Presence of two forms of fumarase (fumarate hydratase E.C. 4.2.1.2) in mammalian cells: immunological characterization and genetic analysis in somatic cell hybrids. Confirmation of the assignment of a gene necessary for the enzyme expression to human chromosome 1

Tolerance of DNA Replication Stress Is Promoted by Fumarate Through Modulation of Histone Demethylation and Enhancement of Replicative Intermediate Processing in Saccharomyces cerevisiae

Fumarase is a well-characterized TCA cycle enzyme that catalyzes the reversible conversion of fumarate to malate. In mammals, fumarase acts as a tumor suppressor, and loss-of-function mutations in the FH gene in hereditary leiomyomatosis and renal cell cancer result in the accumulation of intracellular fumarate-an inhibitor of α-ketoglutarate-dependent dioxygenases. Fumarase promotes DNA repair by nonhomologous end joining in mammalian cells through interaction with the histone variant H2A.Z, and inhibition of KDM2B, a H3 K36-specific histone demethylase. Here, we report that Saccharomyces cerevisiae fumarase, Fum1p, acts as a response factor during DNA replication stress, and fumarate enhances survival of yeast lacking Htz1p (H2A.Z in mammals). We observed that exposure to DNA replication stress led to upregulation as well as nuclear enrichment of Fum1p, and raising levels of fumarate in cells via deletion of FUM1 or addition of exogenous fumarate suppressed the sensitivity to DNA replication stress of htz1Δ mutants. This suppression was independent of modulating nucleotide pool levels. Rather, our results are consistent with fumarate conferring resistance to DNA replication stress in htz1Δ mutants by inhibiting the H3 K4-specific histone demethylase Jhd2p, and increasing H3 K4 methylation. Although the timing of checkpoint activation and deactivation remained largely unaffected by fumarate, sensors and mediators of the DNA replication checkpoint were required for fumarate-dependent resistance to replication stress in the htz1Δ mutants. Together, our findings imply metabolic enzymes and metabolites aid in processing replicative intermediates by affecting chromatin modification states, thereby promoting genome integrity.

Anti-fumarase antibody promotes the dropout of photoreceptor inner and outer segments in diabetic macular oedema

AIMS/HYPOTHESIS

In diabetic macular oedema (DMO), blood components passing through the disrupted blood-retinal barrier cause neuroinflammation, but the mechanism by which autoantibodies induce neuroglial dysfunction is unknown. The aim of this study was to identify a novel autoantibody and to evaluate its pathological effects on clinically relevant photoreceptor injuries.

METHODS

Biochemical purification and subsequent peptide fingerprinting were applied to identify autoantigens. The titres of autoantibodies in DMO sera were quantified and their associations with clinical variables were evaluated. Two animal models (i.e. passive transfer of autoantibodies and active immunisation) were characterised with respect to autoimmune mechanisms underlying photoreceptor injuries.

RESULTS

After screening serum IgG from individuals with DMO, fumarase, a Krebs cycle enzyme expressed in inner segments, was identified as an autoantigen. Serum levels of anti-fumarase IgG in participants with DMO were higher than those in diabetic participants without DMO (p < 0.001) and were related to photoreceptor damage and visual dysfunction. Passively transferred fumarase IgG from DMO sera in concert with complement impaired the function and structure of rodent photoreceptors. This was consistent with complement activation in the damaged photoreceptors of mice immunised with fumarase. Fumarase was recruited to the cell surface by complement and reacted to this autoantibody. Subsequently, combined administration of anti-fumarase antibody and complement elicited mitochondrial disruption and caspase-3 activation.

CONCLUSIONS/INTERPRETATION

This study has identified anti-fumarase antibody as a serum biomarker and demonstrates that the generation of this autoantibody might be a pathological mechanism of autoimmune photoreceptor injuries in DMO.

