Succinate:quinone oxidoreductases--what can we learn from Wolinella succinogenes quinol:fumarate reductase?

The structure of Wolinella succinogenes quinol:fumarate reductase by X-ray crystallography has been determined at 2.2-A resolution [Lancaster et al. (1999), Nature 402, 377-385]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron-sulphur clusters, and two haem b groups) a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. By combining the results from site-directed mutagenesis, functional and electrochemical characterisation, and X-ray crystallography, a residue was identified which is essential for menaquinol oxidation. [Lancaster et al. (2000), Proc. Natl. Acad. Sci. USA 97, 13051-13056]. The location of this residue in the structure suggests that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem-containing succinate:quinone oxidoreductases could be associated with the generation of a transmembrane electrochemical potential. Based on crystallographic analysis of three different crystal forms of the enzyme and the results from site-directed mutagenesis, we have derived a mechanism of fumarate reduction and succinate oxidation [Lancaster et al. (2001) Eur. J. Biochem. 268, 1820-1827], which should be generally relevant throughout the superfamily of succinate:quinone oxidoreductases.

Generation of a proton potential by succinate dehydrogenase of Bacillus subtilis functioning as a fumarate reductase

The membrane fraction of Bacillus subtilis catalyzes the reduction of fumarate to succinate by NADH. The activity is inhibited by low concentrations of 2-(heptyl)-4-hydroxyquinoline-N-oxide (HOQNO), an inhibitor of succinate: quinone reductase. In sdh or aro mutant strains, which lack succinate dehydrogenase or menaquinone, respectively, the activity of fumarate reduction by NADH was missing. In resting cells fumarate reduction required glycerol or glucose as the electron donor, which presumably supply NADH for fumarate reduction. Thus in the bacteria, fumarate reduction by NADH is catalyzed by an electron transport chain consisting of NADH dehydrogenase (NADH:menaquinone reductase), menaquinone, and succinate dehydrogenase operating in the reverse direction (menaquinol:fumarate reductase). Poor anaerobic growth of B. subtilis was observed when fumarate was present. The fumarate reduction catalyzed by the bacteria in the presence of glycerol or glucose was not inhibited by the protonophore carbonyl cyanide m-chlorophenyl hydrazone (CCCP) or by membrane disruption, in contrast to succinate oxidation by O2. Fumarate reduction caused the uptake by the bacteria of the tetraphenyphosphonium cation (TPP+) which was released after fumarate had been consumed. TPP+ uptake was prevented by the presence of CCCP or HOQNO, but not by N,N'-dicyclohexylcarbodiimide, an inhibitor of ATP synthase. From the TPP+ uptake the electrochemical potential generated by fumarate reduction was calculated (Deltapsi = -132 mV) which was comparable to that generated by glucose oxidation with O2 (Deltapsi = -120 mV). The Deltapsi generated by fumarate reduction is suggested to stem from menaquinol:fumarate reductase functioning in a redox half-loop.

Mitochondrial mutations differentially affect aging, mutability and anesthetic sensitivity in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, mutations have been previously isolated that affect the activities of Complex I (gas-1) and Complex II (mev-1), two of the five membrane-bound complexes that control electron flow in mitochondrial respiration. We compared the effects of gas-1 and mev-1 mutations on different traits influenced by mitochondrial function. Mutations in Complex I and II both increased sensitivity to free radicals as measured during development and in aging animals. However, gas-1 and mev-1 mutations differentially affected mutability and anesthetic sensitivity. Specifically, gas-1 was anesthetic hypersensitive but not hypermutable while mev-1 was hypermutable but displayed normal responses to anesthetics. These results indicate that Complexes I and II may differ in their effects on behavior and development, and are consistent with the wide variation in phenotypes that result from mitochondrial changes in other organisms.

