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PubMed2005

Isopentenyl diphosphate isomerase. Mechanism-based inhibition by diene analogues of isopentenyl diphosphate and dimethylallyl diphosphate.

Isopentenyl diphosphate isomerase (IDI) catalyzes the interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This is an essential step in the mevalonate entry into the isoprenoid biosynthetic pathway. The isomerization catalyzed by type I IDI involves protonation of the carbon-carbon double bond in IPP or DMAPP to form a tertiary carbocation, followed by deprotonation. Diene analogues for DMAPP (E-2-OPP and Z-2-OPP) and IPP (4-OPP) were synthesized and found to be potent active-site-directed irreversible inhibitors of the enzyme. X-ray analysis of the E.I complex between Escherichia coli IDI and 4-OPP reveals the presence of two isomers that differ in the stereochemistry of the newly formed C3-C4 double bond in the hydrocarbon chain of the inhibitor. In both adducts C5 of the inhibitor is joined to the sulfur of C67. In these structures the methyl group formed upon protonation of the diene moiety in 4-OPP is located near E116, implicating that residue in the protonation step.

Carbon-Carbon Double Bond Isomerases↗
PubMed2006

Escherichia coli type I isopentenyl diphosphate isomerase: structural and catalytic roles for divalent metals.

Isopentenyl diphosphate isomerase (IDI) catalyzes the essential conversion of isopentenyl diphosphate (IPP) to dimethylallyl diphosphate (DMAPP) in the mevalonate entry into the isoprenoid biosynthetic pathway. Two convergently evolved forms of IDI are known. Type I IDI, which is found in Eukarya and many Bacteria, catalyzes the isomerization of IPP and DMAPP by a protonation-deprotonation mechanism. The enzyme requires two divalent metal ions for activity. An X-ray structure of type I IDI from crystals soaked with (N,N-dimethylamino)-1-ethyl diphosphate (NIPP), a potent transition-state analogue for the carbocationic intermediate in the isomerization reaction, shows one of the metals in a His(3)Glu(2) hexacoordinate binding site, while the other forms a bridge between the diphosphate moiety of the substrate and the enzyme (Wouters, J.; et al. J. Biol. Chem. 2003, 278, 11903). Reconstitution of metal-free recombinant Escherichia coli type I IDI with several divalent metals-Mg(2+), Mn(2+), Zn(2+), Co(2+), Ni(2+), and Cd(2+)-generated active enzyme. Freshly purified IDI contained substoichiometric levels of a single metal ion, presumably bound in the hexacoordinate site. When NIPP was added to the disruption and purification buffers of enzyme, the purified protein contained 0.72 equiv of Mg(2+), 0.92 equiv of Zn(2+), and 0.10 equiv of Mn(2+). These results are consistent with a structure in which Mg(2+) facilitates diphosphate binding and Zn(2+) or Mn(2+) occupies the hexacoordinate site.

Buffers↗
PubMed1987

Activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase does not respond to ubiquinone uptake in cultured cells.

The cellular content of ubiquinone was increased approx. 10-fold by incubation of neuroblastoma cells in medium containing exogenous ubiquinone. Under these conditions the activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, assayed after preincubation of cell homogenates with or without fluoride, was not suppressed. Similar results were obtained with human skin fibroblast cultures to which free ubiquinone or low-density lipoprotein-ubiquinone complex had been added. Consistent with the lack of suppression of HMG-CoA reductase, the rate of incorporation of [1-14C] acetate into ubiquinone was not diminished in cells exposed to exogenous ubiquinone. Measurements of [3H]mevalonolactone incorporation into cellular ubiquinones indicated that exogenous ubiquinone did not affect ubiquinone synthesis at a point in the pathway distal to the formation of mevalonate. The results suggest that cultured mammalian cells lack an end-product 'feedback' mechanism for regulation of HMG-CoA reductase in response to ubiquinone uptake.

Animals↗
PubMed2005

Drug-induced expansion and differentiation of V gamma 9V delta 2 T cells in vivo: the role of exogenous IL-2.