Fumarase: From the TCA Cycle to DNA Damage Response and Tumor Suppression

Fumarase is an enzyme of the tricarboxylic acid (TCA) cycle in mitochondria, but in recent years, it has emerged as a participant in the response to DNA double strand breaks (DSBs) in the nucleus. In fact, this enzyme is dual-targeted and can be also readily detected in the mitochondrial and cytosolic/nuclear compartments of all the eukaryotic organisms examined. Intriguingly, this evolutionary conserved cytosolic population of fumarase, its enzymatic activity and the associated metabolite fumarate, are required for the cellular DNA damage response (DDR) to double-strand breaks. Here we review findings from yeast and human cells regarding how fumarase and fumarate may precisely participate in the DNA damage response. In yeast, cytosolic fumarase is involved in the homologous recombination (HR) repair pathway, through its function in the DSB resection process. One target of this regulation is the resection enzyme Sae2. In human cells, fumarase is involved in the non-homologous end joining (NHEJ) repair pathway. Fumarase is phosphorylated by the DNA-dependent protein kinase (DNA-PK) complex, which induces the recruitment of fumarase to the DSB and local generation of fumarate. Fumarate inhibits the lysine demethylase 2B (KDM2B), thereby facilitating the dimethylation of histone H3, which leads to the repair of the break by the NHEJ pathway. Finally, we discuss the question how fumarase may function as a tumor suppressor via its metabolite substrate fumarate. We offer a number of models which can explain an apparent contradiction regarding how fumarate absence/accumulation, as a function of subcellular location and stage can determine tumorigenesis. Fumarate, on the one hand, a positive regulator of genome stability (its absence supports genome instability and tumorigenesis) and, on the other hand, its accumulation drives angiogenesis and proliferation (thereby supporting tumor establishment).

TET proteins and the control of cytosine demethylation in cancer

The discovery that ten-eleven translocation (TET) proteins are α-ketoglutarate-dependent dioxygenases involved in the conversion of 5-methylcytosines (5-mC) to 5-hydroxymethylcytosine (5-hmC), 5-formylcytosine and 5-carboxycytosine has revealed new pathways in the cytosine methylation and demethylation process. The description of inactivating mutations in TET2 suggests that cellular transformation is in part caused by the deregulation of this 5-mC conversion. The direct and indirect deregulation of methylation control through mutations in DNA methyltransferase and isocitrate dehydrogenase (IDH) genes, respectively, along with the importance of cytosine methylation in the control of normal and malignant cellular differentiation have provided a conceptual framework for understanding the early steps in cancer development. Here, we review recent advances in our understanding of the cytosine methylation cycle and its implication in cellular transformation, with an emphasis on TET enzymes and 5-hmC. Ongoing clinical trials targeting the activity of mutated IDH enzymes provide a proof of principle that DNA methylation is targetable, and will trigger further therapeutic applications aimed at controlling both early and late stages of cancer development.

Expression and properties of the mitochondrial and cytosolic forms of fumarase in germinating maize seeds

Fumarase (EC 4.2.1.2) catalyzes reversible interconversion of malate and fumarate. It is usually associated with the tricarboxylic acid cycle in mitochondria, although the cytosolic form has also been detected. We investigated the expression of two fumarase genes and activities of the mitochondrial and cytosolic isoforms of fumarase in maize (Zea mays) scutellum during germination. Both isoforms were purified to electrophoretic homogeneity. The cytosolic form had low optimum pH (6.5) and high affinity to malate (Km 5 μM) when compared with the mitochondrial form (optimum pH 7.0, Km 50 μM). The cytosolic form was strongly activated by Mg(2+) and even more by Mn(2+) , whereas the mitochondrial form was moderately activated by Mg(2+) and Mn(2+) was less effective. The highest fumarase activity in scutellum and a high expression of the gene encoding the cytosolic form were observed during the maximal activity of the glyoxylate cycle. In leaves, the localization of fumarase is only mitochondrial and only one fumarase gene is expressed. It is concluded that the function of cytosolic fumarase in maize scutellum can be related to metabolism of succinate formed in the glyoxylate cycle.

Fumarase: a paradigm of dual targeting and dual localized functions

The enzyme fumarase is a conserved protein in all organisms with regard to its sequence, structure and function. This enzyme participates in the tricarboxylic acid cycle in mitochondria which is essential for cellular respiration in eukaryotes. However, a common theme conserved from yeast to humans is the existence of a cytosolic form of fumarase; hence this protein is dual localized. We have coined identical (or nearly identical) proteins situated in different subcellular locations 'echoforms' or 'echoproteins'. Fumarase was the first example of a dual localized protein whose mechanism of distribution was found to be based on a single translation product. Consequently, fumarase has become a paradigm for three unique eukaryotic cellular phenomena related to protein dual localization: (a) distribution between mitochondria and the cytoplasm involves reverse translocation; (b) targeting to mitochondria involves translation coupled import; and (c) there are two echoforms possessing distinct functions in the respective subcellular compartments. Here we describe and discuss these fumarase related phenomena and in addition point out approaches for studying dual function of distributed proteins, in particular compartment-specific depletion. In the case of fumarase, the cytoplasmic function was only recently discovered; the enzyme was found to participate in the cellular response to DNA double strand breaks. Strikingly, upon DNA damage the protein is transported from the cytosol to the nucleus, where by virtue of its enzymatic activity it participates in the DNA damage response.

