondary increase in carnitine transport. Similar to other fatty acid oxidation disorders involving the carnitine cycle, urinary dicarboxylic acids were not abnormally elevated during illnesses. The combination of elevated plasma carnitine levels and absence of dicarboxylic aciduria may help to distinguish the hepatic form of carnitine palmitoyltransferase-1 deficiency from other defects in fatty acid oxidation.

Infantile form of carnitine palmitoyltransferase II deficiency with hepatomuscular symptoms and sudden death. Physiopathological approach to carnitine palmitoyltransferase II deficiencies

Reported cases of carnitine palmitoyltransferase II (CPT II) deficiency are characterized only by a muscular symptomatology in young adults although the defect is expressed in extra-muscular tissues as well as in skeletal muscle. We describe here a CPT II deficiency associating hypoketotic hypoglycemia, high plasma creatine kinase level, heart beat disorders, and sudden death in a 3-mo-old boy. CPT II defect (-90%) diagnosed in fibroblasts is qualitatively similar to that (-75%) of two "classical" CPT II-deficient patients previously studied: It resulted from a decreased amount of CPT II probably arising from its reduced biosynthesis. Consequences of CPT II deficiency studied in fibroblasts differed in both sets of patients. An impaired oxidation of long-chain fatty acids was found in the proband but not in patients with the "classical" form of the deficiency. The metabolic and clinical consequences of CPT II deficiency might depend, in part, on the magnitude of residual CPT II activity. With 25% residual activity CPT II would become rate limiting in skeletal muscle but not in liver, heart, and fibroblasts. As observed in the patient described herein, CPT II activity ought to be more reduced to induce an impaired oxidation of long-chain fatty acids in these tissues.