2,4-Dienoyl-coenzyme A reductase deficiency: a possible new disorder of fatty acid oxidation

Peroxisomal beta-oxidation and polyunsaturated fatty acids

Peroxisomes are capable of oxidizing a variety of substrates including (poly)unsaturated enoyl-CoA esters. The beta-oxidation of unsaturated enoyl-CoA esters in peroxisomes, and also in mitochondria, is not just chain-shortening but also involves the metabolizing of pre-existing carbon-to-carbon double bonds. In addition to the enzymes of the beta-oxidation spiral itself, this metabolism requires the participation of auxiliary enzymes: delta 3, delta 2-enoyl-CoA isomerase; 2,4-dienoyl-CoA reductase; 2-enoyl-CoA hydratase 2 or 3-hydroxyacyl-CoA epimerase; and delta 3,5 delta 2,4-dienoyl-CoA isomerase. Many of these enzymes are present as isoforms, and can be found located in multiple subcellular compartments, for example, peroxisomes, mitochondria or the endoplasmic reticulum, while some of the activities are integral parts of multifunctional enzymes of beta-oxidation systems.

Disorders of mitochondrial long-chain fatty acid oxidation

The oxidation of long-chain fatty acids requires a series of enzymes which are located in or on the mitochondrial membranes. These include carnitine palmitoyltransferases I and II, a carnitine-acylcarnitine translocase and, newly discovered, very long-chain acyl-CoA dehydrogenase and the mitochondrial trifunctional protein. These last two chain-shorten acyl-CoA esters to the point where they can be transferred to the more soluble medium- and short-chain-specific enzymes within the mitochondrial matrix. The disorders of long-chain fatty acid oxidation show a rather similar range of clinical and biochemical features, though with different emphasis in the different conditions. Patients with severe defects usually present early with acute attacks of hypoketotic hypoglycaemia and impaired liver function, or with cardiomyopathy or cardiac arrhythmia. In milder variants, skeletal myopathy with intermittent myoglobinuria develops later in life. 3-Hydroxyacyl-CoA dehydrogenase deficiency is unusual in producing peripheral neuropathy and retinitis pigmentosa. Treatment is based on the avoidance of fasting and replacement of normal dietary fat by medium-chain triglyceride, the medium-chain fatty acids entering the mitochondria in a carnitine-independent manner and bypassing the long-chain part of the spiral. Diagnosis must ultimately be based on direct assay of the enzyme involved, but preliminary indicators may come from determination of carnitine and intermediate metabolites in plasma, urinary organic acid profiling, and radioisotopic screening assays with lymphocytes or cultured fibroblasts.

Isolation and characterization of cDNA for human 120 kDa mitochondrial 2,4-dienoyl-coenzyme A reductase

2,4-Dienoyl-CoA reductase (EC 1.3.1.34) participates in beta-oxidation of (poly)unsaturated enoyl-CoAs and it appears in mammalian mitochondria as two isoforms with molecular masses of 120 and 60 kDa [Hakkola and Hiltunen (1993) Eur. J. Biochem. 215, 199-204]. The 120 kDa isomer is a homotetrameric enzyme, and here we report cDNA cloning of its subunit from human. cDNA clones were isolated by reverse transcriptase-PCR from a fibrosarcoma cell line and by screening from a human liver lambda gt11 cDNA library. The 1128 bp clone contained an open reading frame of 1008 bp encoding a polypeptide of 335 amino acid residues with a predicted molecular mass of 36066 Da. This polypeptide represents the immature monomer of the 120 kDa enzyme, and it contains a predicted N-terminal mitochondrial targeting signal. The amino acid (nucleotide) sequence of human 2,4-dienoyl-CoA reductase shows 82.7% (81.7%) similarity (identity) to the corresponding sequence from the rat. Northern-blot analysis gave a single mRNA species of 1.2 kb in several human tissues, the amounts present in the tissues tested ranking as follows: heart approximately liver approximately pancreas > kidney >> skeletal muscle approximately lung. Immunoblotting of human and rat liver samples with an antibody to the subunit of the rat 120 kDa isoform indicates that the mature human enzyme is larger than its counterpart in the rat. The comparison of amino acid sequences for rat and human enzymes proposes that the difference in the size is 10 amino acid residues. The results show that the rat and human reductases are similar in many characteristics and that the reductase is expressed in human tissues capable of beta-oxidation of fatty acids.