YLoc--an interpretable web server for predicting subcellular localization

Predicting subcellular localization has become a valuable alternative to time-consuming experimental methods. Major drawbacks of many of these predictors is their lack of interpretability and the fact that they do not provide an estimate of the confidence of an individual prediction. We present YLoc, an interpretable web server for predicting subcellular localization. YLoc uses natural language to explain why a prediction was made and which biological property of the protein was mainly responsible for it. In addition, YLoc estimates the reliability of its own predictions. YLoc can, thus, assist in understanding protein localization and in location engineering of proteins. The YLoc web server is available online at www.multiloc.org/YLoc.

Molecular analysis and prenatal diagnosis of human fumarase deficiency

Fumarase deficiency is a rare autosomal recessive disorder of the citric acid cycle causing severe neurological impairment. The cDNA for both the rat and human enzymes has been cloned previously and shown to encode a coding region of 1.46 kb. To scan for mutations in fumarase-deficient patients we amplified the coding region of fumarase from fibroblast/lymphoblast cDNA employing the oligonucleotide primers designed from the published human and rat cDNA sequence. We then directly sequenced the polymerase chain reaction product. In seven unrelated patients, we detected four missense mutations (A265T, D383V, F269C, K187R), a nonsense mutation (W458X), a 3-bp AAA insertion that introduces an additional lysine residue at codon 435, and a spontaneous new mutation resulting in a 74-bp deletion (66del74). Seven at-risk pregnancies were monitored with one prenatal diagnosis of fumarase deficiency by molecular analysis and favorable outcome of the other pregnancies as predicted by enzyme assay of cultured fetal cells or molecular analysis.

Mutation of the fumarase gene in two siblings with progressive encephalopathy and fumarase deficiency

We report an inborn error of the tricarboxylic acid cycle, fumarase deficiency, in two siblings born to first cousin parents. They presented with progressive encephalopathy, dystonia, leucopenia, and neutropenia. Elevation of lactate in the cerebrospinal fluid and high fumarate excretion in the urine led us to investigate the activities of the respiratory chain and of the Krebs cycle, and to finally identify fumarase deficiency in these two children. The deficiency was profound and present in all tissues investigated, affecting the cytosolic and the mitochondrial fumarase isoenzymes to the same degree. Analysis of fumarase cDNA demonstrated that both patients were homozygous for a missense mutation, a G-955-->C transversion, predicting a Glu-319-->Gln substitution. This substitution occurred in a highly conserved region of the fumarase cDNA. Both parents exhibited half the expected fumarase activity in their lymphocytes and were found to be heterozygous for this substitution. The present study is to our knowledge the first molecular characterization of tricarboxylic acid deficiency, a rare inherited inborn error of metabolism in childhood.

Signals on proteins, intracellular targeting and inborn errors of organellar metabolism

Newly synthesized polypeptides contain signals that direct them to the appropriate intracellular organelles and the organelles contain receptors that recognize the signals. Protein synthesis occurs either on free ribosomes or on ribosomes bound to the endoplasmic reticulum. The proteins synthesized on bound ribosomes are co-translationally translocated into the lumen of the endoplasmic reticulum and contain or acquire targeting information for retention in the endoplasmic reticulum or for sorting to lysosomes and other compartments of the secretory and endocytic pathways. Proteins synthesized on free ribosomes remain in the cytosol or contain signals for import into the nucleus, mitochondria or peroxisomes. The nature of the targeting signals and the mechanisms of import are discussed briefly. Examples are given of inborn errors of metabolism caused by incorrect or impaired incorporation of proteins into mitochondria, lysosomes or peroxisomes.

