Characterization of composite aminodeoxyisochorismate synthase and aminodeoxyisochorismate lyase activities of anthranilate synthase.

Anthranilate synthase [chorismate pyruvatelyase (amino-accepting), E.C.4.1.3.27] catalyzes the formation of anthranilate (o-aminobenzoate) and pyruvic acid from chorismate and glutamine. A mutant form of the enzyme from Salmonella typhimurium accumulates a compound that we had isolated and identified as trans-6-amino-5-[(1-carboxyethenyl)-oxy]-1,3- cyclohexadiene-1-carboxylic acid, commonly called aminodeoxyisochorismate (ADIC). Here we report that ADIC is formed by a reversible, Mg(2+)-dependent ADIC synthase activity of anthranilate synthase that can be functionally uncoupled from a Mg(2+)-dependent ADIC lyase activity of the enzyme by single amino acid substitutions in the TrpE subunit of the anthranilate synthase complex of S. typhimurium. Both of the component activities of the enzyme are sensitive to feedback inhibition by L-tryptophan. Purified ADIC is quantitatively converted to anthranilate and pyruvic acid by the ADIC lyase activity of wild-type anthranilate synthase. ADIC also serves as a substrate for the formation of chorismate by the enzyme in the absence of glutamine and (NH4)2SO4. The rate of ADIC formation by the mutant enzyme and the steady-state parameters for ADIC utilization by the wild-type enzyme are consistent with a role for ADIC as an enzyme-bound intermediate that does not accumulate during the course of the anthranilate synthase reaction. The altered catalytic specificity of mutant anthranilate synthase enzymes suggests a potential role for ADIC in secondary metabolism.     

Designer enediynes generate DNA breaks, interstrand cross-links, or both, with concomitant changes in the regulation of DNA damage responses

The ability of the radiomimetic anticancer enediyne C-1027 to induce ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR)-independent damage responses was discovered to reside in its unique ability to concurrently generate robust amounts of double-strand breaks (DSBs) and interstrand cross-links (ICLs) in cellular DNA. Furthermore, a single substitution to the chromophore's benzoxazolinate moiety shifted DNA damage to primarily ICLs and an ATR- but not ATM-dependent damage response. In contrast, single substitutions of the chromophore's beta-amino acid component shifted DNA damage to primarily DSBs, consistent with its induction of conventional ATM-dependent damage responses of the type generated by ionizing radiation and other radiomimetics. Thus, phosphatidylinositol 3-kinase-like protein kinase regulation of DNA damage responses is dictated by the relative proportions of DSBs and ICLs.     

Folate synthesis in plants: the p-aminobenzoate branch is initiated by a bifunctional PabA-PabB protein that is targeted to plastids

It is not known how plants synthesize the p-aminobenzoate (PABA) moiety of folates. In Escherichia coli, PABA is made from chorismate in two steps. First, the PabA and PabB proteins interact to catalyze transfer of the amide nitrogen of glutamine to chorismate, forming 4-amino-4-deoxychorismate (ADC). The PabC protein then mediates elimination of pyruvate and aromatization to give PABA. Fungi, actinomycetes, and Plasmodium spp. also synthesize PABA but have proteins comprising fused domains homologous to PabA and PabB. These bipartite proteins are commonly called "PABA synthases," although it is unclear whether they produce PABA or ADC. Genomic approaches identified Arabidopsis and tomato cDNAs encoding bipartite proteins containing fused PabA and PabB domains, plus a putative chloroplast targeting peptide. These cDNAs encode functional enzymes, as demonstrated by complementation of an E. coli pabA pabB double mutant and a yeast PABA-synthase deletant. The partially purified recombinant Arabidopsis protein did not produce PABA unless the E. coli PabC enzyme was added, indicating that it forms ADC, not PABA. The enzyme behaved as a monomer in size-exclusion chromatography and was not inhibited by physiological concentrations of PABA, its glucose ester, or folates. When the putative targeting peptide was fused to GFP and expressed in protoplasts, the fusion protein appeared only in chloroplasts, indicating that PABA synthesis is plastidial. In the pericarp of tomato fruit, the PabA-PabB mRNA level fell drastically as ripening advanced, but there was no fall in total PABA content, which stayed between 0.7 and 2.3 nmol.g(-1) fresh weight.     