Spectrophotometric assay of 2,4-dienoyl coenzyme A reductase with 5-phenyl-2,4-pentadienoyl-coenzyme A as substrate

The spectrophotometric assay of 2,4-dienoyl coenzyme A (CoA) reductase (EC 1.1.1.34) was modified to improve the linearity and sensitivity of this method. 5-Phenyl-2,4-pentadienoyl-CoA, which has an absorbance maximum at 340 nm with an extinction coefficient of 44,300 M-1 cm-1, was synthesized and used as substrate. This compound is reduced by nicotinamide adenine dinucleotide phosphate (NADPH)-dependent 2,4-dienoyl-CoA reductase to 5-phenyl-3-pentenoyl-CoA. When a tissue homogenate serves as an enzyme source, the product is further metabolized by delta 3, delta 2-enoyl-CoA isomerase (EC 5.3.3.8) to 5-phenyl-2-pentenoyl-CoA, which is hydrated to 5-phenyl-3-hydroxypentanoyl-CoA by enoyl-CoA hydratase (EC 4.2.1.17). The modified assay method, which measures the decrease in absorbance at 340 nm due to the reduction of 5-phenyl-2,4-pentadienoyl-CoA and the oxidation of NADPH, is linear for a longer period of time and is twice as sensitive as the conventional assay with 2,4-decadienoyl-CoA as substrate.

The changing face of disorders of fatty acid oxidation

OBJECTIVE

To review the current understanding of the rapidly changing field of disorders of fatty acid metabolism and to discuss the future directions for research.

DESIGN

A literature review of the basic biochemistry of the beta-oxidation pathway and clinical cases of defects of fatty acid metabolism are presented, and the diagnosis and treatment of such defects are discussed.

MATERIAL AND METHODS

In many cases, a correct diagnosis will be made only if these disorders are specifically considered and appropriate tests are obtained, because results of screening tests for other organic acidemias are often normal in these entities.

RESULTS

The first disorder of fatty acid metabolism was described only 20 years ago. Since then, at least 15 different inborn errors of metabolism that affect beta-oxidation have been identified, most in the past 10 years. Within the past 5 years, investigators have realized that a deficiency of one of these enzymes, medium-chain acyl coenzyme A dehydrogenase, may be one of the most common inborn errors of metabolism. This disorder may have a frequency equal to that of phenylketonuria in some populations in the United States and northern Europe. Approximately 1 to 3% of all unexplained deaths during infancy and childhood are probably related to disorders of beta-oxidation. Diagnosis of these disorders can be difficult because of the intermittent nature of the excretion of characteristic compounds. The mainstay of therapy for defects of beta-oxidation is avoidance of fasting.

CONCLUSION

All patients with a suspected defect of fatty acid metabolism should be assessed and monitored by a specialist trained in the care of such patients. Continued improvements in the ability to diagnose and treat these disorders will be directly linked to new advances in the basic research on these enzymes. Movements to screen newborns for medium-chain acyl coenzyme A dehydrogenase are under way in some medical centers. Proposed tests include metabolite analysis or direct mutation analysis (or both) from blood spots from newborn screening cards already obtained for every newborn in the United States.

New developments in the diagnosis and investigation of mitochondrial fatty acid oxidation disorders

Since the discovery of muscle carnitine palmitoyltransferase deficiency in 1973, a dozen separate defects of mitochondrial fatty acid beta-oxidation in man have been identified. With the exception of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, which occurs with a frequency approaching 1:10,000 among Caucasians of Northern European origin, the other defects are quite rare. Collectively, however, they are common causes of disease resembling Reye syndrome in early life, and some have a later and more chronic presentation with cardiomyopathy and skeletal muscle weakness. They also represent a small, but significant, proportion of cases of sudden and unexplained death within the first 2 years of life. Diagnosis of these disorders has become increasingly sophisticated, with the advent of new analytical technologies and an increased awareness of the appropriate clinical and laboratory investigations needed in order to evaluate potential defects of this pathway. The combination of provocative testing (e.g., carnitine loading, phenylpropionic acid loading, long-chain fat loading) and advanced analytical techniques for the measurement of blood and urinary metabolites (e.g., tandem fast atom bombardment-mass spectrometry, stable isotope dilution gas chromatography-mass spectrometry) permits a specific diagnosis in the case of several, although not all, of the disorders of this pathway. Methods for the measurement of all of the enzymes of beta-oxidation are now available to enhance this diagnostic capability. There remain, however, many patients in whom clinical and laboratory signs point to a defect in beta-oxidation, but in whom no specific diagnosis has yet been made.(ABSTRACT TRUNCATED AT 250 WORDS)

The existence of two mitochondrial isoforms of 2,4-dienoyl-CoA reductase in the rat

