3-Hydroxy-3-methylglutaryl-CoA lyase is present in mouse and human liver peroxisomes

Identification and Validation of a PEX5-Dependent Signature for Prognostic Prediction in Glioma

Gliomas, the most prevalent and lethal form of brain cancer, are known to exhibit metabolic alterations that facilitate tumor growth, invasion, and resistance to therapies. Peroxisomes, essential organelles responsible for fatty acid oxidation and reactive oxygen species (ROS) homeostasis, rely on the receptor PEX5 for the import of metabolic enzymes into their matrix. However, the prognostic significance of peroxisomal enzymes for glioma patients remains unclear. In this study, we elucidate that PEX5 is indispensable for the cell growth, migration, and invasion of glioma cells. We establish a robust prognosis model based on the expression of peroxisomal enzymes, whose localization relies on PEX5. This PEX5-dependent signature not only serves as a robust prognosis model capable of accurately predicting outcomes for glioma patients, but also effectively distinguishes several clinicopathological features, including the grade, isocitrate dehydrogenase (IDH) mutation, and 1p19q codeletion status. Furthermore, we developed a nomogram that integrates the prognostic model with other clinicopathological factors, demonstrating highly accurate performance in estimating patient survival. Patients classified into the high-risk group based on our prognostic model exhibited an immunosuppressive microenvironment. Finally, our validation reveals that the elevated expression of GSTK1, an antioxidant enzyme within the signature, promotes the cell growth and migration of glioma cells, with this effect dependent on the peroxisomal targeting signal recognized by PEX5. These findings identify the PEX5-dependent signature as a promising prognostic tool for gliomas.

Proteomic analysis of a hom-disrupted, cephamycin C overproducing Streptomyces clavuligerus

BACKGROUND

Streptomyces clavuligerus is prolific producer of cephamycin C, a medically important antibiotic. In our former study, cephamycin C titer was 2-fold improved by disrupting homoserine dehydrogenase (hom) gene of aspartate pahway in Streptomyces clavuligerus NRRL 3585.

OBJECTIVE

In this article, we aimed to provide a comprehensive understanding at the proteome level on potential complex metabolic changes as a consequence of hom disruption in Streptomyces clavuligerus AK39.

METHODS

A comparative proteomics study was carried out between the wild type and its hom disrupted AK39 strain by 2 Dimensional Electrophoresis-Matrix Assisted Laser Desorption and Ionization Time-Of-Flight Mass Spectrometry (2DE MALDI-TOF/MS) and Nanoscale Liquid Chromatography- Tandem Mass Spectrometry (nanoLC-MS/MS) analyses. Clusters of Orthologous Groups (COG) database was used to determine the functional categories of the proteins. The theoretical pI and Mw values of the proteins were calculated using Expasy pI/Mw tool.

RESULTS

"Hypothetical/Unknown" and "Secondary Metabolism" were the most prominent categories of the differentially expressed proteins. Upto 8.7-fold increased level of the positive regulator CcaR was a key finding since CcaR was shown to bind to cefF promoter thereby direcly controlling its expression. Consistently, CeaS2, the first enzyme of CA biosynthetic pathway, was 3.3- fold elevated. There were also many underrepresented proteins associated with the biosynthesis of several Non-Ribosomal Peptide Synthases (NRPSs), clavams, hybrid NRPS/Polyketide synthases (PKSs) and tunicamycin. The most conspicuously underrepresented protein of amino acid metabolism was 4-Hydroxyphenylpyruvate dioxygenase (HppD) acting in tyrosine catabolism. The levels of a Two Component System (TCS) response regulator containing a CheY-like receiver domain and an HTH DNA-binding domain as well as DNA-binding protein HU were elevated while a TetR-family transcriptional regulator was underexpressed.

CONCLUSION

The results obtained herein will aid in finding out new targets for further improvement of cephamycin C production in Streptomyces clavuligerus.