The expanding clinical spectrum of mitochondrial diseases

The mitochondrion is the only extranuclear organelle containing DNA (mtDNA). As such, genetically determined mitochondrial diseases may result from a molecular defect involving the mitochondrial or the nuclear genome. The first is characterized by maternal inheritance and the second by Mendelian inheritance. Ragged-red fibers (RRF) are commonly seen with primary lesions of mtDNA, but this association is not invariant. Conversely, RRF are seldom associated with primary lesions of nuclear DNA. Large-scale rearrangements (deletions and insertions) and point mutations of mtDNA are commonly associated with RRF and lactic acidosis, e.g. Kearns-Sayre syndrome (KSS) (major large-scale rearrangements), Pearson syndrome (large-scale rearrangements), myoclonus epilepsy with RRF (MERRF) (point mutation affecting tRNA(lys) gene), mitochondrial myopathy, lactic acidosis, and stroke-like episodes (MELAS) (two point mutations affecting tRNA(leu)(UUR) gene) and a maternally-inherited myopathy with cardiac involvement (MIMyCa) (point mutation affecting tRNA(leu)(UUR) gene). However, RRF and lactic acidosis are absent in Leber hereditary optic neuropathy (LHON) (one point mutation affecting ND4 gene, two point mutations affecting ND1 gene, and one point mutation affecting the apocytochrome b subunit of complex III), and the condition associated with maternally inherited sensory neuropathy (N), ataxia (A), retinitis pigmentosa (RP), developmental delay, dementia, seizures, and limb weakness (NARP) (point mutation affecting ATPase subunit 6 gene). The point mutations in MELAS, MIMyCa, and MERRF, and the large-scale mtDNA rearrangements in KSS and Pearson syndrome have a broader biochemical impact since these molecular defects involve the translational sequence of mitochondrial protein synthesis. The nuclear defects involving mitochondrial function generally are not associated with RRF. The biochemical classification of mitochondrial diseases principally catalogues these nuclear defects. This classification divides mitochondrial diseases into five categories. Primary and secondary deficiencies of carnitine are examples of a substrate transport defect. A lipid storage myopathy is often present. Disturbances of pyruvate or fatty acid metabolism are examples of substrate utilization defects. Only four defects of the Krebs cycle are known: fumarase deficiency, dihydrolipoyl dehydrogenase deficiency, alpha-ketoglutarate dehydrogenase deficiency, and combined defects of muscle succinate dehydrogenase and aconitase. Luft disease is the singular example of a defect in oxidation-phosphorylation coupling. Defects of respiratory chain function are manifold. Two clinical syndromes predominate, one involving limb weakness, and the other primarily affecting brain function. Leigh syndrome may result from different enzyme defects, most notably pyruvate dehydrogenase complex deficiency, cytochrome c oxidase deficiency, complex I deficiency, and complex V deficiency associated with the recently described NARP point mutation. A new group of mitochondrial diseases has emerged.(ABSTRACT TRUNCATED AT 400 WORDS)

Rat liver mitochondrial and cytosolic fumarases with identical amino acid sequences are encoded from a single mRNA with two alternative in-phase AUG initiation sites

By use of anticytosolic fumarase antibody, a cDNA clone was isolated from a rat liver cDNA library in the expression vector lambda gt11 and in the pBR 322 vector. A clone with an insert of about 1.7 kbp was isolated. Nucleotide sequence analysis of the insert revealed that the cDNA contained a noncoding region composed of 25 nucleotides in the 5' terminus, the coding region composed of 1,521 nucleotides, and the 3' nontranslated region composed of 43 nucleotides followed by a poly(A)+ tail. The open reading frame encoded a polypeptide of 507 amino acid residues (predicted Mr = 54,462), which contained an additional sequence composed of 41 amino acid residues on the N-terminus of the mitochondrial mature fumarase (the presequence). Thus, this reading frame was concluded to encode the precursor of mitochondrial fumarase. The amino acid sequence predicted from the nucleotide sequence contained all the amino acid sequences of 12 proteolytic polypeptides obtained by digestion of purified mitochondrial fumarase with V8 protease. The total amino acid sequence of the mitochondrial fumarase also contained all the sequences of 14 proteolytic peptides prepared from the cytosolic fumarase, indicating that the amino acid sequences of these two isozymes are identical. Furthermore, the results obtained by hybrid-selected translation, Northern blot and primer-extension analyses using appropriate cDNA segments prepared from fumarase cDNA (1.7 kbp) as the probe or primer suggested a possibility that both precursors of the mitochondrial and cytosolic fumarases were synthesized with one species of mRNA having base sequence coding presequence of the mitochondrial fumarase by unknown post-transcriptional mechanism(s). Rat liver cells may contain a specific RNA(18S) modulating the translational activity of mRNA for fumarase. This RNA(s), which was contained in poly(A)- fraction, was partially purified by high-performance gel filtration. The partially purified RNA(s) suppressed the translational activity of the cytosolic fumarase, whereas the translational activity of the mitochondrial one was accelerated by this RNA(s).