Succinate dehydrogenase and other respiratory pathways in thylakoid membranes of Synechocystis sp. strain PCC 6803: capacity comparisons and physiological function

Respiration in cyanobacterial thylakoid membranes is interwoven with photosynthetic processes. We have constructed a range of mutants that are impaired in several combinations of respiratory and photosynthetic electron transport complexes and have examined the relative effects on the redox state of the plastoquinone (PQ) pool by using a quinone electrode. Succinate dehydrogenase has a major effect on the PQ redox poise, as mutants lacking this enzyme showed a much more oxidized PQ pool. Mutants lacking type I and II NAD(P)H dehydrogenases also had more oxidized PQ pools. However, in the mutant lacking type I NADPH dehydrogenase, succinate was essentially absent and effective respiratory electron donation to the PQ pool could be established after addition of 1 mM succinate. Therefore, lack of the type I NADPH dehydrogenase had an indirect effect on the PQ pool redox state. The electron donation capacity of succinate dehydrogenase was found to be an order of magnitude larger than that of type I and II NAD(P)H dehydrogenases. The reason for the oxidized PQ pool upon inactivation of type II NADH dehydrogenase may be related to the facts that the NAD pool in the cell is much smaller than that of NADP and that the NAD pool is fully reduced in the mutant without type II NADH dehydrogenase, thus causing regulatory inhibition. The results indicate that succinate dehydrogenase is the main respiratory electron transfer pathway into the PQ pool and that type I and II NAD(P)H dehydrogenases regulate the reduction level of NADP and NAD, which, in turn, affects respiratory electron flow through succinate dehydrogenase.

Fumarate reductase is essential for Helicobacter pylori colonization of the mouse stomach

Fumarate reductase (FRD) is the key enzyme in fumarate respiration induced by anaerobic growth of bacteria. In Helicobacter pylori, this enzyme appears to be constitutively expressed under microaerobic conditions and is not essential for its survival in vitro. In this study, the role of FRD in the colonization of H. pylori was investigated using a mouse model. The frdA gene coding for subunit A of FRD, and two control genes, copA and copP associated with the export of copper out of H. pylori, were inactivated by insertion of the chloramphenicol acetyltransferase cassette into these individual genes. The isogenic mutants of H. pylori strain AH244 were obtained by natural transformation. Seventy-five ICR mice (15 mice/group) were orogastrically dosed with either the wild type H. pylori strain AH244, its isogenic mutants, or Brucella broth (negative control). Five mice from each group were killed at 2, 4 and 8 weeks post-inoculation (WPI), respectively. H. pylori colonization was not detected in mouse gastric mucosa infected with the frdA mutant at any time point in the study by both quantitative culture and PCR. In contrast, the mice inoculated with either wild type AH244, copA or copPH. pylori mutants became readily infected. These data indicate that FRD plays a crucial role in H. pylori survival in the gastric mucosa of mice. Given that FRD, present in all H. pylori strains, is immunogenic in H. pylori -infected patients and H. pylori growth in vitro can be inhibited by three anthelmintics (morantel, oxantel and thiabendazole), this enzyme could potentially be used both as a novel drug target as well as in the development of vaccines for H. pylori prevention and eradication.

The contribution of mitochondrial respiratory complexes to the production of reactive oxygen species

This work was focused on distinguishing the contribution of mitochondrial redox complexes to the production of reactive oxygen species (ROS) during cellular respiration. We were able to accurately measure, for the first time, the basal production of ROS under uncoupled conditions by using a very sensitive method, based on the fluorescent probe dichlorodihydrofluorescein diacetate. The method also enabled the detection of the ROS generated by the oxidation of the endogenous substrates in the mitochondrial preparations and could be applied to both mitochondria and live cells. Contrary to the commonly accepted view that complex III (ubiquinol:cytochrome c reductase) is the major contributor to mitochondrial ROS production, we found that complex I (NADH-ubiquinone reductase) and complex II (succinate-ubiquinone reductase) are the predominant generators of ROS during prolonged respiration under uncoupled conditions. Complex II, in particular, appears to contribute to the basal production of ROS in cells.