More Than One HMG-CoA Lyase: The Classical Mitochondrial Enzyme Plus the Peroxisomal and the Cytosolic Ones

There are three human enzymes with HMG-CoA lyase activity that are able to synthesize ketone bodies in different subcellular compartments. The mitochondrial HMG-CoA lyase was the first to be described, and catalyzes the cleavage of 3-hydroxy-3-methylglutaryl CoA to acetoacetate and acetyl-CoA, the common final step in ketogenesis and leucine catabolism. This protein is mainly expressed in the liver and its function is metabolic, since it produces ketone bodies as energetic fuels when glucose levels are low. Another isoform is encoded by the same gene for the mitochondrial HMG-CoA lyase (HMGCL), but it is located in peroxisomes. The last HMG-CoA lyase to be described is encoded by a different gene, HMGCLL1, and is located in the cytosolic side of the endoplasmic reticulum membrane. Some activity assays and tissue distribution of this enzyme have shown the brain and lung as key tissues for studying its function. Although the roles of the peroxisomal and cytosolic HMG-CoA lyases remain unknown, recent studies highlight the role of ketone bodies in metabolic remodeling, homeostasis, and signaling, providing new insights into the molecular and cellular function of these enzymes.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

A liver-specific defect of Acyl-CoA degradation produces hyperammonemia, hypoglycemia and a distinct hepatic Acyl-CoA pattern

Most conditions detected by expanded newborn screening result from deficiency of one of the enzymes that degrade acyl-coenzyme A (CoA) esters in mitochondria. The role of acyl-CoAs in the pathophysiology of these disorders is poorly understood, in part because CoA esters are intracellular and samples are not generally available from human patients. We created a mouse model of one such condition, deficiency of 3-hydroxy-3-methylglutaryl-CoA lyase (HL), in liver (HLLKO mice). HL catalyses a reaction of ketone body synthesis and of leucine degradation. Chronic HL deficiency and acute crises each produced distinct abnormal liver acyl-CoA patterns, which would not be predictable from levels of urine organic acids and plasma acylcarnitines. In HLLKO hepatocytes, ketogenesis was undetectable. Carboxylation of [2-(14)C] pyruvate diminished following incubation of HLLKO hepatocytes with the leucine metabolite 2-ketoisocaproate (KIC). HLLKO mice also had suppression of the normal hyperglycemic response to a systemic pyruvate load, a measure of gluconeogenesis. Hyperammonemia and hypoglycemia, cardinal features of many inborn errors of acyl-CoA metabolism, occurred spontaneously in some HLLKO mice and were inducible by administering KIC. KIC loading also increased levels of several leucine-related acyl-CoAs and reduced acetyl-CoA levels. Ultrastructurally, hepatocyte mitochondria of KIC-treated HLLKO mice show marked swelling. KIC-induced hyperammonemia improved following administration of carglumate (N-carbamyl-L-glutamic acid), which substitutes for the product of an acetyl-CoA-dependent reaction essential for urea cycle function, demonstrating an acyl-CoA-related mechanism for this complication.

Characterization of a novel HMG-CoA lyase enzyme with a dual location in endoplasmic reticulum and cytosol

A novel lyase activity enzyme is characterized for the first time: HMG-CoA lyase-like1 (er-cHL), which is a close homolog of mitochondrial HMG-CoA lyase (mHL). Initial data show that there are nine mature transcripts for the novel gene HMGCLL1, although none of them has all its exons. The most abundant transcript is called “variant b,” and it lacks exons 2 and 3. Moreover, a three-dimensional model of the novel enzyme is proposed. Colocalization studies show a dual location of the er-cHL in the endoplasmic reticulum (ER) and cytosol, but not in mitochondria or peroxisomes. Furthermore, the dissociation experiment suggests that it is a nonendoplasmic reticulum integral membrane protein. The kinetic parameters of er-cHL indicate that it has a lower V_max and a higher substrate affinity than mHL. Protein expression and lyase activity were found in several tissues, and were particularly strong in lung and kidney. The occurrence of er-cHL in brain is surprising, as mHL has not been found there. Although mHL activity is clearly associated with energy metabolism, the results suggest that er-cHL is more closely related to another metabolic function, mostly at the pulmonary and brain level.