Two biochemically distinct classes of fumarase in Escherichia coli

Biochemical studies with strains of Escherichia coli that are amplified for the products of the three fumarase genes, fumA (FUMA), fumB (FUMB) and fumC (FUMC), have shown that there are two distinct classes of fumarase. The Class I enzymes include FUMA, FUMB, and the immunologically related fumarase of Euglena gracilis. These are characteristically thermolabile dimeric enzymes containing identical subunits of Mr 60,000. FUMA and FUMB are differentially regulated enzymes that function in the citric acid cycle (FUMA) or to provide fumarate as an anaerobic electron acceptor (FUMB), and their affinities for fumarate and L-malate are consistent with these roles. The Class II enzymes include FUMC, and the fumarases of Bacillus subtilis, Saccharomyces cerevisiae and mammalian sources. They are thermostable tetrameric enzymes containing identical subunits Mr 48,000-50,000. The Class II fumarases share a high degree of sequence identity with each other (approx. 60%) and with aspartase (approx. 38%) and argininosuccinase (approx. 15%), and it would appear that these are all members of a family of structurally related enzymes. It is also suggested that the Class I enzymes may belong to a wider family of iron-dependent carboxylic acid hydro-lyases that includes maleate dehydratase and aconitase. Apart from one region containing a Gly-Ser-X-X-Met-X-X-Lys-X-Asn consensus sequence, no significant homology was detected between the Class I and Class II fumarases.

Purification and structural comparisons of the cytosolic and mitochondrial fumarases from baker's yeast

1. The cytosolic and mitochondrial fumarases (EC 4.2.1.2) from baker's yeast (Saccharomyces cerevisiae) have been purified to homogeneity. 2. Subunit molecular weights for the cytosolic and mitochondrial isoenzymes were 53,000 and 48,000 respectively. 3. Peptide maps obtained after digestion of the two isoenzymes with trypsin were almost identical but showed significant small differences. The same was true of peptide maps obtained after digestion with the glutamic acid-specific proteinase from S. aureus.

Fumarase deficiency: a new cause of mitochondrial encephalomyopathy

We observed a deficiency of both the mitochondrial and cytosolic forms of fumarase in a male infant with mitochondrial encephalomyopathy who presented at one month of age with failure to thrive, developmental delay, hypotonia, cerebral atrophy, lactic and pyruvic acidemia, and fumaric aciduria. The patient died at eight months of age. Isolated skeletal-muscle mitochondria showed selective defects in the oxidation of glutamate (31 ng atoms of oxygen consumed per minute per milligram of mitochondrial protein, as compared with 94 +/- 19 [mean +/- SD] in five controls) and of succinate (18 vs. 145 +/- 18 ng atoms of oxygen per minute per milligram of protein), whereas isolated liver mitochondria oxidized these and other substrates normally. Fumarase activity was virtually absent in both liver mitochondria (53 vs. 2878 +/- 248 nmol per minute per milligram of protein [5 controls]) and skeletal-muscle mitochondria (23 vs. 1997 +/- 717 nmol per minute per milligram [12 controls]). Seventeen other mitochondrial enzymes had normal activity in both liver and muscle mitochondrial extracts. Fumarase activity was also significantly reduced in homogenates of liver tissue (less than 1 vs. 90 +/- 25 mumol per minute per gram of wet weight [five controls]) and skeletal muscle (less than 1 vs. 21 +/- 4 mumol per minute per gram [five controls]), indicating a deficiency of both mitochondrial and cytosolic fumarases. Organ differences in intramitochondrial accumulation of fumarate may have accounted for the selective oxidative defects observed in the skeletal-muscle mitochondria but not liver mitochondria. All these findings are consistent with a profound combined fumarase deficiency.

Mechanism of synthesis and localization of mitochondrial and cytosolic fumarases in rat liver