The electron transport chain in anaerobically functioning eukaryotes

Many lower eukaryotes can survive anaerobic conditions via a fermentation pathway that involves the use of the reduction of endogenously produced fumarate as electron sink. This fumarate reduction is linked to electron transport in an especially adapted, anaerobically functioning electron-transport chain. An aerobic energy metabolism with Krebs cycle activity is accompanied by electron transfer from succinate to ubiquinone via complex II of the respiratory chain. On the other hand, in an anaerobic metabolism, where fumarate functions as terminal electron acceptor, electrons are transferred from rhodoquinone to fumarate, which is the reversed direction. Ubiquinone cannot replace rhodoquinone in the process of fumarate reduction in vivo, as ubiquinone can only accept electrons from complex II and cannot donate them to fumarate. Rhodoquinone, with its lower redox potential than ubiquinone, is capable of donating electrons to fumarate. Eukaryotic fumarate reductases were shown to interact with rhodoquinone (a benzoquinone), whereas most prokaryotic fumarate reductases interact with the naphtoquinones menaquinone and demethylmenaquinone. Fumarate reductase, the enzyme essential for the anaerobic functioning of many eukaryotes, is structurally very similar to succinate dehydrogenase, the Krebs cycle enzyme catalysing the reverse reaction. In prokaryotes these enzymes are differentially expressed depending on the external conditions. Evidence is now emerging that also in eukaryotes two different enzymes exist for succinate oxidation and fumarate reduction that are differentially expressed.

Physiological function and regulation of flavocytochrome c3, the soluble fumarate reductase from Shewanella putrefaciens NCIMB 400

Shewanella putrefaciens produces a soluble flavocytochrome c under anaerobic growth conditions. This protein shares sequence similarity with the catalytic subunits of membrane-bound fumarate reductases from Escherichia coli and other bacteria and the purified protein has fumarate reductase activity. It is shown here that this enzyme, flavocytochrome c3, is essential for fumarate respiration in vivo since disruption of the chromosomal fccA gene, which encodes flavocytochrome c3, leads to a specific loss of the ability to grow with fumarate as terminal electron acceptor. Growth with nitrate, trimethylamine N-oxide (TMAO) and other acceptors was unaffected. The fccA gene is transcribed as a 2 kb monocistronic mRNA. An adjacent reading frame that bears limited sequence similarity to one of the membrane anchor subunits of E. coli fumarate reductase is not co-transcribed with fccA. Expression of the fccA gene is regulated by anaerobiosis and by the availability of alternative electron acceptors, particularly nitrate and TMAO. DNA sequences have been identified that are required for this regulation.

Mutations in sdh (succinate dehydrogenase genes) alter the thiamine requirement of Salmonella typhimurium

Mutants lacking the first enzyme in de novo purine synthesis (PurF) can synthesize thiamine if increased levels of pantothenate are present in the culture medium (J. L. Enos-Berlage and D. M. Downs, J. Bacteriol. 178:1476-1479, 1996). Derivatives of purF mutants that no longer required pantothenate for thiamine-independent growth were isolated. Analysis of these mutants demonstrated that they were defective in succinate dehydrogenase (Sdh), an enzyme of the tricarboxylic acid cycle. Results of phenotypic analyses suggested that a defect in Sdh decreased the thiamine requirement of Salmonella typhimurium. This reduced requirement correlated with levels of succinyl-coenzyme A (succinyl-CoA), which is synthesized in a thiamine pyrophosphate-dependent reaction. The effect of succinyl-CoA on thiamine metabolism was distinct from the role of pantothenate in thiamine synthesis.

Inhibition of succinate:ubiquinone reductase and decrease of ubiquinol in nephrotoxic cysteine S-conjugate-induced oxidative cell injury