Transfer of metabolites across the peroxisomal membrane

Peroxisomes perform a large variety of metabolic functions that require a constant flow of metabolites across the membranes of these organelles. Over the last few years it has become clear that the transport machinery of the peroxisomal membrane is a unique biological entity since it includes nonselective channels conducting small solutes side by side with transporters for 'bulky' solutes such as ATP. Electrophysiological experiments revealed several channel-forming activities in preparations of plant, mammalian, and yeast peroxisomes and in glycosomes of Trypanosoma brucei. The properties of the first discovered peroxisomal membrane channel - mammalian Pxmp2 protein - have also been characterized. The channels are apparently involved in the formation of peroxisomal shuttle systems and in the transmembrane transfer of various water-soluble metabolites including products of peroxisomal β-oxidation. These products are processed by a large set of peroxisomal enzymes including carnitine acyltransferases, enzymes involved in the synthesis of ketone bodies, thioesterases, and others. This review discusses recent data pertaining to solute permeability and metabolite transport systems in peroxisomal membranes and also addresses mechanisms responsible for the transfer of ATP and cofactors such as an ATP transporter and nudix hydrolases.

Role of peroxisomes in the biosynthesis and secretion of β-lactams and other secondary metabolites

Peroxisomes are eukaryotic organelles surrounded by a single bilayer membrane, containing a variety of proteins depending on the organism; they mainly perform degradation reactions of toxic metabolites (detoxification), catabolism of linear and branched-chain fatty acids, and removal of H(2)O(2) (formed in some oxidative processes) by catalase. Proteins named peroxins are involved in recruiting, transporting, and introducing the peroxisomal matrix proteins into the peroxisomes. The matrix proteins contain the peroxisomal targeting signals PTS1 and/or PTS2 that are recognized by the peroxins Pex5 and Pex7, respectively. Initial evidence indicated that the penicillin biosynthetic enzyme isopenicillin N acyltransferase (IAT) of Penicillium chrysogenum is located inside peroxisomes. There is now solid evidence (based on electron microscopy and/or biochemical data) confirming that IAT and the phenylacetic acid- and fatty acid-activating enzymes are also located in peroxisomes. Similarly, the Acremonium chrysogenum CefD1 and CefD2 proteins that perform the central reactions (activation and epimerization of isopenicillin N) of the cephalosporin pathway are targeted to peroxisomes. Growing evidence supports the conclusion that some enzymes involved in the biosynthesis of mycotoxins (e.g., AK-toxin), and the biosynthesis of signaling molecules in plants (e.g., jasmonic acid or auxins) occur in peroxisomes. The high concentration of substrates (in many cases toxic to the cytoplasm) and enzymes inside the peroxisomes allows efficient synthesis of metabolites with interesting biological or pharmacological activities. This compartmentalization poses additional challenges to the cell due to the need to import the substrates into the peroxisomes and to export the final products; the transporters involved in these processes are still very poorly known. This article focuses on new aspects of the metabolic processes occurring in peroxisomes, namely the degradation and detoxification processes that lead to the biosynthesis and secretion of secondary metabolites.

Differential HMG-CoA lyase expression in human tissues provides clues about 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a rare human autosomal recessive disorder caused by deficiency of 3-hydroxy-3-methylglutaryl CoA lyase (HL). This mitochondrial enzyme catalyzes the common final step of leucine degradation and ketogenesis. Acute symptoms include vomiting, seizures and lethargy, accompanied by metabolic acidosis and hypoketotic hypoglycaemia. Such organs as the liver, brain, pancreas, and heart can also be involved. However, the pathophysiology of this disease is only partially understood. We measured mRNA levels, protein expression and enzyme activity of human HMG-CoA lyase from liver, kidney, pancreas, testis, heart, skeletal muscle, and brain. Surprisingly, the pancreas is, after the liver, the tissue with most HL activity. However, in heart and adult brain, HL activity was not detected in the mitochondrial fraction. These findings contribute to our understanding of the enzyme function and the consequences of its deficiency and suggest the need for assessment of pancreatic damage in these patients.

