Molecular cloning and nucleotide sequence of cDNA encoding the entire precursor of rat liver medium chain acyl coenzyme A dehydrogenase

Medium-chain acyl-CoA deficiency: outlines from newborn screening, in silico predictions, and molecular studies

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is a disorder of fatty acid oxidation characterized by hypoglycemic crisis under fasting or during stress conditions, leading to lethargy, seizures, brain damage, or even death. Biochemical acylcarnitines data obtained through newborn screening by liquid chromatography-tandem mass spectrometry (LC-MS/MS) were confirmed by molecular analysis of the medium-chain acyl-CoA dehydrogenase (ACADM) gene. Out of 324.000 newborns screened, we identified 14 MCADD patients, in whom, by molecular analysis, we found a new nonsense c.823G>T (p.Gly275∗) and two new missense mutations: c.253G>C (p.Gly85Arg) and c.356T>A (p.Val119Asp). Bioinformatics predictions based on both phylogenetic conservation and functional/structural software were used to characterize the new identified variants. Our findings confirm the rising incidence of MCADD whose existence is increasingly recognized due to the efficacy of an expanded newborn screening panel by LC-MS/MS making possible early specific therapies that can prevent possible crises in at-risk infants. We noticed that the “common” p.Lys329Glu mutation only accounted for 32% of the defective alleles, while, in clinically diagnosed patients, this mutation accounted for 90% of defective alleles. Unclassified variants (UVs or VUSs) are especially critical when considering screening programs. The functional and pathogenic characterization of genetic variants presented here is required to predict their medical consequences in newborns.

Clinical, biochemical and genetic analyses in two Korean patients with medium-chain acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is an autosomal recessive hereditary metabolic disorder of mitochondrial fatty acid β-oxidation. It is characterized by hypoketotic hypoglycemia, hyperammonemia, seizure, coma, and sudden infant death syndrome-like illness. The most frequently isolated mutation in the acyl-CoA dehydrogenase, medium-chain (ACADM) gene of Caucasian patients with MCADD is c.985A>G, but ethnic variations exist in the frequency of this mutation. Here, we describe 2 Korean pediatric cases of MCADD, which was detected during newborn screening by tandem mass spectrometry and confirmed by molecular analysis. The levels of medium-chain acylcarnitines, including octanoylcarnitine (C8), hexanoylcarnitine (C6), and decanoylcarnitine (C10), were typically elevated. Molecular studies revealed that Patient 1 was a compound heterozygote for c.449_452delCTGA (p.Thr150ArgfsX4) and c.461T>G (p.L154W) mutations, and Patient 2 was a compound heterozygote for c.449_452delCTGA (p.Thr150ArgfsX4) and c.1189T>A (p.Y397N) mutations. We detected asymptomatic patients with MCADD by using a newborn screening test and confirmed it by ACADM mutation analysis. This report presents evidence of the biochemical and molecular features of MCADD in Korean patients and, to the best of our knowledge, this is the first report of the c.461T>G mutation in the ACADM gene.

The benefit of medium-chain triglyceride therapy on the cardiac function of SHRs is associated with a reversal of metabolic and signaling alterations

The spontaneously hypertensive rat (SHR) is a model of cardiomyopathy that displays a genetic defect in cardiac fatty acid (FA) translocase/CD36, a plasma membrane long-chain FA transporter. Therapy with medium-chain FAs, which do not require CD36-facilitated transport, has been shown to improve cardiac function and hypertrophy in SHRs despite persistent hypertension. However, little is known about the underlying molecular mechanisms. The aim of this study was to document the impact of medium-chain triglyceride (MCT) therapy in SHRs on the expression level and activity of metabolic enzymes and signaling pathways. Four-week-old male SHRs were administered MCT (SHR-MCT) or long-chain triglyceride (SHR-LCT) for 16 wk. We used Wistar-Kyoto (WKY) rats as controls (WKY-MCT and WKY-LCT). The SHR-MCT group displayed improved cardiac dysfunction [as assessed by left ventricular (LV) end-diastolic pressure and the positive and negative first derivatives of LV pressure/ P value], a shift in the β-myosin heavy chain (MHC)-to-α-MHC ratio, and cardiac hypertrophy compared with the SHR-LCT group without an effect on blood pressure. Administration of MCT of SHRs reversed the LCT-induced reduction in the cardiac FA metabolic enzymatic activities of long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and medium-chain acyl-CoA dehydrogenase (MCAD). In the SHR-MCT group, the protein expression and transcriptional regulation of myocardial peroxisome proliferator-activated receptor-α, which regulates the transcription of LCHAD and MCAD genes, corresponded to the changes seen in those enzymatic activities. Furthermore, MCT intake caused an inhibition of JNK activation in SHR hearts. Collectively, the observed changes in the myocardial activity of metabolic enzymes and signaling pathways may contribute to the improved cardiac dysfunction and hypertrophy in SHRs following MCT therapy.