Fumarases in the mitochondrial and cytosolic fractions of rat liver were separately purified and crystallized. These two fumarases were not distinguishable in physicochemical, catalytic, or immunochemical properties. The sequences of seven amino acids in the C-terminal portions of the two fumarases were shown using carboxypeptidase P to be identical, i.e.-Val-Asp-Glu-Thr-Ala-Leu-Lys-. The amino acid sequence of the N-terminal portion of the mitochondrial fumarase was determined by the Edman method as Ala-Gln-Gln-Asn-Phe-Glu-Ile-Pro-Asp-, but that of the cytosolic fumarase could not be determined by the Edman method, since the N-terminal amino acid was blocked. The N-terminal amino acid of the cytosolic fumarase was identified as N-acetyl-alanine by analysis of the acidic amino acid produced by digestion of the enzyme protein with pronase E, carboxypeptidase A and B. Then the sequence of five amino acids in the N-terminal portion was determined by analyzing the acidic peptide obtained by limited proteolysis of the enzyme protein with carboxypeptidase A as Ac-Ala-Ser-Gln-Asn-Ser-. Peptide mapping of the tryptic peptides obtained from the mitochondrial and cytosolic fumarases showed no difference in the amino acid sequences of the two except in their N-terminal portions. The turnover rates of the mitochondrial and cytosolic fumarases were determined by injecting L-[U-14C]leucine into rat and following the decay of specific radioactivity incorporated into immunoprecipitates from the partially purified enzyme. The half-life of the cytosolic fumarase was estimated as 4.8 days from the decay curve of its specific radioactivity. The decay curve of the specific radioactivity of the mitochondrial fumarase, obtained after a single injection of L-[U-14]leucine, was quite unusual: its specific radioactivity remained constant for about 7 days after pulse labeling, and then decreased exponentially with a half-life of 9.7 days. Similar amounts of cytosolic and mitochondrial fumarase were found in the livers of the rat, mouse, rabbit, dog, chicken, snake, frog, and carp, respectively. Similar subcellular distributions of the enzyme were also found in the kidney, heart, and skeletal muscle of rats, and in hepatoma cells (AH-109A). However, in rat brain no fumarase activity was detected in the cytosolic fraction. Two putative precursor polypeptides of rat liver fumarase were synthesized when rat liver RNA was translated in vitro in a rabbit reticulocyte lysate system.(ABSTRACT TRUNCATED AT 400 WORDS)

Assignment of human ferritin genes to chromosomes 11 and 19q13.3----19qter

Extracts of hamster-human and mouse-human hybrids, some with translocations involving chromosome 19, have been assayed for both human spleen ferritin (rich in L subunits) and human heart ferritin (rich in H subunits). Hybrid lines retaining part of the long arm of chromosome 19 including the region 19q13.3----19qter produced human "L" type ferritin. This confirms the previous assignment of the "ferritin gene" to chromosome 19 (Caskey et al. 1983). However, lines retaining chromosome 11 were found to contain human "H" type ferritin suggesting that the gene for the "H" subunit is on this chromosome. The presence of chromosome 6 was not necessary for the expression of either "H" or "L" type human ferritin. It thus seems unlikely that the gene for idiopathic haemochromatosis is a ferritin gene.

Purification and structural comparisons of the cytosolic and mitochondrial isoenzymes of fumarase from pig liver

A method has been developed for the purification of cytosolic and mitochondrial isoenzymes of fumarase from total homogenates of pig liver. Separation of the isoenzymes from one another was achieved using chromatofocusing. The isoenzymes were pure as judged by production of single bands on electrophoresis in the presence of sodium dodecyl sulphate; they appeared to have identical or very similar subunit molecular weights. The isoenzymes differed in electrophoretic properties under nondenaturing conditions. One-dimensional peptide maps of fragments produced from the two isoenzymes by chemical cleavage at cysteine residues were identical; maps obtained after digestion with the V8 proteinase from Staphylococcus aureus showed small differences at short times of digestion which could have reflected variations in rates of hydrolysis rather than structural differences. However, two-dimensional peptide maps of digests obtained by treatment of the isoenzymes with trypsin followed by chymotrypsin had 58 peptides in common, but showed two peptides unique to the mitochondrial isoenzyme and five peptides unique to the cytosolic form. Using the dansylation procedure, the mitochondrial isoenzyme was shown to have N-terminal alanine and the cytosolic form to have N-terminal glutamic acid or glutamine. We conclude that the isoenzymes of fumarase are identical over nearly all of their amino acid sequences but differ at their N-termini; the extent of these differences is yet to be established. These results are consistent with the claim (Edwards, Y.H. and Hopkinson,D.A. (1979) Ann. Human Genet. Lond. 42, 303-313) that the isoenzymes are determined at the same genetic locus, but they raise interesting questions about the biosynthesis of the isoenzymes.

The subcellular distribution of fumarase isozymes in rat liver

Between 13 and 25% of the fumarase activity of rat liver was found to be cytosolic in origin the remainder being localised in the mitochondria. Electrophoretic analysis on cellulose acetate showed that mitochondria do not contain detectable levels of cytosolic isozyme or vice versa.