Molecular analysis of Taiwanese patients with 3-hydroxy-3-methylglutaryl CoA lyase deficiency

BACKGROUND

3-Hydroxy-3-methylglutaryl CoA lyase deficiency (HL deficiency) is a rare autosomal recessive mitochondrial disease characterized by a deficiency in the enzyme 3-Hydroxy-3-methylglutaryl CoA lyase (HMGCL). Here, we report on novel mutations identified in the HMGCL gene in 2 Taiwanese patients with HL deficiency.

METHODS

Analysis of organic acids in urine was performed using gas chromatography-mass spectrometry to confirm HL deficiency in the two subjects. The mutations in their HMGCL genes then were determined by direct sequencing. In addition, the effect of a splice site mutation was determined using reverse transcription-polymerase chain reactions (RT-PCR).

RESULTS

A total of 3 novel mutations in the HMGCL gene were revealed by molecular analysis: one missense mutation (c.494G>T, p.Arg165Gln) and 2 splice site mutations (IVS3+1G>A, IVS6-1G>A). The results of RT-PCR revealed that an IVS3+1G>A mutation leads to skipping of exon3. We also calculated that the incidence of HL deficiency in Taiwan is <1 per 1,000,000 live births.

CONCLUSIONS

The results of this study suggest that unique HMGCL gene mutations exist in Taiwanese HL deficiency patients. Therefore, HMGCL gene profiling may be useful in genetic counseling for families affected by HL deficiency.

Age-related subproteomic analysis of mouse liver and kidney peroxisomes

BACKGROUND

Despite major recent advances in the understanding of peroxisomal functions and how peroxisomes arise, only scant information is available regarding this organelle in cellular aging. The aim of this study was to characterize the changes in the protein expression profile of aged versus young liver and kidney peroxisome-enriched fractions from mouse and to suggest possible mechanisms underlying peroxisomal aging. Peroxisome-enriched fractions from 10 weeks, 18 months and 24 months C57bl/6J mice were analyzed by quantitative proteomics.

RESULTS

Peroxisomal proteins were enriched by differential and density gradient centrifugation and proteins were separated by two-dimensional electrophoresis (2-DE), quantified and identified by mass spectrometry (MS). In total, sixty-five proteins were identified in both tissues. Among them, 14 proteins were differentially expressed in liver and 21 proteins in kidney. The eight proteins differentially expressed in both tissues were involved in beta-oxidation, alpha-oxidation, isoprenoid biosynthesis, amino acid metabolism, and stress response. Quantitative proteomics, clustering methods, and prediction of transcription factors, all indicated that there is a decline in protein expression at 18 months and a recovery at 24 months.

CONCLUSION

These results indicate that some peroxisomal proteins show a tissue-specific functional response to aging. This response is probably dependent on their differential regeneration capacity. The differentially expressed proteins could lead several cellular effects: such as alteration of fatty acid metabolism that could alert membrane protein functions, increase of the oxidative stress and contribute to decline in bile salt synthesis. The ability to detect age-related variations in the peroxisomal proteome can help in the search for reliable and valid aging biomarkers.