Population spectrum of ACADM genotypes correlated to biochemical phenotypes in newborn screening for medium-chain acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most frequent inherited defect of fatty acid oxidation, with a significant morbidity and mortality in undiagnosed patients. Adverse outcomes can effectively be prevented by avoiding metabolic stress and following simple dietary measures. Therefore, prospective newborn screening (NBS) is being proposed for this condition. However, technical validation of MCADD population screening and assessment of its overall benefit require broadening of the as-yet-scarce knowledge of the MCADD genetic heterogeneity unraveled by NBS and its phenotypic consequences. Here, we describe the entire spectrum of sequence variations occurring in newborns with MCADD in the population of Bavaria, Germany, in relation to the biochemical phenotype. Among 524,287 newborns, we identified 62 cases of MCADD, indicating a birth incidence of 1 in 8,456. In all of the 57 newborns available for analysis, two alterations within the MCADD gene (ACADM) were identified. The most prevalent alteration c.985A>G (Lys329Glu) occurred in 27 (47%) newborns in the homozygous and in 18 (32%) in the heterozygous state (63% of defective alleles). The mild folding variant c.199T>C (Tyr67His) was identified in nine individuals, six of them being compound heterozygous with c.985A>G (Lys329Glu). Neither of the prevalent alterations were found in the remaining nine newborns. A total of 18 sequence variations were identified; 13 of them were novel: eight missense mutations, one nonsense mutation, two splice variants, and two small deletions. The remaining five were previously reported in MCADD patients. The ACADM heterogeneity uncovered was larger as anticipated from previous c.985A>G (Lys329Glu) carrier screening data. In addition, we show that MCADD appears to occur as frequently in Turkish newborns as in the native German population. Our data validate that biochemical NBS for MCADD is a highly specific procedure for disease detection, with the identification of a significant share of milder biochemical phenotypes, such as c.199T>C (Tyr67His). These show statistically lower acylcarnitine markers, allowing us to distinguish subgroups within the spectrum of ACADM sequence variations that correlate to biochemical MCADD disease expression. Our data might provide technical and medical guidance for decision making in the worldwide efforts to introduce MCADD population screening.

Medium-chain acyl-CoA dehydrogenase deficiency in gene-targeted mice

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common inherited disorder of mitochondrial fatty acid β-oxidation in humans. To better understand the pathogenesis of this disease, we developed a mouse model for MCAD deficiency (MCAD^−/−) by gene targeting in embryonic stem (ES) cells. The MCAD^−/− mice developed an organic aciduria and fatty liver, and showed profound cold intolerance at 4 °C with prior fasting. The sporadic cardiac lesions seen in MCAD^−/− mice have not been reported in human MCAD patients. There was significant neonatal mortality of MCAD^−/− pups demonstrating similarities to patterns of clinical episodes and mortality in MCAD-deficient patients. The MCAD-deficient mouse reproduced important aspects of human MCAD deficiency and is a valuable model for further analysis of the roles of fatty acid oxidation and pathogenesis of human diseases involving fatty acid oxidation.

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is one of the most common inherited disorders of metabolism. This defect in fatty acid oxidation can lead to severe and sometimes fatal disease, especially in young children because they are unable to tolerate a fasting episode. Metabolic complications include very low blood glucose concentrations and generation of toxic by-products. This disorder can result in sudden infant death. Using a process known as gene targeting in mouse embryonic stem cells, the authors have developed a mouse model with the same enzyme deficiency. This mouse model of MCAD deficiency develops many of the same disease characteristics found in affected children. The MCAD-deficient mouse model shows a high rate of newborn loss, intolerance to cold, and the characteristic biochemical changes in the blood, tissues, and urine that are very similar to those found in the human disease counterpart. The MCAD-deficient mouse model will allow researchers to better understand disease mechanisms so that new preventive measures or therapies can be developed.

Nucleotide sequence of a portion of the camphor-degrading gene cluster from Rhodococcus sp. NCIMB 9784

Rhodococcus sp. NCIMB 9784 is a camphor-degrading Gram-positive organism originally isolated from activated sewage sludge. A 5.4kbp portion of a proposed camphor degradation gene cluster from this organism was cloned and its nucleotide sequence determined. Four open reading frames (ORFs) were identified encoding proteins possibly involved in camphor metabolism; sequence alignment of the translation products suggested that the ORFs encode for a ferredoxin reductase, acyl-CoA ligase, epimerase and an acyl-CoA dehydrogenase. The last three activities are thought to be involved in the poorly understood late stage of camphor degradation. Our findings are entirely consistent with the proposed formation of a branched 9-carbon acid intermediate (3,4,4-trimethyl-5-oxo-trans-2-hexenoic acid) which has been isolated from the fermentation broth of camphor-grown cells.

Expression of genes participating in regulation of fatty acid and glucose utilization and energy metabolism in developing rat hearts

The heart is a unique organ that can use several fuels for energy production. During development, the heart undergoes changes in fuel supply, and it must be able to respond to these changes. We have examined changes in the expression of several genes that regulate fuel transport and metabolism in rat hearts during early development. At birth, there was increased expression of fatty acid transporters and enzymes of fatty acid metabolism that allow fatty acids to become the major source of energy for cardiac muscle during the first 2 wk of life. At the same time, expression of genes that control glucose transport and oxidation was downregulated. After 2 wk, expression of genes for glucose uptake and oxidation was increased, and expression of genes for fatty acid uptake and utilization was decreased. Expression of carnitine palmitoyltransferase I (CPT I) isoforms during development was different from published data obtained from rabbit hearts. CPT Iα and Iβ isoforms were both highly expressed in hearts before birth, and both increased further at birth. Only after the second week did CPT Iα expression decrease appreciably below the level of CPT Iβ expression. These results represent another example of different expression patterns of CPT I isoforms among various mammalian species. In rats, changes in gene expression followed nutrient availability during development and may render cardiac fatty acid oxidation less sensitive to factors that influence malonyl-CoA content (e.g., fluctuations in glucose concentration) and thereby favor fatty acid oxidation as an energy source for cardiomyocytes in early development.