A phosphopantetheinylating polyketide synthase producing a linear polyene to initiate enediyne antitumor antibiotic biosynthesis

The enediynes, unified by their unique molecular architecture and mode of action, represent some of the most potent anticancer drugs ever discovered. The biosynthesis of the enediyne core has been predicted to be initiated by a polyketide synthase (PKS) that is distinct from all known PKSs. Characterization of the enediyne PKS involved in C-1027 (SgcE) and neocarzinostatin (NcsE) biosynthesis has now revealed that (i) the PKSs contain a central acyl carrier protein domain and C-terminal phosphopantetheinyl transferase domain; (ii) the PKSs are functional in heterologous hosts, and coexpression with an enediyne thioesterase gene produces the first isolable compound, 1,3,5,7,9,11,13-pentadecaheptaene, in enediyne core biosynthesis; and (iii) the findings for SgcE and NcsE are likely shared among all nine-membered enediynes, thereby supporting a common mechanism to initiate enediyne biosynthesis.     

Rapid PCR amplification of minimal enediyne polyketide synthase cassettes leads to a predictive familial classification model

A universal PCR method for the rapid amplification of minimal enediyne polyketide synthase (PKS) genes and the application of this methodology to clone remaining prototypical genes from producers of structurally determined enediynes in both family types are presented. A phylogenetic analysis of the new pool of bona fide enediyne PKS genes, consisting of three from 9-membered producers (neocarzinostatin, C1027, and maduropeptin) and three from 10-membered producers (calicheamicin, dynemicin, and esperamicin), reveals a clear genotypic distinction between the two structural families from which to form a predictive model. The results from this study support the postulation that the minimal enediyne PKS helps define the structural divergence of the enediyne core and provides the key tools for generating enediyne hybrid genes/molecular scaffolds; by using the model, a classification is also provided for the unknown enediyne PKS genes previously identified via genome scanning.     

Studies of the Mechanism of Anthranilate Synthase Reaction

The enzyme anthranilate synthase catalyzes the formation of anthranilate form either chorismate and glutamine or chorismate and ammonia. In the aromatization of chorismate, a hydroxyl group and an enolpyruvyl group must be eliminated. Elimination of the enolpyruvyl group of chorismate is accompanied by protonation to form pyruvate. The source of this proton was investigated by performing the enzymatic reaction in 99.7 per cent D(2)O.The isolated pyruvate contained close to an atom of deuterium in the methyl group. High resolution mass spectra also revealed that about 6 per cent of the deuterio pyruvate contains a -CHD(2) species. Thus, the results obtained conclusively demonstrate that in the formation of the pyruvate, the third hydrogen of the methyl group arises from water and not by intramolecular shift of a hydrogen from the ring of chorismate.     

p-Aminobenzoate synthesis in Escherichia coli: purification and characterization of PabB as aminodeoxychorismate synthase and enzyme X as aminodeoxychorismate lyase.

The Escherichia coli pabA and pabB genes have been overexpressed separately and in tandem. Using purified PabB, we have confirmed the recent suggestion that PabB needs an additional protein, enzyme X, to convert chorismate and NH3 to p-aminobenzoate (PABA). With chorismate and NH3, pure PabB generates an intermediate presumed to be 4-amino-4-deoxychorismate based upon UV/visible spectroscopy and enzymatic and nonenzymatic transformations. The PabB-catalyzed interconversion of chorismate and isolated aminodeoxychorismate is readily reversible. With pure PabB as a stoichiometric assay reagent, enzyme X was purified approximately 800-fold to near homogeneity as an apparent homodimer of 50 kDa from E. coli. Enzyme X shows no activity on chorismate but quantitatively converts the preformed aminodeoxychorismate into p-aminobenzoate and pyruvate, acting thereby as an aminodeoxychorismate lyase.     

Kedarcidin chromophore: an enediyne that cleaves DNA in a sequence-specific manner.