Feasibility of pathways for transfer of acyl groups from mitochondria to the cytosol to form short chain acyl-CoAs in the pancreatic beta cell

The mitochondria of pancreatic beta cells are believed to convert insulin secretagogues into products that are translocated to the cytosol where they participate in insulin secretion. We studied the hypothesis that short chain acyl-CoA (SC-CoAs) might be some of these products by discerning the pathways of SC-CoA formation in beta cells. Insulin secretagogues acutely stimulated 1.5-5-fold increases in acetoacetyl-CoA, succinyl-CoA, malonyl-CoA, hydroxymethylglutaryl-CoA (HMG-CoA), and acetyl-CoA in INS-1 832/13 cells as judged from liquid chromatography-tandem mass spectrometry measurements. Studies of 12 relevant enzymes in rat and human pancreatic islets and INS-1 832/13 cells showed the feasibility of at least two redundant pathways, one involving acetoacetate and the other citrate, for the synthesis SC-CoAs from secretagogue carbon in mitochondria and the transfer of their acyl groups to the cytosol where the acyl groups are converted to SC-CoAs. Knockdown of two key cytosolic enzymes in INS-1 832/13 cells with short hairpin RNA supported the proposed scheme. Lowering ATP citrate lyase 88% did not inhibit glucose-induced insulin release indicating citrate is not the only carrier of acyl groups to the cytosol. However, lowering acetoacetyl-CoA synthetase 80% partially inhibited glucose-induced insulin release indicating formation of SC-CoAs from acetoacetate in the cytosol is important for insulin secretion. The results indicate beta cells possess enzyme pathways that can incorporate carbon from glucose into acetyl-CoA, acetoacetyl-CoA, and succinyl-CoA and carbon from leucine into these three SC-CoAs plus HMG-CoA in their mitochondria and enzymes that can form acetyl-CoA, acetoacetyl-CoA, malonyl-CoA, and HMG-CoA in their cytosol.

Biochemistry of mammalian peroxisomes revisited

In this review, we describe the current state of knowledge about the biochemistry of mammalian peroxisomes, especially human peroxisomes. The identification and characterization of yeast mutants defective either in the biogenesis of peroxisomes or in one of its metabolic functions, notably fatty acid beta-oxidation, combined with the recognition of a group of genetic diseases in man, wherein these processes are also defective, have provided new insights in all aspects of peroxisomes. As a result of these and other studies, the indispensable role of peroxisomes in multiple metabolic pathways has been clarified, and many of the enzymes involved in these pathways have been characterized, purified, and cloned. One aspect of peroxisomes, which has remained ill defined, is the transport of metabolites across the peroxisomal membrane. Although it is clear that mammalian peroxisomes under in vivo conditions are closed structures, which require the active presence of metabolite transporter proteins, much remains to be learned about the permeability properties of mammalian peroxisomes and the role of the four half ATP-binding cassette (ABC) transporters therein.

The complete genome sequence of Mycobacterium avium subspecies paratuberculosis

We describe here the complete genome sequence of a common clone of Mycobacterium avium subspecies paratuberculosis (Map) strain K-10, the causative agent of Johne's disease in cattle and other ruminants. The K-10 genome is a single circular chromosome of 4,829,781 base pairs and encodes 4,350 predicted ORFs, 45 tRNAs, and one rRNA operon. In silico analysis identified >3,000 genes with homologs to the human pathogen, M. tuberculosis (Mtb), and 161 unique genomic regions that encode 39 previously unknown Map genes. Analysis of nucleotide substitution rates with Mtb homologs suggest overall strong selection for a vast majority of these shared mycobacterial genes, with only 68 ORFs with a synonymous to nonsynonymous substitution ratio of >2. Comparative sequence analysis reveals several noteworthy features of the K-10 genome including: a relative paucity of the PE/PPE family of sequences that are implicated as virulence factors and known to be immunostimulatory during Mtb infection; truncation in the EntE domain of a salicyl-AMP ligase (MbtA), the first gene in the mycobactin biosynthesis gene cluster, providing a possible explanation for mycobactin dependence of Map; and Map-specific sequences that are likely to serve as potential targets for sensitive and specific molecular and immunologic diagnostic tests. Taken together, the availability of the complete genome sequence offers a foundation for the study of the genetic basis for virulence and physiology in Map and enables the development of new generations of diagnostic tests for bovine Johne's disease.