Expression and purification of His-tagged rat mitochondrial medium-chain acyl-CoA dehydrogenase wild-type and Arg256 mutant proteins

Mitochondrial medium-chain acyl-CoA dehydrogenase is a key enzyme for the beta-oxidation of fatty acids, and the deficiency of this enzyme in patient has been previously reported. We cloned the gene of rat mitochondrial medium-chain acyl-CoA dehydrogenase into a bacterial expression vector pLM1 with six continuous histidine codons attached to the 3' of the gene. The cloned gene was overexpressed in Escherichia coli and the soluble protein was purified with a nickel Hi-Trap chelating metal affinity column in 88% yield to apparent homogeneity. The specific activity of the purified His-tagged rat mitochondrial medium-chain acyl-CoA dehydrogenase was 4.0 U/mg. Arg256 is a highly conserved amino acid, which may play an important role in enzymatic reaction based on the crystal structure of medium-chain acyl-CoA dehydrogenase. We constructed four mutant expression plasmids of the enzyme using site-directed mutagenesis. Mutant proteins were overexpressed in E. coli and purified with a nickel metal affinity column. Kinetic studies of wild-type and mutant proteins were carried out, and the result confirmed that Arg256 is a very important residue of rat mitochondrial medium-chain acyl-CoA dehydrogenase. Our overexpression in E. coli and one-step purification of the highly active rat mitochondrial medium-chain acyl-CoA dehydrogenase greatly facilitated our further investigation of this enzyme, and our result from site-directed mutagenesis increased our understanding of medium-chain acyl-CoA dehydrogenase.

The peroxisome proliferator-activated receptor alpha-selective activator ciprofibrate upregulates expression of genes encoding fatty acid oxidation and ketogenesis enzymes in rat brain

Activated peroxisome proliferator activated receptor alpha (PPAR alpha) protects against the cellular inflammatory response, and is central to fatty acid-mediated upregulation of the gene encoding the key ketogenic enzyme mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHS). We have previously demonstrated both PPAR alpha and mHS expression in brain, implying that brain-targeted PPAR alpha activators may likewise up-regulate mHS expression in brain. Thus, to attempt pharmacological activation of brain PPAR alpha in vivo, we have administered to rats two drugs with previously defined actions in rat brain, namely the PPAR alpha-selective activator ciprofibrate and the pan-PPAR activator valproate. Using the sensitive and discriminatory RNase protection co-assay, we demonstrate that both ciprofibrate and valproate induce mHS expression in liver, the archetypal PPAR alpha-expressing organ. Furthermore, ciprofibrate potently increases mHS mRNA abundance in rat brain, together with lesser increases in two other PPAR alpha-regulated mRNAs. Thus we demonstrate, for the first time, up-regulation of expression of PPAR alpha-dependent genes including mHS in brain, with implications in the increased elimination of neuro-inflammatory lipids and concomitant increased production of neuro-protective ketone bodies.

The ketogenic diet upregulates expression of the gene encoding the key ketogenic enzyme mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase in rat brain

The ketogenic diet is a clinically and experimentally effective anti-epileptic treatment whose molecular mechanism(s) of action remain to be elucidated. As a first step in defining its effects on regulation of fatty acid oxidation and ketogenesis at the genetic level, we have administered to rats: (1) a calorie-restricted ketogenic diet (KCR); (2) a calorie-restricted normal diet (NCR); or (3) a normal diet ad libitum (NAL). We have used RNase protection to co-assay diet-induced changes in abundance of the mRNA encoding the critical enzyme of ketogenesis from acetyl-CoA namely mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHS) in liver and brain, together with mRNAs encoding three other key enzymes of fatty acid oxidation. We demonstrate that NCR-fed rats exhibit a significant 2-fold increase in liver mHS mRNA compared to NAL-fed rats, and that KCR-fed rats exhibit a significant 2-fold increase in both liver and brain mHS mRNA compared to NAL-fed rats. Our results demonstrate, for the first time, the effect of a ketogenic diet on gene expression in brain, and suggest possible anti-epileptic mechanisms for future investigation.

Identification of Caenorhabditis elegans isovaleryl-CoA dehydrogenase and structural comparison with other acyl-CoA dehydrogenases

Isovaleryl-CoA dehydrogenase (IVD) is a flavoenzyme, which catalyzes the conversion of isovaleryl-CoA to 3-methylcrotonyl-CoA in the leucine catabolism pathway and transfers electrons to the electron-transferring flavoprotein (ETF). IVDs from human and rat have been identified and characterized previously. In this study, the gene coding for Caenorhabditis elegans IVD has been identified from a published cDNA sequence and molecular modeling has been performed using the human IVD atomic coordinates. The coding sequence for the mature form of the enzyme was expressed in Escherichia coli, and the recombinant nematode IVD enzyme was purified to essential homogeneity. Its spectrum is typical of recombinant FAD-containing acyl-CoA dehydrogenases and shows a minor broad absorption band at 650-700 nm characteristic of an IVD:CoA persulfide charge-transfer complex. Following treatment of the enzyme with sodium dithionite to remove the bound CoA persulfide, the K(m) values for isovaleryl-, butyryl-, valeryl-, and hexanoyl-CoA were estimated to be 2.5, 36.2, 10.5, and 33.8 microM, respectively, using the ETF fluorescence reduction assay. The catalytic efficiency (k(cat)/K(m)) for these substrates was 56.9, 1.3, 13.7, and 3.2 microM(-1). min(-1) per mole of FAD, respectively. The apparent binding constant (K(D app)) of the recombinant IVD determined spectrally for isovaleryl-CoA was 0.34 microM. These kinetic parameters confirm that isovaleryl-CoA is the preferred substrate for the purified enzyme. The variability in the protein structure among known and putative IVDs from various species is discussed in the context of possible mechanisms for modulating enzyme activity.