Human Vgamma9Vdelta2 T cells recognize nonpeptidic Ags generated by the 1-deoxy-d-xylulose 5-phosphate (many eubacteria, algae, plants, and Apicomplexa) and mevalonate (eukaryotes, archaebacteria, and certain eubacteria) pathways of isoprenoid synthesis. The potent Vgamma9Vdelta2 T cell reactivity 1) against certain cancer cells or 2) induced by infectious agents indicates that therapeutic augmentations of Vgamma9Vdelta2 T cell activities may be clinically beneficial. The functional characteristics of Vgamma9Vdelta2 T cells from Macaca fascicularis (cynomolgus monkey) are very similar to those from Homo sapiens. We have found that the i.v. administration of nitrogen-containing bisphosphonate or pyrophosphomonoester drugs into cynomolgus monkeys combined with s.c. low-dose (6 x 10(5) U/animal) IL-2 induces a large pool of CD27+ and CD27- effector/memory T cells in the peripheral blood of treated animals. The administration of these drugs in the absence of IL-2 is substantially less effective, indicating the importance of additional exogenous costimuli. Shortly after the costimulatory IL-2 treatment, only gammadelta (but not alphabeta) T cells expressed the CD69 activation marker, indicating that Vgamma9Vdelta2 T lymphocytes are more responsive to low-dose IL-2 than alphabeta T cells. Up to 100-fold increases in the numbers of peripheral blood Vgamma9Vdelta2 T cells were observed in animals receiving the gammadelta stimulatory drug plus IL-2. Moreover, the expanded Vgamma9Vdelta2 T cells were potent Th1 effectors capable of releasing large amounts of IFN-gamma. These results may be relevant for designing novel (or modifying current) immunotherapeutic trials with nitrogen-containing bisphosphonate or pyrophosphomonoester drugs.

2,3-Diphosphoglycerate↗
PubMed1991

Plasma membrane sphingomyelin and the regulation of HMG-CoA reductase activity and cholesterol biosynthesis in cell cultures.

We have examined the mechanism of the inhibition of cholesterol synthesis in cells treated with exogenous sphingomyelinase. Treatment of rat intestinal epithelial cells (IEC-6), human skin fibroblasts (GM-43), and human hepatoma (HepG2) cells in culture with sphingomyelinase resulted in a concentration- and time-dependent inhibition of the activity of HMG-CoA reductase, a key regulatory enzyme in cholesterol biosynthesis. The following observations were obtained with IEC-6 cells. Free fatty acid synthesis or general cellular protein synthesis was unaffected by the addition of sphingomyelinase. Addition of sphingomyelinase to the in vitro reductase assay had no effect on activity, suggesting that an intact cell system is required for the action of sphingomyelinase. The products of sphingomyelin hydrolysis, e.g., ceramide and phosphocholine, had no effect on reductase activity. Sphingosine, a further product of ceramide metabolism, caused a stimulation of reductase activity. Examination of the incorporation of [3H]acetate into the nonsaponifiable lipid fractions in the presence of sphingomyelinase showed no changes in the percent distribution of radioactivity in the post-mevalonate intermediates of the cholesterol biosynthetic pathway, but there was increased radioactivity associated with the polar sterol fraction. Pretreatment of cells with ketoconazole, a known inhibitor of oxysterol formation, prevented the inhibition of reductase activity by sphingomyelinase and decreased the incorporation of [3H]acetate in the polar sterol fraction. Ketoconazole had no effect on exogenous sphingomyelinase activity in vitro in the presence or absence of cells. Endogenous sphingomyelinase activity was also unaffected by ketoconazole. Addition of inhibitors of endogenous sphingomyelinase activity, e.g., chlorpromazine, desipramine, and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7), to the culture medium caused a dose-dependent stimulation of reductase activity. However, these agents had no effect on the inhibition of reductase activity by exogenous sphingomyelinase. Treatment of cells with small unilamellar vesicles of dioleyl phosphatidylcholine or high density lipoprotein3 resulted in increased efflux of cholesterol and stimulation of reductase activity. Under similar conditions, the inhibitory effect of exogenous sphingomyelinase on reductase activity was prevented by incubation with small unilamellar vesicles of phosphatidylcholine or high density lipoprotein. These results support the hypothesis that alteration of the ratio of sphingomyelin:cholesterol in the plasma membrane plays a modulatory role on the flow of membrane cholesterol to a site where it may be converted to a putative regulatory molecule, possibly an oxysterol.

Animals↗
PubMed1983

The modulating effect of an inhibitor of cholesterolgenesis present in bovine milk upon the synthesis of cholesterol, dolichol and ubiquinone.

Bovine milk contains two inhibitors of hepatic cholesterol genesis. One of these, identified as orotic acid, influences the early segment of the cholesterol biosynthetic pathway and suppresses the conversion of acetate to mevalonate. In this study the other inhibitor was shown to curtail the formation of compounds past farnesyl pyrophosphate on the squalene-cholesterol branch of the pathway. Thus cholesterol synthesis may be suppressed while the production of two other products of the branched pathway, dolichol and ubiquinone, is allowed to continue. The possible role of these ingested regulators in the metabolism of the young until they achieve sufficient development is discussed.