Molecular basis of 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a human autosomal recessive metabolic disorder that usually appears within the first year of life. The causes of this aciduria are lethal mutations in the gene encoding for 3-hydroxy-3-methylglutaryl coenzyme A lyase (HL). HL is a mitochondrial matrix enzyme that catalyzes the last step of ketogenesis and leucine catabolism. This gene has been mapped to chromosome 1 at locus 1pter-p33 and its genomic organisation comprises 9 exons whose sizes vary between 64-678 bp. The human cDNA sequence was reported in 1993 with the first genetic study of two Acadian-French Canadian siblings. To date, 24 mutations in 36 patients have been described; most of them are single-base substitutions causing amino acid replacements and a variety of splicing defects. In the population studied two mutations appear predominant: g.122GA (8 patients and 15 alleles) frequent in Saudi Arabia, and g.109GT (6 patients and 12 alleles), prevalent in Spain. At least seven mutations are clustered in the second half of exon 2 affecting aminoacids E37, R41 and D42 and conforming a possible hot spot. The genotype-phenotype correlation is difficult to establish since the probands received different treatments, and the onset of an acute episode frequently depends on external factors such as fasting or acute illness.

Pathways and control of ketone body metabolism: on the fringe of lipid biochemistry

Ketone bodies become major body fuels during fasting and consumption of a high-fat, low-carbohydrate (ketogenic) diet. Hyperketonemia is associated with potential health benefits. Ketone body synthesis (ketogenesis) is the last recognizable step of lipid energy metabolism, a pathway that links dietary lipids and adipose triglycerides to the Krebs cycle and respiratory chain and has three highly regulated control points: (1) adipocyte lipolysis, (2) mitochondrial fatty acids entry, controlled by the inhibition of carnitine palmityl transferase I by malonyl coenzyme A (CoA) and (3) mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase, which catalyzes the irreversible first step of ketone body synthesis. Each step is suppressed by an elevated circulating insulin level or insulin/glucagon ratio. The utilization of ketone bodies (ketolysis) also determines circulating ketone body levels. Consideration of ketone body metabolism reveals the mechanisms underlying the extreme fragility of dietary ketosis to carbohydrate intake and highlights areas for further study.

Motif refinement of the peroxisomal targeting signal 1 and evaluation of taxon-specific differences

Eukaryote peroxisomes, plant glyoxysomes and trypanosomal glycosomes belong to the microbody family of organelles that compartmentalise a variety of biochemical processes. The interaction between the PTS1 signal and its cognate receptor Pex5 initiates the major import mechanism for proteins into the matrix of these organelles. Relying on the analysis of amino acid sequence variability of known PTS1-targeted proteins and PTS1-containing peptides that interact with Pex5 in the yeast two-hybrid assay, on binding site studies of the Pex5-ligand complex crystal structure, 3D models and sequences of Pex5 proteins from various taxa, we derived the requirements for a C-terminal amino acid sequence to interact productively with Pex5. We found evidence that, at least the 12 C-terminal residues of a given substrate protein are implicated in PTS1 signal recognition. This motif can be structurally and functionally divided into three regions: (i) the C-terminal tripeptide, (ii) a region interacting with the surface of Pex5 (about four residues further upstream), and (iii) a polar, solvent-accessible and unstructured region with linker function (the remaining five residues). Specificity differences are confined to taxonomic subgroups (metazoa and fungi) and are connected with amino acid type preferences in region 1 and deviating hydrophobicity patterns in region 2.