Isoforms of 2,4-dienoyl-CoA reductase (EC 1.3.1.34), which is the key enzyme in the beta-oxidation of fatty acids with double bonds, have been studied in rat heart and liver. Electrofocusing and adsorption chromatography on hydroxyapatite were used to separate the reductase activity in tissue homogenates into two peaks, one peak in each experiment being identified as the previously purified mitochondrial reductase. The novel activity was partially purified from rat liver by means of ammonium sulphate precipitation, anion-exchange chromatography on DEAE-cellulose (DE-52), hydrophobic chromatography on Phenyl-Sepharose and dye-ligand binding chromatography (Blue Sepharose). Taking into account the contribution of the different reductases to the total activity in rat liver, the overall purification for the novel isoform was 1900-fold. Ultracentrifugation on a sucrose gradient gave an M(r) of 50,000 and size-exclusion chromatography on Superdex 200 an M(r) of 60,000. The antibody against the previously characterised reductase did not cross-react with this novel isoform, but the distribution of the activity peaks in heart and liver tissue, and an electrofocusing experiment with isolated mitochondria, both pointed to a mitochondrial origin. The novel reductase was estimated to account for 80% (50%) of the total reductase activity in rat heart (liver) homogenate measured with 2,4-hexadienoyl-CoA. The present results, together with those previously published, suggest that mammals have at least three reductase isoforms: two in mitochondria and a third one in peroxisomes, but the peroxisomal activity has not been characterised so far.

Beta-oxidation of unsaturated fatty acids in humans. Isoforms of delta 3, delta 2-enoyl-CoA isomerase

This investigation was undertaken in order to elucidate the human enzymes which participate in metabolism of the double bonds of unsaturated fatty acids during beta-oxidation. The results indicate that the human monofunctional delta 3, delta 2-enoyl-CoA isomerase (EC 5.3.3.8) with the native M(r) of 70,000 differed significantly from its rat counterpart [Palosaari et al. (1990) J. Biol. Chem. 265, 3347-3353]; the isoelectric point of the human isoform was over three pH-units more acidic, it showed different chromatographic behaviour, the human enzyme did not show any clear-cut substrate chain-length specificity and only a weak immunological cross-reactivity was detected with the antibody to rat liver mitochondrial short-chain enzyme. This explains the failure of attempts to apply the rat data directly to human beings. Another isomerase activity from human liver was found to be a part of the isomerase-hydratase-dehydrogenase polypeptide showing immunological cross-reactivity with the previously characterized peroxisomal multifunctional enzyme (MFE) from rat liver.

Carnitine-acylcarnitine translocase deficiency with severe hypoglycemia and auriculo ventricular block. Translocase assay in permeabilized fibroblasts

Deficiency of the enzymes of mitochondrial fatty acid oxidation and related carnitine dependent steps have been shown to be one of the causes of the fasting-induced hypoketotic hypoglycemia. We describe here carnitine-acylcarnitine translocase deficiency in a neonate who died eight days after birth. The proband showed severe fasting-induced hypoketotic hypoglycemia, high plasma creatine kinase, heartbeat disorder, hypothermia, and hyperammonemia. The plasma-free carnitine on day three was only 3 microM, and 92% of the total carnitine (37 microM) was present as acylcarnitine. Treatments with intravenous glucose, carnitine, and medium-chain triglycerides had been tried without improvements. Measurements in fibroblasts confirmed deficient oxidation of palmitate and showed normal activities of the carnitine palmitoyltransferases I and II and of the three acyl-CoA dehydrogenases. A total deficiency of the carnitine-acyl-carnitine translocase was found in fibroblasts using the carnitine acetylation assay (1986. Biochem. J. 236:143-148). This assay has been further simplified by seeking conditions permitting application to permeabilized fibroblasts and lymphocytes.

Peroxisomal beta-oxidation of polyunsaturated fatty acids

Peroxisomes have been shown to play an important role in the oxidative degradation of (poly)unsaturated fatty acids, and contain the enzyme activities needed for the metabolism of double bonds of unsaturated fatty acids in connection with this physiological function. Our understanding of the metabolic pathways and enzyme activities involved in the degradation of unsaturated acyl-CoAs has undergone a re-evaluation recently, and though many open questions still remain significant progress has been made, especially concerning the reactions metabolizing double bonds. The enzyme activities to be discussed here are 2,4-dienoyl-CoA reductase; 3/2-enoyl-CoA isomerase; 2-enoyl-CoA hydratase 2; 5-enoyl-CoA reductase and 3,5/2,4-dienoyl-CoA isomerase. Some of these activities are integral parts of the multifunctional proteins of beta-oxidation systems, which must also be taken into account in this context.

Radioactive assay of 2,4-dienoyl-coenzyme A reductase

A radioactive method for assaying 2,4-dienoyl-CoA reductase, also referred to as 4-enoyl-CoA reductase (EC 1.3.1.34), is described. The assay measures the incorporation of tritium from [4B-3H]NADPH into 2-trans,4-cis-decadienoyl-CoA or 2-trans,4-trans-decadienoyl-CoA which, after cleavage of the thioester bond with hydroxylamine, can be separated from the radioactive coenzyme by extraction with toluene. This assay is at least 30 times more sensitive than the spectrophotometric assay, even though rates determined by the radioactive method are 10 times lower than rates obtained spectrophotometrically due to a primary kinetic isotope effect. The linearity of this assay with respect to time and protein concentration is sufficient for determining 2,4-dienoyl-CoA reductase activities in extracts from small samples of human fibroblasts, which were found to contain reductase activities between 1.8 and 5.8 mU/mg of protein.