Kedarcidin chromophore is a 9-membered enediyne, recently isolated from an actinomycete strain. In vivo studies show this molecule to be extremely active against P388 leukemia and B16 melanoma. Cytotoxicity assays on the HCT116 colon carcinoma cell line result in an IC50 value of 1 nM. In vitro experiments with phi X174, pM2 DNA, and 32P-end-labeled restriction fragments demonstrate that this chromophore binds and cleaves duplex DNA with a remarkable sequence selectivity producing single-strand breaks. The cleavage chemistry requires reducing agents and oxygen similar to the other naturally occurring enediynes. Certain cations (Ca2+ and Mg2+) prevent strand cleavage. High-resolution 1H NMR studies on the chromophore in the presence of calcium chloride implicate the 2-hydroxynaphthoyl moiety in DNA binding. Interestingly, the kedarcidin chromophore appears structurally related to neocarzinostatin yet recognizes specific DNA sequences in a manner similar to calicheamicin gamma 1I, an enediyne with a significantly different structure. Moreover, kedarcidin and calicheamicin share a DNA preferred site, the TCCTN-mer. These observations indicate that the individual structural features of these agents are not solely responsible for their DNA selectivity. Rather, a complementarity between their overall tertiary structure and the local conformation of the DNA at the binding sites must play a significant role in the recognition process.     

Metabolic strategies for the degradation of the neuromodulator agmatine in mammals

Agmatine (1-amino-4-guanidinobutane), a precursor for polyamine biosynthesis, has been identified as an important neuromodulator with anticonvulsant, antineurotoxic and antidepressant actions in the brain. In this context it has emerged as an important mediator of addiction/satiety pathways associated with alcohol misuse. Consequently, the regulation of the activity of key enzymes in agmatine metabolism is an attractive strategy to combat alcoholism and related addiction disorders.Agmatine results from the decarboxylation of L-arginine in a reaction catalyzed by arginine decarboxylase (ADC), and can be converted to either guanidine butyraldehyde by diamine oxidase (DAO) or putrescine and urea by the enzyme agmatinase (AGM) or the more recently identified AGM-like protein (ALP). In rat brain, agmatine, AGM and ALP are predominantly localised in areas associated with roles in appetitive and craving (drug-reinstatement) behaviors. Thus, inhibitors of AGM or ALP are promising agents for the treatment of addictions. In this review, the properties of DAO, AGM and ALP are discussed with a view to their role in the agmatine metabolism in mammals.     

The structures of anthranilate synthase of Serratia marcescens crystallized in the presence of (i) its substrates, chorismate and glutamine, and a product, glutamate, and (ii) its end-product inhibitor, L-tryptophan

The crystal structure of anthranilate synthase (AS) from Serratia marcescens, a mesophilic bacterium, has been solved in the presence of its substrates, chorismate and glutamine, and one product, glutamate, at 1.95 A, and with its bound feedback inhibitor, tryptophan, at 2.4 A. In comparison with the AS structure from the hyperthermophile Sulfolobus solfataricus, the S. marcescens structure shows similar subunit structures but a markedly different oligomeric organization. One crystal form of the S. marcescens enzyme displays a bound pyruvate as well as a putative anthranilate (the nitrogen group is ambiguous) in the TrpE subunit. It also confirms the presence of a covalently bound glutamyl thioester intermediate in the TrpG subunit. The tryptophan-bound form reveals that the inhibitor binds at a site distinct from that of the substrate, chorismate. Bound tryptophan appears to prevent chorismate binding by a demonstrable conformational effect, and the structure reveals how occupancy of only one of the two feedback inhibition sites can immobilize the catalytic activity of both TrpE subunits. The presence of effectors in the structure provides a view of the locations of some of the amino acid residues in the active sites. Our findings are discussed in terms of the previously described AS structure of S. solfataricus, mutational data obtained from enteric bacteria, and the enzyme's mechanism of action.     

Melatonin attenuates myocardial ischemia‐reperfusion injury via improving mitochondrial fusion/mitophagy and activating the AMPK‐OPA1 signaling pathways