Investigation of the oligomeric status of the peroxisomal isoform of human 3-hydroxy-3-methylglutaryl-CoA lyase

Unprocessed 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase, retaining the mitochondrial signal sequence, has been proposed to correspond to a peroxisomal isoform. Using a modified expression plasmid and purification protocol, it is now possible to isolate substantial amounts (>10mg) of highly purified peroxisomal HMG-CoA lyase. These improvements facilitate more detailed protein chemistry approaches for characterization of the enzyme, which exhibits substantial (eightfold) dithiothreitol (DTT) stimulation of activity. The C323S mutant shows little DTT activation. Superose gel filtration chromatography data have prompted other investigators to hypothesize that the peroxisomal isoform is a monomer. This study confirms the elution properties presented in that earlier report, but also demonstrates anomalous elution up on Superose chromatography. Elution properties observed using a polyacrylamide resin (Bio-Gel P100) suggest a dimeric, rather than monomeric, enzyme. This observation has been further tested by protein chemistry experiments. The peroxisomal enzyme forms a covalently linked dimeric species upon crosslinking with dibromopropanone or o-phenylenedimaleimide or upon disulfide formation as a result of incubation with diamide. Cysteine-323 is required for intersubunit covalent crosslinking. Crosslinking efficiency is not dependent on HMG-CoA lyase protein concentration nor is it influenced by the presence of varying concentrations of an unrelated protein, such as ovalbumin. Sedimentation equilibrium analyses do not indicate a monomeric form of either human mitochondrial or human peroxisomal HMG-CoA lyase; the results suggest that these proteins are predominantly dimers. The retention of the basic N-terminal mitochondrial signal sequence in the peroxisomal HMG-CoA lyase isoform may influence elution from Superose gel filtration media but does not alter the oligomeric status of the enzyme.

Type III 3-methylglutaconic aciduria (optic atrophy plus syndrome, or Costeff optic atrophy syndrome): identification of the OPA3 gene and its founder mutation in Iraqi Jews

Type III 3-methylglutaconic aciduria (MGA) (MIM 258501) is a neuro-ophthalmologic syndrome that consists of early-onset bilateral optic atrophy and later-onset spasticity, extrapyramidal dysfunction, and cognitive deficit. Urinary excretion of 3-methylglutaconic acid and of 3-methylglutaric acid is increased. The disorder has been reported in approximately 40 patients of Iraqi Jewish origin, allowing the mapping of the disease to chromosome 19q13.2-q13.3, by linkage analysis. To isolate the causative gene, OPA3, we sequenced four genes within the critical interval and identified, in the intronic sequence of a gene corresponding to cDNA clone FLJ22187, a point mutation that segregated with the type III MGA phenotype. The FLJ22187-cDNA clone, which we identified as the OPA3 gene, consists of two exons and encodes a peptide of 179 amino acid residues. Northern blot analysis revealed a primary transcript of approximately 5.0 kb that was ubiquitously expressed, most prominently in skeletal muscle and kidney. Within the brain, the cerebral cortex, the medulla, the cerebellum, and the frontal lobe, compared to other parts of the brain, had slightly increased expression. The intronic G-->C mutation abolished mRNA expression in fibroblasts from affected patients and was detected in 8 of 85 anonymous Israeli individuals of Iraqi Jewish origin. Milder mutations in OPA3 should be sought in patients with optic atrophy with later onset, even in the absence of additional neurological abnormalities.

Identification, purification and characterization of an acetoacetyl-CoA thiolase from rat liver peroxisomes

Acetoacetyl-CoA specific thiolases catalyse the cleavage of acetoacetyl-CoA into two molecules of acetyl-CoA and the synthesis (reverse reaction) of acetoacetyl-CoA. The formation of acetoacetyl-CoA is the first step in cholesterol and ketone body synthesis. In this report we describe the identification of a novel acetoacetyl-CoA thiolase and its purification from isolated rat liver peroxisomes by column chromatography. The enzyme, which is a homotetramer with a subunit molecular mass of 42 kDa, could be distinguished from the cytosolic and mitochondrial acetoacetyl-CoA thiolases by its chromatographic behaviour, kinetic characteristics and partial internal amino-acid sequences. The enzyme did not catalyse the cleavage of medium or long chain 3-oxoacyl-CoAs. The enzyme cross-reacted with polyclonal antibodies raised against cytosolic acetoacetyl-CoA thiolase. The latter property was exploited to confirm the peroxisomal localization of the novel thiolase in subcellular fractionation experiments. The peroxisomal acetoacetyl-CoA thiolase most probably catalyses the first reaction in peroxisomal cholesterol and dolichol synthesis. In addition, its presence in peroxisomes along with the other enzymes of the ketogenic pathway indicates that the ketogenic potential of peroxisomes needs to be re-evaluated.