Acetates↗
PubMed2001

Inherited disorders of cholesterol biosynthesis.

Defects of cholesterol biosynthesis comprise a heterogeneous group of disorders, most of which have only been recently described and more are likely to follow in the near future. Mevalonic aciduria (MVA) and hyperimmunoglobulinemia D syndrome (HIDS) are due to allelic defects in mevalonate kinase, an enzyme located proximally in the pathway of cholesterol and nonsterol isoprene biosynthesis. Clinically, patients affected with these disorders present with recurrent febrile attacks. This is the only manifestation in most patients with HIDS, and, in the case of classical mevalonic aciduria, is part of a severe multisystemic disease, including malformations, severe failure to thrive and neurological abnormalities. The other recognized defects of cholesterol biosynthesis are due to enzyme defects located distally in the pathway beyond the branching points of nonsterol isoprene biosynthesis and solely affecting cholesterol biosynthesis. Patients with these disorders all present with complex malformation syndromes involving different organ systems. The main characteristics of CHILD syndrome and Conradi-Huenermann syndrome are skeletal defects and ichthyosiform skin involvement. Smith-Lemli-Opitz syndrome and desmosterolosis are generalized malformation syndromes involving many different organs including the central nervous system.The diagnosis of MVA and HIDS is based on determination of mevalonic acid in urine followed by determination of enzyme activity, whereas the search for the distally located defects of cholesterol biosynthesis requires sterol analysis in blood or tissues by GCMS. Rational therapeutic approaches have been described for HIDS, MVA and Smith-Lemli-Opitz syndrome.

Abnormalities, Multiple↗
PubMed2003

Simvastatin modulates expression of the PON1 gene and increases serum paraoxonase: a role for sterol regulatory element-binding protein-2.

BACKGROUND: The HDL-associated enzyme paraoxonase protects LDLs from oxidative stress. 3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) appear to favorably influence the atherosclerotic process by different mechanisms. The present study examined the influence of simvastatin on paraoxonase expression and serum paraoxonase levels. METHODS AND RESULTS: Simvastatin upregulated in a dose-dependent manner the activity of the promoter of the paraoxonase gene in expression cassettes transfected into HepG2 cells. Upregulation could be blocked by mevalonate and other intermediates of the cholesterol biosynthetic pathway. Simvastatin increased nuclear factors, notably sterol regulatory element-binding protein-2, capable of binding to the paraoxonase promoter; this was also blocked by mevalonate. Sterol regulatory element-binding protein-2 upregulated promoter activity in vitro. Patients treated with statin showed a significant increase in serum concentrations and activities of paraoxonase. CONCLUSIONS: The data indicate that simvastatin can modulate expression in vitro of the antioxidant enzyme paraoxonase and is associated with increased serum paraoxonase concentration and activity. It is consistent with effects of simvastatin treatment, which have the potential to influence beneficially antiatherogenic mechanisms at the HDL level. The study provides evidence for 1 molecular mechanism by which paraoxonase gene expression could be regulated.

Aryldialkylphosphatase↗
PubMed2002

Studies on the nonmevalonate terpene biosynthetic pathway: metabolic role of IspH (LytB) protein.

Isopentenyl diphosphate and dimethylallyl diphosphate serve as the universal precursors for the biosynthesis of terpenes. Although their biosynthesis by means of mevalonate has been studied in detail, a second biosynthetic pathway for their formation by means of 1-deoxy-D-xylulose 5-phosphate has been discovered only recently in plants and certain eubacteria. Earlier in vivo experiments with recombinant Escherichia coli strains showed that exogenous 1-deoxy-D-xylulose can be converted into 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate by the consecutive action of enzymes specified by the xylB and ispCDEFG genes. This article describes the transformation of exogenous [U-(13)C(5)]1-deoxy-D-xylulose into a 5:1 mixture of [U-(13)C(5)]isopentenyl diphosphate and [U-(13)C(5)]dimethylallyl diphosphate by an E. coli strain engineered for the expression of the ispH (lytB) gene in addition to recombinant xylB and ispCDEFG genes.

Bacterial Proteins↗
PubMed1999

Enhancement of sterol synthesis by the monoterpene perillyl alcohol is unaffected by competitive 3-hydroxy-3-methylglutaryl-CoA reductase inhibition.