Optic atrophy 1 (OPA1)‐related mitochondrial fusion and mitophagy are vital to sustain mitochondrial homeostasis under stress conditions. However, no study has confirmed whether OPA1‐related mitochondrial fusion/mitophagy is activated by melatonin and, consequently, attenuates cardiomyocyte death and mitochondrial stress in the setting of cardiac ischemia‐reperfusion (I/R) injury. Our results indicated that OPA1, mitochondrial fusion, and mitophagy were significantly repressed by I/R injury, accompanied by infarction area expansion, heart dysfunction, myocardial inflammation, and cardiomyocyte oxidative stress. However, melatonin treatment maintained myocardial function and cardiomyocyte viability, and these effects were highly dependent on OPA1‐related mitochondrial fusion/mitophagy. At the molecular level, OPA1‐related mitochondrial fusion/mitophagy, which was normalized by melatonin, substantially rectified the excessive mitochondrial fission, promoted mitochondria energy metabolism, sustained mitochondrial function, and blocked cardiomyocyte caspase‐9‐involved mitochondrial apoptosis. However, genetic approaches with a cardiac‐specific knockout of OPA1 abolished the beneficial effects of melatonin on cardiomyocyte survival and mitochondrial homeostasis in vivo and in vitro. Furthermore, we demonstrated that melatonin affected OPA1 stabilization via the AMPK signaling pathway and that blockade of AMPK repressed OPA1 expression and compromised the cardioprotective action of melatonin. Overall, our results confirm that OPA1‐related mitochondrial fusion/mitophagy is actually modulated by melatonin in the setting of cardiac I/R injury. Moreover, manipulation of the AMPK‐OPA1‐mitochondrial fusion/mitophagy axis via melatonin may be a novel therapeutic approach to reduce cardiac I/R injury.     

Sulfonamide resistance mechanism in Escherichia coli: R plasmids can determine sulfonamide-resistant dihydropteroate synthases.

Several natural isolate E. coli strains highly resistant to sulfonamides and antibiotics are shown to contain a sulfonamide-resistant dihydropteroate synthase (2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine-diphosphate:4-aminobenzoate 2-amino-4-hydroxydihydropteridine-6-methenyltransferase, EC 2.5.1.15) in addition to the normal sensitive enzyme. The resistant dihydropteroate synthases examined are determined by an R plasmid and are smaller and less heat stable than the normal sulfonamide-sensitive enzyme. One synthase resistant to any sulfonamide tested, and to sulfanilic and arsanilic acids, was still inhibited by several non-sulfonamide analogs of p-aminobenzoate. Citrobacter and Klebsiella pneumoniae strains also show similar mechanisms of sulfonamide resistance.     

Cell-specific regulation of apoptosis by designed enediynes.

The naturally occurring enediyne antibiotics are a unique class of antitumor drugs that combine reactive enediynes with additional structural features conferring affinity for DNA. Dynemicin A, in which an enediyne core is attached to an anthraquinone group capable of DNA intercalation, readily cleaves double-stranded DNA. This activity is thought to be the basis of its potent antitumor cytotoxicity. To investigate cell-specific mechanisms of cytotoxicity in the absence of DNA affinity, we have synthesized a variety of dynemicin-like enediynes that lack the anthraquinone moiety. We have found that the cytotoxicity of these compounds is dependent on their chemical instability and their enantiomeric form. Their selective toxicity results from a potent induction of apoptosis primarily in human leukemic cells. A group of synthetic enediynes were designed to be highly stable. These compounds were found to inhibit apoptotic cell death. This inhibition was observed in competition with the chemically unstable enediynes, including dynemicin and calicheamicin. The stable synthetic enediynes could also block the apoptotic morphology induced by unrelated cytotoxic agents such as cycloheximide, actinomycin D, and ultraviolet radiation. The results suggest that the cellular target(s) of synthetic enediynes may play a central role in regulating programmed cell death; a specific receptor-ligand interaction is proposed.     

Nitric oxide and central antihypertensive drugs: one more difference between catecholamines and imidazolines