Hepatic mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme a synthase deficiency

There are at least two isoenzymes of 3-hydroxy-3-methylglutaryl (HMG)-CoA synthase (EC 4.1.3.5) located in the mitochondrial matrix and the cytoplasm of hepatocytes, respectively. The mitochondrial enzyme is necessary for the synthesis of ketone bodies, which are important fuels during fasting. We report a child with a deficiency of this isoenzyme. He presented at 16 mo with hypoglycemia. There was no rise in ketone bodies during fasting or after a long chain fat load but there was a small rise after a leucine load. Measurement of beta-oxidation flux in fibroblasts was normal. Using antibodies specific for mitochondrial HMG-CoA synthase, no immunoreactive material could be detected on Western blotting. Total HMG-CoA synthase activity in liver homogenate was only slightly lower than in control samples. Presumably, as there was no mitochondrial HMG-CoA synthase enzyme protein, this activity arose from the cytoplasmic or other (e.g. peroxisomal) isoenzymes. With avoidance of fasting, our patient has had no problems since presentation and is developing normally at 4 y of age.

HMG CoA lyase deficiency: identification of five causal point mutations in codons 41 and 42, including a frequent Saudi Arabian mutation, R41Q

The hereditary deficiency of 3-hydroxy-3-methylglutaryl (HMG) CoA lyase (HL; OMIM 246450 [http://www3.ncbi.nlm.nih. gov:80/htbin-post/Omim/dispmim?246450]) results in episodes of hypoketotic hypoglycemia and coma and is reported to be frequent and clinically severe in Saudi Arabia. We found genetic diversity among nine Saudi HL-deficient probands: six were homozygous for the missense mutation R41Q, and two were homozygous for the frameshift mutation F305fs(-2). In 32 non-Saudi HL-deficient probands, we found three R41Q alleles and also discovered four other deleterious point mutations in codons 41 and 42: R41X, D42E, D42G, and D42H. In purified mutant recombinant HL, all four missense mutations in codons 41 and 42 cause a marked decrease in HL activity. We developed a screening procedure for HL missense mutations that yields residual activity at levels comparable to those obtained using purified HL peptides. Codons 41 and 42 are important for normal HL catalysis and account for a disproportionate 21 (26%) of 82 of mutant alleles in our group of HL-deficient probands.

Characterization and isolation of enzymes that hydrolyze short-chain acyl-CoA in rat-liver mitochondria

In this study we investigated the presence of short-chain acyl-CoA hydrolases in rat liver mitochondria. One acetyl-CoA-hydrolyzing enzyme with a molecular mass of about 48 kDa was purified to apparent homogeneity as judged by SDS/PAGE. Immunoprecipitation experiments with antibodies raised to the purified protein showed that this enzyme corresponds to a minor portion of the total mitochondrial acetyl-CoA hydrolase activity, most (about 90%) of the total activity being due to an enzyme which was labile and required Triton X-100 for its stability. Neither of these acetyl-CoA-hydrolyzing enzymes appeared to be induced by treatment of rats with di(2-ethylhexyl)phthalate, a peroxisome proliferator which mediates induction of several cytosolic and mitochondrial long-chain acyl-CoA thioesterases. In addition, an enzyme that hydrolyzed acetoacetyl-CoA was partially purified; it was induced about 3.5-fold by di(2-ethylhexyl)phthalate treatment. In conclusion, these results demonstrate that rat liver mitochondria contain several enzymes capable of hydrolyzing short-chain acyl-CoA, indicating that regulation of the metabolism of short-chain acyl-CoAs and formation of ketone bodies, could be complex.