Cloning of genomic and cDNA for mouse isovaleryl-CoA dehydrogenase (IVD) and evolutionary comparison to other known IVDs

Isovaleryl-CoA dehydrogenase (IVD) is an intramitochondrial homotetrameric flavoenzyme that catalyzes the conversion of isovaleryl-CoA to 3-methylcrotonyl-CoA in the leucine catabolism pathway. Deficiency of IVD in humans causes isovaleric acidemia, which shows tremendous clinical variability for reasons that are unknown. To help better understand this disorder, we have cloned and sequenced the mouse IVD genomic and cDNAs. The mouse IVD gene spans approximately 17 kb and contains 12 coding exons organized identically to the human gene. It maps to mouse chromosome 2 in the area of band 2E4-E5, corresponding to the syntenic region of human chromosome 15. Mouse IVD predicted amino acid sequences are 95.8 and 89.6% identical to that of the rat and human sequences, respectively, with conservation of key functional residues. We have now identified IVD sequences from seven species. Comparison of these sequences shows that the rat and mouse proteins are the most closely related, both of which, in turn, share highest homology to human. All of the mammalian enzymes appear to be more closely related than any of the IVDs on other branches of the phylogram, while the fly and worm IVDs are the most divergent. The invertebrate IVDs are more closely related to the mammalian enzymes than to those from two plant species.

Sesamin, a sesame lignan, is a potent inducer of hepatic fatty acid oxidation in the rat

The effects of sesamin, one of the most abundant lignans in sesame seed, on hepatic fatty acid oxidation were examined in rats that were fed experimental diets containing various amounts (0%, 0.1%, 0.2%, and 0.5%) of sesamin (a 1:1 mixture of sesamin and episesamin) for 15 days. Dietary sesamin dose-dependently increased both mitochondrial and peroxisomal palmitoyl-coenzyme A (CoA) oxidation rates. Mitochondrial activity almost doubled in rats on the 0.5% sesamin diet. Peroxisomal activity increased more than 10-fold in rats fed a 0.5% sesamin diet in relation to rats on the sesamin-free diet. Dietary sesamin greatly increased the hepatic activity of fatty acid oxidation enzymes, including carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increased the activity of 2,4-dienoyl-CoA reductase and delta3,delta2-enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for beta-oxidation of unsaturated fatty acids dose-dependently. Examination of hepatic mRNA levels using specific cDNA probes showed a sesamin-induced increase in the gene expression of mitochondrial and peroxisomal fatty acid oxidation enzymes. Among these various enzymes, peroxisomal acyl-CoA oxidase and bifunctional enzyme gene expression were affected most by dietary sesamin (15- and 50-fold increase by the 0.5% dietary level). Sesamin-induced alterations in the activity and gene expression of carnitine palmitoyltransferase I and acyl-CoA oxidase were in parallel with changes in the mitochondrial and peroxisomal palmitoyl-CoA oxidation rate, respectively. In contrast, dietary sesamin decreased the hepatic activity and mRNA abundance of fatty acid synthase and pyruvate kinase, the lipogenic enzymes. However, this lignan increased the activity and gene expression of malic enzyme, another lipogenic enzyme. An alteration in hepatic fatty acid metabolism may therefore account for the serum lipid-lowering effect of sesamin in the rat.

The roles of threonine-136 and glutamate-137 of human medium chain acyl-CoA dehydrogenase in FAD binding and peptide folding using site-directed mutagenesis: creation of an FAD-dependent mutant, T136D

We studied the roles of Thr-136 (T136) and Glu-137 (E137) in the biogenesis of medium chain acyl-CoA dehydrogenase (MCAD) by altering the former to Ser (T136S), Asp (T136D), or Leu (T136L) and the latter to Asp (E137D), Gln (E137Q), or Lys (E137K). After import into mitochondria, T136S and E137D were assembled into the native tetramer as efficiently as the wild-type. The tetrameric assembly of four other variants with a nonconservative substitution was severely impaired. When expressed in Escherichia coli as the mature subunit, the amounts of the catalytically active forms of T136S and E137D were comparable to wild-type, whereas four nonconservative variants were lost as aggregates. Of these nonconservative variants, only T136D formed catalytically active tetramer when the culture broth and buffers were supplemented with riboflavin and FAD, respectively. Culturing T136L or E137K at a lower temperature (28 degreesC) did not increase the yield at all, suggesting the severity of disruption of biogenesis. These results, together with the previous crystallographic findings, indicate that the T136 hydroxyl is a major FAD-binding site, and that E137 carboxyl plays a key role in the beta-domain folding, through salt bridge formation with K164. These findings also support the notion that the isoalloxazine ring plays a critical role in the MCAD folding, presumably exerting nucleating effects.

Comparative effects of alpha- and gamma-linolenic acids on rat liver fatty acid oxidation

It has been reported that both n-3 and n-6 octadecatrienoic acids can increase hepatic fatty acid oxidation activity. It remains unclear, however, whether different enzymes in fatty acid oxidation show a similar response to n-3 and n-6 octadecatrienoic acids. The activity of hepatic fatty acid oxidation enzymes in rats fed an oil mixture rich in alpha-linolenic acid (18:3n-3) and borage oil rich in gamma-linolenic acid (18:3n-6) was therefore compared to that in rats fed an oil mixture rich in linoleic acid (18:2n-6) and a saturated fat (palm oil) in this study. Linseed oil served as the source of 18:3n-3 for the oil mixture rich in this octadecatrienoic acid and contained 30.6% 18:3n-3 but not 18:3n-6. Borage oil contained 25.7% 18:3n-6 and 4.5% 18:3n-3. Groups of seven rats each were fed diets containing 15% various fats for 15 d. The oxidation rate of palmitoyl-CoA in the peroxisomes was higher in rats fed a fat mixture rich in 18:3n-3 (3.03 nmol/min/mg protein) and borage oil (2.89 nmol/min/mg protein) than in rats fed palm oil (2.08 nmol/min/mg protein) and a fat mixture rich in 18:2n-6 (2.15 nmol/min/mg protein). The mitochondrial palmitoyl-CoA oxidation rate was highest in rats fed a fat mixture rich in 18:3n-3 (1.93 nmol/min/mg protein), but no significant differences in this parameter were seen among the other groups (1.25-1.46 nmol/min/mg protein). Compared to palm oil and fat mixtures rich in 18:2n-6, a fat mixture rich in 18:3n-3 and borage oil significantly increased the hepatic activity of carnitine palmitoyltransferase and acyl-CoA oxidase. Compared to palm oil and a fat mixture rich in 18:2n-6, a fat mixture rich in 18:3n-3, but not fats rich in 18:3n-6, significantly decreased 3-hydroxyacyl-CoA dehydrogenase activity. Compared to palm oil and a fat mixture rich in 18:2n-6, borage oil profoundly decreased mitochondrial acyl-CoA dehydrogenase activity, but a fat mixture rich in 18:3n-3 increased it. 2,4-Dienoyl-CoA reductase activity was significantly lower in rats fed palm oil than in other groups. Compared to other fats, borage oil significantly increased delt3,delta2-enoyl-CoA isomerase activity. Activity was also significantly higher in rats fed 18:2n-6 oil than in those fed palm oil. It was confirmed that both dietary 18:3n-6 and 18:3n-3 increased fatty acid oxidation activity in the liver. These two dietary octadecatrienoic acids differ considerably, however, in how they affect individual fatty acid oxidation enzymes.