Monoterpenes such as limonene and perillyl alcohol (PA) are currently under investigation for their chemotherapeutic properties which have been tied to their ability to affect protein isoprenylation. Because PA affects the synthesis of isoprenoids, such as ubiquinone, and cholesterol is the end product of the synthetic pathway from which this isoprenoid pathway branches, we investigated the effects of this compound upon cholesterol metabolism in the colonic adenocarcinoma cell line SW480. PA (1 mM) inhibited incorporation of 14C-mevalonate into 21-26 kDa proteins by 25% in SW480 cells. Cholesterol (CH) biosynthesis was assessed by measuring the incorporation of 14C-acetate and 14C-mevalonate into 27-carbon-sterols. Cells treated with PA (1 mM) exhibited a fourfold increase in the incorporation of 14C-acetate but not 14C-mevalonate into cholesterol. Mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) reductase, at 2 microM concentration, inhibited CH synthesis from 14C-acetate by 80%. Surprisingly, concurrent addition of mevinolin and PA did not significantly alter the stimulatory effects of PA. As observed differences in 14C-acetate and 14C-mevalonate precursor labeling could indicate PA affects early pathway events, the effects of this monoterpene on HMG-CoA reductase activity were evaluated. Unexpectedly, 1 mM PA did not stimulate activity of this enzyme. Consistent with its action as a reversibly bound inhibitor, in washed microsomes, 2 microM mevinolin pretreatment increased reductase protein expression causing a 12.7 (+/- 2.4)-fold compensatory HMG-CoA reductase activity increase; concurrent treatment with 1 mM PA attenuated this to a 5.3 (+/- 0.03)-fold increase. Gas chromatographic analysis confirmed CH was the major lipid present in the measured thin-layer chromatography spot. Since 14C-acetate incorporation into free fatty acid and phospholipid pools was not significantly affected by PA treatment, nonspecific changes in whole acetate pool sizes were not indicated. Because increases in endogenous CH synthesis should result in compensatory changes in exogenous sterol utilization, the effects of PA upon low density lipoprotein (LDL) receptor activity were evaluated. Consistent with the observed increases in CH synthesis, 1 mM PA decreased 125I-LDL internalization to 50% of the fetal bovine serum control; concurrent addition of 2 microM mevinolin attenuated this effect to a reduction of 80% of the control value. Data suggest that in certain colonic tumor cells PA strongly affects cholesterol metabolism via a mechanism of action that is insensitive to the HMG-CoA reductase inhibitor mevinolin.

Animals↗
PubMed2002

Isoprenoid biosynthesis in higher plants and in Escherichia coli: on the branching in the methylerythritol phosphate pathway and the independent biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate.

In the bacterium Escherichia coli, the mevalonic-acid (MVA)-independent 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway is characterized by two branches leading separately to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). The signature of this branching is the retention of deuterium in DMAPP and the deuterium loss in IPP after incorporation of 1-[4-(2)H]deoxy-d-xylulose ([4-(2)H]DX). Feeding tobacco BY-2 cell-suspension cultures with [4-(2)H]DX resulted in deuterium retention in the isoprene units derived from DMAPP, as well as from IPP in the plastidial isoprenoids, phytoene and plastoquinone, synthesized via the MEP pathway. This labelling pattern represents direct evidence for the presence of the DMAPP branch of the MEP pathway in a higher plant, and shows that IPP can be synthesized from DMAPP in plant plastids, most probably via a plastidial IPP isomerase.

Cell Line↗
PubMed1994

Light-Stimulated Carotenoid Biosynthesis during Transformation of Maize Etioplasts Is Regulated by Increased Activity of Isopentenyl Pyrophosphate Isomerase.

Light-stimulated carotenoid biosynthesis associated with the transformation of etioplasts to chloroplasts was investigated after dark-grown maize (Zea mays) seedlings were transferred into light. These studies focused on the enzymes of the pathway to detect those enzyme activities that were stimulated in the light and thus that were responsible for increased biosynthesis of carotenoids. In preliminary experiments, norflurazon, an inhibitor of phytoene desaturase, was used to prevent phytoene being further metabolized to carotenoids. Light-dependent stimulation of phytoene accumulation indicated that the light-regulated steps are located in the pathway leading to phytoene synthesis. The use of the 14C- labeled precursors mevalonic acid, isopentenyl pyrophosphate, and farnesyl pyrophosphate pointed to increased activity of an enzyme involved in the biosynthetic steps between isopentenyl pyrophosphate and farnesyl pyrophosphate. Determination of the activities of all five enzymes of the pathway involved in the sequence from mevalonic acid to phytoene revealed that the only enzyme activity stimulated by light was isopentenyl pyrophosphate isomerase. Over a 3-h period of illumination, this enzyme activity, like carotenoid biosynthesis, was stimulated 2.8-fold.