NO is known to be involved in the peripheral and central regulation of the cardiovascular function. It plays a neuromodulatory role via a direct action on presynaptic nerve terminals, stimulating the release of gamma-aminobutyric acid, glutamate, and norepinephrine. Our aim was to study the possible role of NO in the cardiovascular effects of the central antihypertensive drugs clonidine, rilmenidine, and alpha-methyl-norepinephrine (alpha-MNA). Sites and mechanisms of the hypotensive action of these drugs were different; clonidine and rilmenidine acted on imidazoline receptors in the nucleus reticularis lateralis, whereas alpha-MNA acted upon alpha(2)-adrenoceptors in the nucleus tractus solitarius. The influence of N:(G)-nitro-L-arginine, an NO synthase inhibitor, on the central hypotensive effects of these drugs was investigated in pentobarbital-anesthetized rabbits. The intracisternal (IC) administration of alpha-MNA (30 microg/kg) induced hypotension (79+/-2 versus 103+/-4 mm Hg) and bradycardia (222+/-8 versus 278+/-4 bpm) (P:<0.05) (n=5). Clonidine (0.07 microg/kg IC) also induced hypotension (69+/-5 versus 99+/-4 mm Hg) and bradycardia (266+/-7 versus 306+/-10 bpm) (P:<0.05) (n=5). In addition to clonidine, rilmenidine (1 microg/kg IC) induced hypotension (64+/-4 versus 97+/-4 mm Hg) and bradycardia (264+/-11 versus 310+/-4 bpm) (P:<0.05) (n=5). Pretreatment with N:(G)-nitro-L-arginine (900 microg/kg IC) completely prevented the hypotensive effect of alpha-MNA but influenced the cardiovascular effects of neither clonidine nor rilmenidine. These results confirm that imidazoline drugs, such as clonidine, rilmenidine, and the catecholamine alpha(2)-adrenoceptor agonist alpha-MNA, have distinct mechanisms of action.     

The crystal structure of anthranilate synthase from Sulfolobus solfataricus: functional implications.

Anthranilate synthase catalyzes the synthesis of anthranilate from chorismate and glutamine and is feedback-inhibited by tryptophan. The enzyme of the hyperthermophile Sulfolobus solfataricus has been crystallized in the absence of physiological ligands, and its three-dimensional structure has been determined at 2.5-A resolution with x-ray crystallography. It is a heterotetramer of anthranilate synthase (TrpE) and glutamine amidotransferase (TrpG) subunits, in which two TrpG:TrpE protomers associate mainly via the TrpG subunits. The small TrpG subunit (195 residues) has the known "triad" glutamine amidotransferase fold. The large TrpE subunit (421 residues) has a novel fold. It displays a cleft between two domains, the tips of which contact the TrpG subunit across its active site. Clusters of catalytically essential residues are located inside the cleft, spatially separated from clustered residues involved in feedback inhibition. The structure suggests a model in which chorismate binding triggers a relative movement of the two domain tips of the TrpE subunit, activating the TrpG subunit and creating a channel for passage of ammonia toward the active site of the TrpE subunit. Tryptophan presumably blocks this rearrangement, thus stabilizing the inactive states of both subunits. The structure of the TrpE subunit is a likely prototype for the related enzymes 4-amino 4-deoxychorismate synthase and isochorismate synthase.     

A kinesin signaling complex mediates the ability of GSK-3β to affect mood-associated behaviors

Lithium has been the gold standard in the treatment of bipolar disorder (BPD) for 60 y. Like lithium, glycogen synthase kinase 3 (GSK-3) inhibitors display both antimanic-like and antidepressant-like effects in some animal models. However, the molecular mechanisms of both lithium and GSK-3 inhibitors remain unclear. Here we show that the GSK-3 inhibitor AR-A014418 regulated α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)-induced GluR1 and GluR2 internalization via phosphorylation of kinesin light chain 2 (KLC2), the key molecule of the kinesin cargo delivery system. Specifically, AMPA stimulation triggered serine phosphorylation of KLC2 and, subsequently, the dissociation of the GluR1/KLC2 protein complex. This suggests that GSK-3 phosphorylation of KLC2 led to the dissociation of AMPA-containing vesicles from the kinesin cargo system. The peptide TAT-KLCpCDK, a specific inhibitor for KLC2 phosphorylation by GSK-3β, reduced the formation of long-term depression. Furthermore, the TAT-KLCpCDK peptide showed antimanic-like effects similar to lithium''s on amphetamine-induced hyperactivity, a frequently used animal model of mania. It also induced antidepressant-like effects in the tail suspension and forced swim tests, two commonly used animal models of depression. Taken together, the results demonstrated that KLC2 is a cellular target of GSK-3β capable of regulating synaptic plasticity, particularly AMPA receptor trafficking, as well as mood-associated behaviors in animal models. The kinesin cargo system may provide valuable novel targets for the development of new therapeutics for mood disorders.     