Identification of the catalytic residue of human short/branched chain acyl-CoA dehydrogenase by in vitro mutagenesis

The acyl-CoA dehydrogenases (ACDs) are a family of related enzymes which catalyze the alpha,beta-dehydrogenation of acyl-CoA esters, transferring electrons to electron transferring flavoprotein. We have recently cloned and characterized the cDNA for human short/branched chain acyl-CoA dehydrogenase (SBCAD). Based on homology with the other ACDs we hypothesized that E381 is the catalytic residue for this enzyme. Alteration of this amino acid to glutamine, glycine or arginine resulted in an inactive enzyme. Substitution of aspartate at this position led to an enzyme with reduced activity compared to the wild type. An E381G/G260E double mutation (which places a glutamate in a position homologous to the catalytic residue identified in other members of this gene family) restored enzyme activity. These data confirm the crucial nature of E381 to the activity of this enzyme and strongly support its role as the alpha-proton abstracting base.

The molecular basis of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency in compound heterozygous patients: is there correlation between genotype and phenotype?

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most commonly recognized defect of mitochondrial beta-oxidation. It is potentially fatal, but shows a wide clinical spectrum. The aim of the present study was to investigate whether any correlation exists between MCAD genotype and disease phenotype. We determined the prevalence of the 14 known and seven previously unknown non-G985 mutations in 52 families with MCAD deficiency not caused by homozygosity for the prevalent G985 mutation. This showed that none of the non-G985 mutations are prevalent, and led to the identification of both disease-causing mutations in 14 families in whom both mutations had not previously been reported. We then evaluated the severity of the mutations identified in these 14 families. Using expression of mutant MCAD in Escherichia coli with or without co-overexpression of the molecular chaperonins GroESL we showed that five of the missense mutations affect the folding and/or stability of the protein, and that the residual enzyme activity of some of them could be modulated to a different extent depending on the amounts of available chaperonins. Thus, some of the missense mutations may result in relatively high levels of residual enzyme activity, whereas the mutations leading to premature stop codons will result in no residual enzyme activity. By correlating the observed types of mutations identified to the clinical/biochemical data in the 14 patients in whom we identified both disease-causing mutations, we show that a genotype/phenotype correlation in MCAD deficiency is not straightforward. Different mutations may contribute with different susceptibilities for disease precipitation, when the patient is subjected to metabolic stress, but other genetic and environmental factors may play an equally important role.

Cloning, sequencing, and expression of clustered genes encoding beta-hydroxybutyryl-coenzyme A (CoA) dehydrogenase, crotonase, and butyryl-CoA dehydrogenase from Clostridium acetobutylicum ATCC 824

The enzymes beta-hydroxybutyryl-coenzyme A (CoA) dehydrogenase (BHBD), crotonase, and butyryl-CoA dehydrogenase (BCD) from Clostridium acetobutylicum are responsible for the formation of butyryl-CoA from acetoacetyl-CoA. These enzymes are essential to both acid formation and solvent formation by clostridia. Clustered genes encoding BHBD, crotonase, BCD, and putative electron transfer flavoprotein alpha and beta subunits have been cloned and sequenced. The nucleotide sequence of the crt gene indicates that it encodes crotonase, a protein with 261 amino acid residues and a calculated molecular mass of 28.2 kDa; the hbd gene encodes BHBD, with 282 residues and a molecular mass of 30.5 kDa. Three open reading frames (bcd, etfB, and etfA) are located between crt and hbd. The nucleotide sequence of bcd indicates that it encodes BCD, which consists of 379 amino acid residues and has high levels of homology with various acyl-CoA dehydrogenases. Open reading frames etfB and etfA, located downstream of bcd, encode 27.2- and 36.1-kDa proteins, respectively, and show homology with the fixAB genes and the alpha and beta subunits of the electron transfer flavoprotein. These findings suggest that BCD in clostridia might interact with the electron transfer flavoprotein in its redox function. Primer extension analysis identified a promoter consensus sequence upstream of the crt gene, suggesting that the clustered genes are transcribed as a transcriptional unit and form a BCS (butyryl-CoA synthesis) operon. A DNA fragment containing the entire BCS operon was subcloned into an Escherichia coli-C. acetobutylicum shuttle vector. Enzyme activity assays showed that crotonase and BHBD were highly overproduced in cell extracts from E. coli harboring the subclone. In C. acetobutylicum harboring the subclone, the activities of the enzymes crotonase, BHBD, and BCD were elevated.