Structural and genetic relationships between cytosolic and mitochondrial isoenzymes

The most common type of genetic relationship between cytosolic and mitochondrial isoenzymes will probably be found to be divergent evolution from a common ancestral form. This is firmly established for the aspartate aminotransferases and less directly so in other cases. The two isoenzymes of aspartate aminotransferase have evolved at roughly equal rates at the level of total amino acid sequence but certain limited surface regions of the mitochondrial form have been much more highly conserved than corresponding regions in the cytosolic protein; these regions probably play a role in topogenesis of the mitochondrial isoenzyme. It is of interest that nearly all mitochondrial proteins are initially synthesised as precursors of molecular weight greater than the mature forms. In the case of aspartate aminotransferase, and possibly of other such isoenzymes, the N-terminus of the mature protein is nearly coincident with that of the cytosolic isoenzyme. Hence during evolution either the gene for the mitochondrial isoenzyme has gained an extra coding region for this N-terminal extension or, less likely, the structural gene for the cytosolic form has suffered a sizeable terminal deletion. Cytosolic and mitochondrial superoxide dismutases have not shared a common ancestral form as shown by the fact that their primary structures are completely unrelated. On the other hand, the mitochondrial and prokaryotic enzymes are clearly related. There is now, however, evidence to suggest that some prokaryotes possess a copper/zinc enzyme related to the eukaryotic cytosolic form. Hence the possibility arises that primitive prokaryotes possessed both proteins. The copper/zinc superoxide dismutase has been retained in the cytosol of eukaryotic cells and a few bacterial species.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-linking of mitochondrial matrix proteins in situ

Different cross-linkers (10 mM) of varying specificity and arm length were found to cross-link mitochondria matrix proteins in situ in 2 min at pH 7.4. As seen by SDS-polyacrylamide electrophoresis, the disappearance of individual protein bands was accompanied by concomitant appearance of polymeric aggregates that failed to enter the 4% spacer gel. The disorganization of the mitochondrial matrix infrastructure either by swelling or sonication of the mitochondria resulted in a decrease in the rate of cross-linking. Leakage of citrate synthase, malate dehydrogenase and fumarase was found to be reduced when cross-linked mitochondria were made permeable with toluene. On lysing the cross-linked mitochondria, a major part of the matrix protein (75%) was found to sediment with the membrane fraction. The activities of citrate synthase malate dehydrogenase and fumarase in rat liver mitochondria were also found to increase in the percipitates with concomitant decrease in their activities in the soluble matrix fraction. These results indicate that the cross-linkers enters the mitochondria and cross-links matrix proteins including Krebs cycle enzymes either to the mitochondrial membranes, or to themselves resulting in very large molecular weight complexes. These results are interpreted to mean that in liver mitochondria, the Krebs cycle enzyme are preferentially located near the membrane.

Assignment to chromosome 16 of a gene necessary for the expression of human mitochondrial glutamate oxaloacetate transaminase (aspartate aminotransferase) (E.C. 2.6.1.1.)

A gene necessary for the expression of human mitochondrial glutamate oxaloacetate transaminase (GOT-2) has been assigned to chromosome 16 on the basis of an immunochemical analysis of human-mouse somatic cell hybrids. Mitochondrial GOT cosegregates with adenine phosphoribosyl transferase (E.C. 2.4.2.7.).

Intracellular distribution of NADP-linked isocitrate dehydrogenase, fumarase and citrate synthase in bovine heart muscle

By fractional extraction of minced bovine heart muscle with iso-osmotic sucrose and phosphate buffer solutions, it is shown that less than 4% of the total citrate synthase in the tissue is in the cytosol. Using citrate synthase as a marker for broken mitochondria, two methods of fractionation of 750 x g supernatants from homogenates of bovine heart muscle show that 10% of the total fumarase and NADP-linked isocitrate dehydrogenase activities are present in the cytoplasm. Homogenates prepared by sonication and osmotic shock and by sand-grinding gave closely similar results as regards enzyme distributions and extent of mitochondrial breakage. The results are compared with those reported for other tissues.

Subcellular distribution of enzymes determined by rapid digitonin fractionation of isolated hepatocytes

Conditions were determined for rapid separation of cytosolic and mitochondrial compartments by digitonin fractionation of rat hepatocytes. The minimum time required for separation of mitochondrial and cytosolic enzyme markers decreased rapidly with increasing temperature. Kyro EOB, a non-ionic detergent, increases the release of cytosolic enzymes, particularly at lower temperatures. Experimental procedures are described for greater than 90% release of cytosolic enzymes and less than 2% release of mitochondrial enzymes in 3s. By using appropriate concentrations of digitonin and Kyro EOB in a fractionation medium maintained at 1 degrees C and a minimum time of exposure to the medium, nearly separate patterns of release were obtained for enzyme markers for the cytosol, mitochondrial matrix and mitochondrial intermembrane space. The distribution of enzymes that exist in more than one of these compartments was quantified by comparing their rates of release with those of marker enzymes. The cytosol/mitochondrial-matrix distributions for such enzymes in hepatocytes from starved rats were 16%/84% for aspartate aminotransferase, 34%/66% for fumarase and 77%/23% for ATP citrate lyase. In hepatocytes from rats that were induced to synthesize ATP citrate lyase by starvation and re-feeding, the ratio had increased to 95%/5%. The maximum cytosol/intermembrane-space ratio for adenylate kinase was 8%/92%. A procedure is also described for treating commercial digitonin that increases its solubility in water from about 1mg/ml to more than 800mg/ml.