R68070, a combined thromboxane/endoperoxide receptor antagonist and thromboxane synthase inhibitor, inhibits human platelet activation in vitro and in vivo: a comparison with aspirin

We have investigated the effects of R68070 on platelet function in vitro and in vivo. The drug inhibits U46619-induced aggregation (IC50 = 1.2 x 10(-6) mol/L), blocks serum thromboxane formation (IC50 = 1 x 10(-7) mol/L), and increases serum prostaglandin (PG)E2 and 6-keto-PGF1 alpha levels, indicating that it combines thromboxane receptor blocking and thromboxane synthase inhibiting properties. The thromboxane-dependent aggregation of blood platelets is blocked by R68070, whereas no inhibition of thromboxane independent pathways occurs. A double-blind, randomized, cross-over study was performed on nine volunteers, comparing 400 mg placebo, 400 mg aspirin, and 400 mg R68070. Thromboxane-dependent aggregations were significantly inhibited by R68070 and by aspirin, the latter still having the most pronounced action. However, R68070 was clearly more powerful than aspirin (P less than .0005) in prolonging the bleeding time. Serum TxB2 formation was completely inhibited with both treatments, whereas serum 6-keto-PGF1 alpha and PGE2 and intralesional 6-keto-PGF1 alpha were inhibited after aspirin and stimulated after R68070. We conclude that R68070 inhibits platelet thromboxane synthase and its thromboxane receptor both in vitro and in vivo; local reorientation of cyclic endoperoxide metabolism toward prostacyclin induces a stronger inhibition of hemostasis than that produced by aspirin.     

Predicting Methadone Maintenance Treatment Outcomes Using the Addiction Severity Index and the MMPI‐2 Content Scales (Negative Treatment Indicators and Cynism Scales)

We studied the ability of the Minnesota Multiphasic Personality Inventory‐2 Content Scales (Negative Treatment Indicators [TRT] and Cynicism [CYN]) and the domain scales of the Addiction Severity Index (ASI) in predicting outcome from a methadone maintenance program. Participants were 108 African American males treated in a VA health care outpatient methadone maintenance treatment program and followed for up to 1 year after admission. Dependent variables were 1) length of stay and the percentage of 2) missed medication days, 3) toxicology urine samples free from illicit drugs, 4) full‐time employment, 5) attendance at scheduled counseling sessions, and 6) counselor ratings of patient progress. A stepwise linear regression equation indicated that low drug severity scores on the ASI and low scores on percentage of missed medication predicted patients who were clean 1 year later; low scores on the psychological domain of the ASI predicted attendance at counseling sessions; a discriminant function analysis (consisting of percent of missed medication, percentage of clean urines, and ratings of patient progress) successfully predicted patient status (i.e., dropouts vs. “active patients”) with 85% accuracy. Although the TRT and CYN were related to some ASI domains, they were not associated with any outcome variable. Results suggest that some ASI scores serve as important indicators of patient progress in methadone maintenance treatment.     

Inhibition of citrate synthase by oleoyl-CoA: a regulatory phenomenon.

Fatty acyl-CoAs are good detergents (dritical micelle concentrations = 3-4 muM) and can inhibit a number of enzymes, including some involved in fatty acid biosynthesis. The regulatory significance of fatty acyl-CoAs as negative effectors has been questioned largely because of the difficulties in distinguishing possible nonspecific detergent effects from more specific regulatory interactions with these enzymes. A new analogue of oleoyl-CoA, oleoyl-(1, N6-etheno)-CoA, is a better detergent (critical micelle concentration = 3.2 muM) than oleoyl-CoA (critical micelle concentration = 4.7 muM). This new analogue is not as good (by an order of magnitude) an inhibitor of citrate synthase [citrate oxaloacetatelyase (pro-3S-CH2-COO-vectoracetyl-CoA); EC 4.1.3.7] nor is it bound as well oleoyl-CoA. Since the only difference between these two compounds is substitution of 1,N6-ethenoadenine for the adenine of CoA, the difference in inhibition and binding implies a specific interaction between the adenine moiety of oleoyl-CoA and citrate synthase. Moreover, since oleoyl-(1,N6-etheno)CoA is a better detergent than oleoyl-CoA, the detergency of oleoyl-CoA is not the sole cause of the fatty acyl-CoA inhibition of citrate synthase. These results support a physiological role for oleoyl-CoA as a negative effector for citrate synthase. An analogous physiological role for fatty acyl-CoA as negative effectors for other enzymes seems reasonable.