Expression in Escherichia coli and characterization of a bile acid-inducible 3 alpha-hydroxysteroid dehydrogenase from Eubacterium sp. strain VPI 12708

We have previously cloned and sequenced three members of a bile acid-inducible gene family from Eubacterium sp. strain VPI 12708 that encode 27,000-M(r) polypeptides. Two copies of these genes (baiA1 and baiA3) are identical, while the third copy (baiA2) encodes a polypeptide sharing 92% amino acid identity with the baiA1 and baiA3 gene products. We have overexpressed the baiA1 gene in Escherichia coli and analyzed the expressed activity. Thin-layer chromatography of 14C-labeled bile acid products from reactions using cell-free extracts revealed a 3 alpha-hydroxysteroid dehydrogenase activity for the BaiA1 protein. The BaiA1 protein could utilize both NAD+ and NADP+, and the preferred steroid substrate was the cholyl-coenzyme A conjugate rather than free cholic acid. These results show that the BaiA proteins are novel 3 alpha-hydroxysteroid dehydrogenases.

Effects of two mutations detected in medium chain acyl-CoA dehydrogenase (MCAD)-deficient patients on folding, oligomer assembly, and stability of MCAD enzyme

We have used expression of human medium chain acyl-CoA dehydrogenase (MCAD) in Escherichia coli as a model system for dissecting the molecular effects of two mutations detected in patients with MCAD deficiency. We demonstrate that the R28C mutation predominantly affects polypeptide folding. The amounts of active R28C mutant enzyme produced could be modulated between undetectable to 100% of the wild-type control by manipulating the level of available chaperonins and the growth temperature. For the prevalent K304E mutation, however, the amounts of active mutant enzyme could be modulated only in a range from undetectable to approximately 50% of the wild-type, and the assembled mutant enzyme displayed a decreased thermal stability. Two artificially constructed mutants (K304Q and K304E/D346K) yielded clearly higher amounts of active MCAD enzyme than the K304E mutant but were also responsive to chaperonin co-overexpression and growth at low temperature. The thermal stability profile of the K304E/D346K double mutant was shifted to even lower temperatures than that of the K304E mutant, whereas that of the K304Q mutant was closely similar to the wild-type. Taken together, the results show that the K304E mutation affects (i) polypeptide folding due to elimination of the positively charged lysine and (ii) oligomer assembly and stability due to replacement of lysine 304 with the negatively charged glutamic acid.

Isoalloxazine ring of FAD is required for the formation of the core in the Hsp60-assisted folding of medium chain acyl-CoA dehydrogenase subunit into the assembly competent conformation in mitochondria

We studied the role of FAD in the intramitochondrial folding and assembly of medium-chain acyl-CoA dehydrogenase (MCAD), a homotetrameric mitochondrial enzyme containing a molecule of non-covalently bound FAD/monomer. In the MCAD molecule, FAD is buried in a crevice containing the active center. We have previously shown that upon import into mitochondria, newly processed MCAD is first incorporated into a high molecular weight (hMr) complex and that the hMr complex mainly consisted of MCAD-heat-shock protein 60 (hsp60) complex (Saijo, T., Welch, W.J., and Tanaka, K (1994) J. Biol. Chem. 269, 4401-4408). In the present study, we incubated in vitro synthesized precursor MCAD with mitochondria isolated from normal and riboflavin-deficient rat liver for 10-60 min and fractionated the solubilized mitochondria using gel filtration. The amount of MCAD in the hMr complex was larger and that of tetramer was smaller in riboflavin-deficient mitochondria than in control at any time point. In addition, riboflavin-deficient mitochondria were solubilized after 10-min import in a buffer containing ATP and were chased in the presence of FAD, FMN, or NAD+ or without any addition. The mitochondrial proteins were analyzed using gel filtration or immunoprecipitated with anti-hsp60 antibody. After 60-min chase in the presence of FAD, the majority of MCAD in the complex with hsp60 was transferred to tetramer, whereas no such transfer occurred after the chase in the absence of FAD. When chase was done in the presence of FMN, a significant amount of MCAD was transferred from the complex with hsp60 to tetramer, but the transfer was not as efficient as in the presence of FAD. The chase in the presence of NAD+ resulted in no transfer. These data suggest that isoalloxazine ring of FAD plays a critical role, exerting nucleating effect, in the hsp60-assisted folding of MCAD subunit into an assembly competent conformation, probably assisting the formation of the core.

Molecular characterization of the cai operon necessary for carnitine metabolism in Escherichia coli

The sequence encompassing the cai genes of Escherichia coli, which encode the carnitine pathway, has been determined. Apart from the already identified caiB gene coding for the carnitine dehydratase, five additional open reading frames were identified. They belong to the caiTABCDE operon, which was shown to be located at the first minute on the chromosome and transcribed during anaerobic growth in the presence of carnitine. The activity of carnitine dehydratase was dependent on the CRP regulatory protein and strongly enhanced in the absence of a functional H-NS protein, in relation to the consensus sequences detected in the promoter region of the cai operon. In vivo expression studies led to the synthesis of five polypeptides in addition to CaiB, with predicted molecular masses of 56,613 Da (CaiT), 42,564 Da (CaiA), 59,311 Da (CaiC), 32,329 Da (CaiD) and 21,930 Da (CaiE). Amino acid sequence similarity or enzymatic analysis supported the function assigned to each protein. CaiT was suggested to be the transport system for carnitine or betaines, CaiA an oxidoreduction enzyme, and CaiC a crotonobetaine/carnitine CoA ligase. CaiD bears strong homology with enoyl hydratases/isomerases. Overproduction of CaiE was shown to stimulate the carnitine racemase activity of the CaiD protein and to markedly increase the basal level of carnitine dehydratase activity. It is inferred that CaiE is an enzyme involved in the synthesis or the activation of the still unknown cofactor required for carnitine dehydratase and carnitine racemase activities. Taken together, these data suggest that the carnitine pathway in E. coli resembles that found in a strain situated between Agrobacterium and Rhizobium.