The role of complex II in the cellular protection against oxidative stress was investigated in freshly isolated rat renal proximal tubular cells (PTC) with the use of the nephrotoxin S-(1,2-dichlorovinyl)-L-cysteine (DCVC). DCVC caused oxidative stress in PTC as determined by flow cytometry with dihydrorhodamine-123; this fluorescent probe is readily oxidized by primary hydroperoxides such as those formed during lipid peroxidation. The oxidative stress could be prevented by inhibition of the beta-lyase-mediated formation and covalent binding to cellular macromolecules of reactive DCVC metabolites, with amino oxyacetic acid (AOA), or by the antioxidant N,N'-diphenyl-p-phenylenediamine. Both AOA and DPPD also prevented cell death. The DCVC-induced oxidative stress was associated with a decrease in the succinate:ubiquinone reductase (SQR) activity of complex II, whereas NADH:ubiquinone reductase activity of complex I remained unaffected. AOA prevented the effect on SQR activity, whereas N,N'-diphenyl-p-phenylenediamine did not. Inhibition of SQR activity with thenoyl trifluoracetone (TTFA) potentiated the DCVC-induced oxidative cell injury, suggesting the involvement of SQR activity in an antioxidant pathway. To investigate this in greater detail, PTC were treated with an inhibitor of cytochrome-c-oxidase, KCN, in a buffer containing glycine, which prevents cell death by KCN. Glycine did not affect cell death by DCVC. KCN prevented the DCVC-induced oxidative stress and cell death. KCN cytoprotection could be prevented by inhibition of SQR activity with oxaloacetate or TTFA, whereas inhibition of either complex I or III with rotenone and antimycin, respectively, did not prevent it. The effect of DCVC on complex II was associated with a decrease in the cellular amount of reduced ubiquinone (QH2); the KCN-mediated cytoprotection was related to a 60% increase of cellular QH2. Rotenone almost completely inhibited ubiquinone reduction even in the presence of KCN, whereas oxaloacetate in combination with KCN resulted in QH2 levels comparable to control. This suggests that the SQR activity by complex II rather than the cellular content of reduced ubiquinone (QH2) is important as a part of the cellular antioxidant machinery in the cyto-protection against oxidative stress.

Oxidative enzyme activity in the rat soleus muscle and its motoneurons during postnatal maturation

The effect of postnatal maturation on changes in the oxidative enzyme (succinate dehydrogenase) activity in the rat soleus muscle and its motoneurons was examined at 3, 6, and 12 weeks of age. The motoneurons innervating the soleus muscle were identified using the fluorescent retrograde neuronal tracer Nuclear Yellow. An inverse relationship between soma size and oxidative enzyme activity of soleus motoneurons was observed at 3 and 6 weeks of age, whereas there was no correlation between them at 12 weeks. Although the oxidative enzyme activity in the soleus muscle increased during postnatal maturation, it showed a decrease in the soleus motoneurons. These data demonstrate that the inverse relationship between soma size and oxidative enzyme activity of rat soleus motoneurons can only be detected in the early postnatal period and that the oxidative enzyme activity in the rat soleus muscle and its motoneurons can change independently during postnatal maturation.

Characterization of the excitotoxic potential of the reversible succinate dehydrogenase inhibitor malonate

Although the mechanism of neuronal death in neurodegenerative diseases remains unknown, it has been hypothesized that relatively minor metabolic defects may predispose neurons to N-methyl-D-aspartate (NMDA) receptor-mediated excitotoxic damage in these disorders. To further investigate this possibility, we have characterized the excitotoxic potential of the reversible succinate dehydrogenase (SDH) inhibitor malonate. After its intrastriatal stereotaxic injection into male Sprague-Dawley rats, malonate produced a dose-dependent lesion when assessed 3 days after surgery using cytochrome oxidase histochemistry. This lesion was attenuated by coadministration of excess succinate, indicating that it was caused by specific inhibition of SDH. The lesion was also prevented by administration of the noncompetitive NMDA antagonist MK-801. MK-801 did not induce hypothermia, and hypothermia itself was not neuroprotective, suggesting that the neuroprotective effect of MK-801 was due to blockade of the NMDA receptor ion channel and not to any nonspecific effect. The competitive NMDA antagonist LY274614 and the glycine site antagonist 7-chlorokynurenate also profoundly attenuated malonate neurotoxicity, further indicating an NMDA receptor-mediated event. Finally, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)-quinoxaline) was ineffective at preventing malonate toxicity at a dose that effectively reduced S-AMPA toxicity, indicating that non-NMDA receptors are involved minimally, if at all, in the production of the malonate lesion. We conclude that inhibition of SDH by malonate results in NMDA receptor-mediated excitotoxic neuronal death. If this mechanism of "secondary" or "weak" excitotoxicity plays a role in neurodegenerative disease, NMDA antagonists and other "antiexcitotoxic" strategies may have therapeutic potential for these diseases.