Further characterization of the human fumarase variant, FH 2--1

1. Further investigation of fumarase using lymphoblastoid cells derived from an individual of the FH 2--1 phenotype has confirmed that the mitochondrial (FHM) and soluble (FHS) forms of fumarase are determined at the same structural locus. 2. The FH 2--1 variant is associated with enzyme deficiency: c. 70% of normal in lymphoblastoid cells and c. 20% of normal in red cells. 3. The pH optimum and apparent Michaelis constant of the variant fumarase were normal but heat-inactivation studies suggest that the isozymes containing polypeptides determined by the variant allele are unstable.

The genetic determination of fumarase isozymes in human tissues

(1) A method for the starch gel electrophoresis of human fumarase has been devised which resolves two groups of isozymes: one group (FHM) is associated with the mitochondria, the other (FHS) is cytosolic. (2) Tissue to tissue variation in the complexity of the isozyme patterns and the examination of red cells fractionated according to their age by density gradient centrifugation suggest that the least anodal isozymes of the FHS group and ofthe FHM group are modified in vivo with the generation of several secondary isozymes. (3) Red cells, which display only FHS isozymes, were screened from 776 European and 100 Nigerian individuals and a single electrophoretic variant (phenotype FH 2-1) was identified in a middle-aged male from England. The same variant was identified in a paternal aunt and the data suggest that the variant repressents heterozygosity for the common (FH1) allele and a rare variant (FH2) allele at an autosomal locus. (4) Fibroblasts and white cells from the FH 2-1 individual showed variation in the FHM and FHS isozymes indicating that the mitochondrial and the soluble forms of fumarase are determined at the same structural locus.

Gene dosage effect for fumarate hydratase (FH; E.C. 4.2.1.2) in partial trisomy 1

A strain of fibroblasts partially trisomic for the larger part of 1q (Norwood and Hoehn, 1974) contains about 1.5 times as much fumarate hydratase (FH) as various control-strains. This gene dosage effect was ascertained by (1) comparative measurements of the specific activity; (2) relating the specific activity of FH to that of reference enzymes, not influenced by the chromosomal anomaly; and (3) by immunoprecipitation methods, using a rabbit antiserum against pig heart FH which cross-reacts with the human enzyme. Among others, this gene dosage effect can be demonstrated numerically by the following parameters: Ratio of the average specific activity of FH in the trisomic strain to that of the control strains: 1.53. Corresponding ratio after dividing FH activity by that of reference enzymes; for acid phosphatase: 1.58, for glutamate dehydrogenase: 1.53. Average ratio of the immunoprecipitation areas obtained upon radial immunodiffusion according to Mancini et al. (1965): 1.56.

Adenylate kinase 2, a mitochondrial enzyme

The subcellular compartmentalization of the isoenzymes of ATP:AMP phosphotransferase (adenylate kinase) was analyzed in HeLa cells, RAG cells, and RAG-human hybrids that express human AK-2. In HeLa cells and in the hybrids, human AK-2 was present in a mitochemical fraction prepared from cell extracts and in mitochondria purified by density gradient centrifugation. Human AK-1 was, as expected, distributed in the soluble cytoplasmic fraction of the cells. The rodent isozymes which are homologous to human AK-1 and AK-2 have been determined.

Human mitochondrial thymidine kinase is coded for by a gene on chromosome 16 of the nucleus

The expression of human mitochondrial thymidine kinase (mt TK) was investigated by polyacrylamide electrophoresis in 19 independent human-mouse somatic cell hybrids which allowed all human chromosomes to be analyzed. In 8 hybrid clones the presence of this enzymatic activity could be demonstrated. Human mt TK segregated concordantly with human adenine phosphoribosyltransferase (APRT) and human chromosome 16. Discordant segregation with all other human chromosomes was demonstrated by karyotype and isozyme analyses. These results suggest that human mt TK is coded for by a gene on chromosome 16 of the nucleus. Thus human mt TK is genetically different from human cytosol thymidine kinase which is coded for by a gene on chromosome 17. The appearance of one heteropolymer band after electrophoretic separation of human and murine mt TK supports the notion that both enzymes have dimeric structures.