Medium-chain acyl-CoA dehydrogenase deficiency: molecular aspects

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is an autosomal recessive disorder which is known to cause Reye-like syndrome in children and sudden infant death. A point mutation of lysine329-to-glutamic acid329 substitution in the MCAD gene was recently identified as the most common mutation in patients with MCAD deficiency. This mutation is responsible for about 90% of mutant MCAD alleles in Caucasians. Patients with this type of mutation have a variety of symptoms, indicating that the clinical heterogeneity of MCAD deficiency may not be caused entirely by genetic heterogeneity. Screening for the mutation among newborns in England, Australia, and United States of America indicates the prevalence of carriers to be 1 in 40-107, suggesting the high incidence of the mutation. Since presymptomatic diagnosis and appropriate dietary management are important in MCAD deficiency to prevent life-threatening complications, the relatively high incidence of this disorder may warrant population screening. The most common MCAD mutation can now be detected by DNA diagnostic methods using Guthrie cards. This makes it possible to screen a population efficiently for this potentially fatal disorder.

Mutations in the medium chain acyl-CoA dehydrogenase (MCAD) gene

Medium chain acyl-CoA dehydrogenase (MCAD) catalyzes the first reaction of the beta-oxidation cycle for 4-10-carbon fatty acids. MCAD deficiency is one of the most frequent inborn metabolic disorders in populations of northwestern European origin. In the compilation of data from a worldwide study of 172 unrelated patients each representing an independent pedigree, a total of 8 different mutations have been identified. Among them, a single prevalent mutation, 985A-->G, was found in 90% of 344 variant alleles. 985A-->G causes glutamate substitution for lysine-304 in the mature MCAD subunit, which causes impairment of tetramer assembly and instability of the protein. Three of 7 rarer mutations have been identified in a few unrelated patients, while the remaining 4 have each been found in only a single pedigree. In addition to tabulating the mutations, the acyl-CoA dehydrogenase gene family, the structure of the MCAD gene and the evolution of 985A-->G mutation are briefly discussed.

Molecular cloning and nucleotide sequence of cDNAs encoding human long-chain acyl-CoA dehydrogenase and assignment of the location of its gene (ACADL) to chromosome 2

Long-chain acyl-CoA dehydrogenase (LCAD) catalyzes the first reaction of the mitochondrial beta-oxidation of fatty acids. We isolated and sequenced three cDNA clones encoding human LCAD precursor (p). The cDNAs encompass a 2217-base region including 5, 1290, and 922 bases in the 5'-noncoding, coding, and 3'-noncoding regions, respectively, and encodes the entire pLCAD of 430 amino acids (Mr: 47,656). The N-terminus of the mature human LCAD is currently unknown, but 30 (Mr 3221) and 400 amino acids (Mr: 44,435) of the sequence are considered to constitute the leader peptide and mature protein, respectively, in analogy to its rat counterpart. Human pLCAD cDNA shares 85.3 and 83.7% identical residues with rat pLCAD cDNA at the amino acid and nucleotide levels, respectively. At the amino acid level, human pLCAD shares 30.4 to 32.7% identical residues with three other human enzymes in the acyl-CoA dehydrogenase family, sharing 57 perfectly conserved residues among them. The human pLCAD gene is assigned to chromosome 2, bands q34-q35.

Structurally different rat liver medium-chain acyl CoA dehydrogenases directed by complementary DNAs differing in their 5'-region

Different forms of rat liver medium-chain acyl CoA dehydrogenase (MCAD) (EC 1.3.99.3) were produced in Escherichia coli carrying expression plasmids (pRMCADm-1 approximately 9) differing at the 5'-region of the cDNA. The proteins expressed could be readily extracted from the cells. The protein (approximately 44 kDa) directed by pRMCADm-3 showed the highest activity and was readily purified to homogeneity. The purified enzyme contained non-covalently bound FAD and was similar to rat liver mitochondrial enzyme in all respects examined. The purified protein (approximately 45 kDa) directed by pRMCADm-1 did not contain FAD and showed no enzymatic activity. Therefore, the leader peptide disturbs the binding of FAD to the apoprotein. The purified protein (approximately 40 kDa) directed by pRMCADm-6 did not contain FAD. Thus, the deletion of the NH2-terminal portion of the apoprotein to some extent results in its inability to combine with FAD.

Spectral properties of 3-ketosteroid-delta 1-dehydrogenase from Nocardia corallina

3-Ketosteroid-delta 1-dehydrogenase from Nocardia corallina is a flavoenzyme that catalyzes 1,2-desaturation of 3-ketosteroid. The dehydrogenase generated complexes with 3-ketosteroids and phenolic steroids such as estradiol with remarkable perturbations of the visible spectrum. The enzyme did not make the adduct with sulfite ion, but could use molecular oxygen as the electron acceptor. The CD spectra of oxidized and steroid-bound enzymes exhibited positive dichroisms in the visible region which resembled those of flavoenzyme oxidases. The dehydrogenase led isosbestically to the stable red semiquinone species with large yields upon photochemical or dithionite reduction (at pH 7.4) in the presence of the steroid product, 1,4-androstadiene-3,17-dione, but in the absence of the steroid the yield of semiquinone was low and the fully reduced enzyme was obtained. Substrate titration also yielded the red flavo-semiquinone stoichiometrically and it was hard to generate the fully reduced form. The reduced enzyme was oxidized with molecular oxygen, but did not oxidize with ferricyanide. An EPR study of these half-reduced forms confirmed the presence of the radical species with the g = 2.004 signal. The dehydrogenase was rapidly reduced with an excess amount of 3-ketosteroid at about 80% yield at pH 7.4 under anaerobic conditions and the reduced species was altered to the stable red semiquinone species. The rate of this reaction was t1/2 = 28 min at pH 7.4, 130 min at pH 9.0 and 34 min at pH 6.4, respectively. These results indicate that the semiquinone species does not act directly in turnover of the dehydrogenase reaction. The results were compared with the spectral properties of general acyl-CoA dehydrogenases and acyl-CoA oxidase toward the mechanism of C1,2-dehydrogenation.