Effect of acute ethanol exposure on cultured fetal rat hepatocytes: relation to mitochondrial function

Studies from our laboratory have shown that short-term ethanol exposure inhibits epidermal growth factor-dependent replication of cultured fetal rat hepatocytes, along with a drop in ATP level, and that these effects could be caused, at least in part, by ethanol-induced oxidative stress. In these prior studies, mitochondrial morphology was abnormal and membrane lipid peroxidation products were increased, along with reduced transmembrane potential and enhanced permeability to sucrose. To define the effects of ethanol on mitochondrial function further, the present study examines the impact of ethanol exposure on mitochondrial electron transport chain components. A 24-hr exposure of cultured fetal rat hepatocytes to ethanol (2.5 mg/ml) reduced mitochondrial complex I activity by 16% (p < 0.05), complex IV by 28% (p < 0.05), and succinate dehydrogenase by 23% (p < 0.05). This reduction was paralleled by lower ADP translocase activity (24%, p < 0.05) and diminished mitochondrial glutathione (GSH) (20%, p < 0.05). Pretreatment with 0.1 mM S-adenosyl methionine, before ethanol exposure, normalized mitochondrial GSH along with activities of complex I, complex IV, and succinate dehydrogenase. A 3-hr exposure of isolated mitochondria (which do not metabolize ethanol) to ethanol (2.5 mg/ml), inhibited the activities of complex I (19%, p < 0.05), complex IV (24%, p < 0.05), and of ATP synthesis (20%, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and functional properties of fumarate reductase

Images

An evaluation of the measurement of the activities of complexes I-IV in the respiratory chain of human skeletal muscle mitochondria

The measurement of individual respiratory chain complexes is an important component of the investigation of diseases due to mitochondrial dysfunction. We have evaluated assays which measure complexes I to IV in human skeletal muscle mitochondria and in addition optimized these assays to provide sensitive and reliable diagnostic techniques, particularly in situations where a partial interruption at a single complex needs to identified. Using several established methods of membrane disruption we have found that optimal activities of complexes I and II are obtained by freeze-thawing the mitochondria in hypotonic potassium phosphate buffer, whereas complex III and IV activities are markedly increased by the addition of the detergent n-dodecyl-beta-D-maltoside. Complex I activity is measured in the presence of 2.5 mg.ml-1 bovine serum albumin, which increases rotenone sensitivity, and we have shown that NADH-cytochrome b5 reductase makes an important contribution to the rotenone-insensitive NADH-ubiquinone oxidoreductase activity. Complex II activity is measured after preincubation of the mitochondrial fraction with succinate to fully activate the complex. Complex I and III activities are dependent upon the length of the isoprenoid chain of the ubiquinone and ubiquinol, respectively. These assays have been used to establish a control range.

Hypertrophic cardiomyopathy with mitochondrial myopathy. A new phenotype of complex II defect

Two brothers, 25 and 19 years old, were affected by asymmetrical hypertrophic cardiomyopathy. The older brother had waddling gait and weakness of the proximal girdle muscles, while the younger had a broad-based gait and weakness of selected limb girdle muscles. EMG exam was myopathic. Serum enzyme, CPK and aldolase were elevated. Histochemical reactions in muscle revealed "core-like" areas, subsarcolemmal rims of mitochondria and lipid accumulation. Succinate-dehydrogenase stain showed a lack of activity in both biopsies, with the exception of intrafusal fibers. Microphotometric quantitative measurements confirmed the defect in both biopsies. Biochemical measurements of several mitochondrial enzymes in muscle showed a reduced activity of succinate-dehydrogenase (33%) and succinate-cytochrome C reductase (36-47%) which are both components of complex II. On myocardial biopsy lipid and mitochondrial abnormalities were found. This mitochondriopathy represents a new phenotype of partial complex II defect.

Diode-like behaviour of a mitochondrial electron-transport enzyme

In mitochondria, electrons derived from the oxidation of succinate by the tricarboxylic acid cycle enzyme succinate-ubiquinone oxido-reductase are transferred directly to the quinone pool. Here we provide evidence that the soluble form of this enzyme (succinate dehydrogenase) behaves as a diode that essentially allows electron flow in one direction only. The gating effect is observed when electrons are exchanged rapidly and directly between fully active succinate dehydrogenase and a graphite electrode. Turnover is therefore measured under conditions of continuously variable electrochemical potential. The otherwise rapid and efficient reduction of fumarate (the reverse reaction) is severely retarded as the driving force (overpotential) is increased. Such behaviour can arise if a rate-limiting chemical step like substrate binding or product release depends on the oxidation state of a redox group on the enzyme. The observation provides, for a biological electron-transport system, a simple demonstration of directionality that is enforced by kinetics as opposed to that which is assumed from thermodynamics.