Identification of a common mutation in patients with medium-chain acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is one of the most common recessively inherited metabolic diseases in man. We have studied fibroblast cultures obtained from three patients with MCAD deficiency by sequencing the entire coding region of MCAD mRNA. A single A to G nucleotide replacement which resulted in lysine329-to-glutamic acid329 substitution of the MCAD protein was identified in all cultures. Furthermore, this point mutation was present in 91% (31 of 34) of mutant MCAD alleles, indicating that the majority of cases with MCAD deficiency are caused by this type of mutation.

The locus for the medium-chain acyl-CoA dehydrogenase gene on chromosome 1 is highly polymorphic

The gene for medium-chain acyl-CoA dehydrogenase (gene symbol ACADM; enzyme symbol MCAD) has been characterized for restriction fragment length polymorphisms (RFLPs) and mapped by linkage analysis to 4.2 cM from D1S2 and 11.7 cM from PGM1. The three RFLP systems described in detail show significant linkage disequilibrium but define four haplotypes with a PIC of 0.58. This makes ACADM informative for linkage mapping and for clinical genetic studies. By linkage studies, the orientation of these three loci relative to the centromere places ACADM most proximal. This is in direct conflict with the regional assignments of ACADM to 1p31 by in situ hybridization and of PGM1 to 1p22.1 by somatic cell studies. We suggest that this somatic cell localization of PGM1 may be incorrect.

Inactivation of two-electron reduced medium chain acyl-CoA dehydrogenase by 2-octynoyl-CoA

The acetylenic thioester, 2-octynoyl-CoA, inactivates medium chain acyl-CoA dehydrogenase from pig kidney by two distinct pathways depending on the redox state of the FAD prosthetic group. Inactivation of the oxidized dehydrogenase occurs with labeling of an active site glutamate residue and elimination of CoASH. Incubation of the reduced dehydrogenase with 2-octynoyl-CoA rapidly forms a kinetically stable dihydroflavin species which is resistant to reoxidation using trans-2-octenoyl-CoA, molecular oxygen, or electron transferring flavoprotein. The reduced enzyme derivative shows extensive bleaching at 446 nm with shoulders at 320 and 380 nm. Denaturation of the reduced derivative in 80% methanol yields a mixture of products which was characterized by HPLC, by uv/vis, and by radiolabeling experiments. Approximately 20% of the flavin is recovered as oxidized FAD, about 40% is retained covalently attached to the protein, and the remainder is distributed between several species eluting after FAD on reverse-phase HPLC. The spectrum of one of these species ressembles that of a N(5)-C(4a) dihydroflavin adduct. These data suggest that a primary reduced flavin species undergoes various rearrangements during release from the protein. The possibility that the inactive modified enzyme represents a covalent adduct between 2-octynoyl-CoA and reduced flavin is discussed. Analogous experiments using enzyme substituted with 1,5-dihydro-5-deaza-FAD show rapid and quantitative reoxidation of the flavin by 0.5 eq of 2-octynoyl-CoA.

Mitochondrial protein import

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

2-octynoyl coenzyme A is a mechanism-based inhibitor of pig kidney medium-chain acyl coenzyme A dehydrogenase: isolation of the target peptide

Pig kidney medium-chain acyl-CoA dehydrogenase (EC 1.3.99.3) is irreversibly and stoichiometrically inactivated by [1-14C]-2-octynoyl coenzyme A. The linkage is stable at pH 2-6, but labile under basic conditions. The inhibitor labels a unique tryptic peptide, Ile-Tyr-Gln-Ile-Tyr-Glu-Gly-Thr-Ala-Gln-Ile-Gln-Arg, close to the C-terminus of the protein. The peptide is labeled at Glu-401 with the acyl moiety of the inhibitor but does not contain detectable coenzyme A. Both the inactivation of the dehydrogenase and the appearance of an absorption band at 800 nm show large primary deuterium isotope effects using 4,4'-dideuterio-2-octynoyl-CoA (7.3 and 6.3, respectively). Thus, 2-octynoyl-CoA is a mechanism-based inactivator of the dehydrogenase and is activated by rate-limiting gamma-proton abstraction. Glutamate-401 may be the base that abstracts the pro-R alpha-proton during the dehydrogenation of normal substrates.

Isolation of cDNA clones coding for rat isovaleryl-CoA dehydrogenase and assignment of the gene to human chromosome 15

Rat liver mRNA encoding the cytoplasmic precursor of mitochondrial isovaleryl-CoA dehydrogenase was highly enriched by polysome immunopurification using a polyclonal monospecific antibody. The purified mRNA was used to prepare a plasmid cDNA library which was screened with two oligonucleotide mixtures encoding two peptides in the amino-terminal portion of mature rat isovaleryl-CoA dehydrogenase. Thirty-one overlapping cDNA clones, spanning a region of 2.1 kbp, were isolated and characterized. The cDNA sequence of a 5'-end clone, rIVD-13 (155 bp), predicts a mitochondrial leader peptide of 30 amino acid residues and the first 18 amino acids of the mature protein. These consecutive 18 residues completely matched the amino-terminal peptide determined by automated Edman degradation of the rat enzyme. The leader peptide contains six arginines, has no acidic residues, and is particularly rich in leucine, alanine, and proline residues. Southern blot analysis of DNAs from human-rodent somatic cell hybrids with an isolated rat cDNA (2 kbp) assigned the isovaleryl-CoA dehydrogenase gene to the long arm of chromosome 15, region q14----qter. The chromosomal assignment was confirmed and further refined to bands q14----q15 by in situ hybridization of the probe to human metaphase cells. This location differs from that of the gene for medium-chain acyl-CoA dehydrogenase, a closely related enzyme, which has been previously assigned to chromosome 1.