Identification of a second gene involved in global regulation of fumarate reductase and other nitrate-controlled genes for anaerobic respiration in Escherichia coli

Fumarate reductase catalyzes the final step of anaerobic electron transport in Escherichia coli when fumarate is used as a terminal electron acceptor. Transcription of the fumarate reductase operon (frdABCD) was repressed when cells were grown in the presence of either of the preferred terminal electron acceptors, oxygen or nitrate, and was stimulated modestly by fumarate. We have previously identified a locus called frdR which pleiotropically affects nitrate repression of fumarate reductase, trimethylamine N-oxide reductase, and alcohol dehydrogenase gene expression and nitrate induction of nitrate reductase expression (L. V. Kalman and R. P. Gunsalus, J. Bacteriol. 170:623-629, 1988). Transformation of various frdR mutants with plasmids identified two complementation groups, indicating that the frdR locus is composed of two genes. One class of mutants was not completely restored to wild-type frdA-lacZ expression or nitrate reductase induction when complemented with multicopy narX+ plasmids, whereas low-copy narX+ plasmid-containing strains were. A second class of frdR mutants was identified and shown to correspond to a previously described gene, narL (frdR2). Complementation of these strains with multicopy narL+ plasmids resulted in superrepression of frdA-lacZ expression and moderate elevation of nitrate reductase expression. Multicopy plasmids containing both narL+ and narX+ or only narL+ were able to complement narL mutants, whereas narX+ plasmids complemented narX mutants only when present in a copy number approximately equal to that of narL. Both narL and narX mutants retained normal oxygen control of frdA-lacZ expression. Both types of mutants are pleiotropic, as evidenced by derepressed levels of the fumarate reductase and trimethylamine N-oxide reductase enzymes and by defective induction of nitrate reductase when cells were grown in the presence of nitrate. These results indicate that both the narL and narX gene products must be present in a defined ratio in the cell. We conclude that these proteins interact to effect normal nitrate control of the anaerobic electron transport-associated operons. From these studies, we propose that narX encodes a nitrate sensor protein while narL encodes a DNA-binding regulatory protein which together function in a manner analogous to other two-component regulatory systems.

The effects of recessive lethal Notch mutations of Drosophila melanogaster on flavoprotein enzyme activities whose inhibitions cause Notch-like phenocopies

The biochemical action of the Notch locus whose mutants cause morphological aberrations in flies, viz., notches of wings and bristle multiplication, has been analyzed by the addition to the food medium of enzyme inhibitors causing phenocopies of Notch and by comparison of enzyme activity patterns of Notch mutants with different degrees of phenotypic expression. Notch phenocopies were induced by inhibitors of enzyme activities in two biochemical pathways: the de novo pyrimidine synthesis by 5-methylorotate (inhibitor of dihydroorotate dehydrogenase) and the choline shunt by amobarbital (inhibits choline dehydrogenase) and methoxyacetate (inhibits sarcosine dehydrogenase). The inhibition of de novo pyrimidine synthesis prevents the production of deoxyuridine-5-phosphate, the substrate for the synthesis of thymidine-5-phosphate via thymidylate synthase, whereas the inhibition of the choline shunt prevents the production of HCHO groups and glycine, both of which are involved in the synthesis of 5,10-methylenetetrahydrofolate, which is a cofactor of thymidylate synthase. It was already known that the inhibition of the latter enzyme in vivo induces Notch phenocopies. Notch mutants with a strong morphological expression show low enzyme activities for dihydroorotate dehydrogenase and choline dehydrogenase. Both are flavoprotein enzymes linked to the respiratory chain. The correspondence between the low enzyme activities in Notch mutants with a strong morphological expression and the phenocopying effect of antimetabolites on these enzymes in the two biochemical pathways involved strongly suggests that the morphological effects of Notch on flies are a consequence of lowered activities of choline dehydrogenase and dihydroorotate dehydrogenase.