Journal Article↗
PubMed1990

Inhibition of cholesterol synthesis from mevalonate by aminotriazole treatment in vivo.

3-Amino-1,2,4-triazole (aminotriazole) is an irreversible inhibitor of catalase which is a marker enzyme of peroxisomes. We studied the effect of aminotriazole treatment on biosyntheses of cholesterol and bile acid in vivo. When catalase activity of peroxisomes of rat liver was inhibited by aminotriazole treatment, bile acid content in the bile was significantly decreased to about 70% of the control, but that in the liver was not changed. Cholesterol content in the bile was significantly decreased to about 80% of the control, while in liver and serum the content was not significantly changed. When [2-14C]mevalonate was administered to rats, radioactivities of cholesterol in the liver, serum and bile were all drastically decreased by aminotriazole treatment, and an unidentified radioactive product was detected. Radioactivity of bile acid in the bile was also greatly decreased. In a similar experiment with [4-14C]cholesterol, aminotriazole treatment had no effect on the radioactivity of either cholesterol or bile acid in the liver, serum and bile. In this case, the unidentified product could not be detected. These results indicate that when catalase activity of liver peroxisomes is suppressed by aminotriazole treatment, biosynthesis of bile acid from exogenous cholesterol is not inhibited, but a step in the pathway of biosynthesis of endogenous cholesterol from mevalonate is inhibited.

Amitrole↗
PubMed1993

Metabolism of Linoleic Acid or Mevalonate and 6-Pentyl-alpha-Pyrone Biosynthesis by Trichoderma Species.

The understanding of the biosynthetic pathway of 6-pentyl-alpha-pyrone in Trichoderma species was achieved by using labelled linoleic acid or mevalonate as a tracer. Incubation of growing cultures of Trichoderma harzianum and T. viride with [U-C]linoleic acid or [5-C]sodium mevalonate revealed that both fungal strains were able to incorporate these labelled compounds (50 and 15%, respectively). Most intracellular radioactivity was found in the neutral lipid fraction. At the initial time of incubation, the radioactivity from [C]linoleic acid was incorporated into 6-pentyl-alpha-pyrone more rapidly than that from [C]mevalonate. No radioactivity incorporation was detected in 6-pentyl-alpha-pyrone when fungal cultures were incubated with [1-C]linoleic acid. These results suggested that beta-oxidation of linoleic acid was a probable main step in the biosynthetic pathway of 6-pentyl-alpha-pyrone in Trichoderma species.

Journal Article↗
PubMed2005

Reconstitution of an apicoplast-localised electron transfer pathway involved in the isoprenoid biosynthesis of Plasmodium falciparum.

In the malaria parasite Plasmodium falciparum isoprenoid precursors are synthesised inside a plastid-like organelle (apicoplast) by the mevalonate independent 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway. The last reaction step of the DOXP pathway is catalysed by the LytB enzyme which contains a [4Fe-4S] cluster. In this study, LytB of P. falciparum was shown to be catalytically active in the presence of an NADPH dependent electron transfer system comprising ferredoxin and ferredoxin-NADP(+) reductase. LytB and ferredoxin were found to form a stable protein complex. These data suggest that the ferredoxin/ferredoxin-NADP(+) reductase redox system serves as the physiological electron donor for LytB in the apicoplast of P. falciparum.

Animals↗
PubMed2006

Bifunctional inhibitors of mevalonate kinase and mevalonate 5-diphosphate decarboxylase.

[structure: see text] A bifunctional inhibitor of mevalonate kinase and mevalonate 5-diphosphate decarboxylase was synthesized. Both enzymes are in the cholesterol biosynthetic pathway and play an important role in regulating cholesterol biosynthesis. The molecule may become a useful lead compound for further development for treating cardiovascular disease and cancer. This study provides a novel example of a single inhibitor blocking two sequential steps simultaneously in the cholesterol biosynthetic pathway.

Carboxy-Lyases↗
PubMed1997

The orphan nuclear receptor LXRalpha is positively and negatively regulated by distinct products of mevalonate metabolism.

LXRalpha is an orphan member of the nuclear hormone receptor superfamily that displays constitutive transcriptional activity. We reasoned that this activity may result from the production of an endogenous activator that is a component of intermediary metabolism. The use of metabolic inhibitors revealed that mevalonic acid biosynthesis is required for LXRalpha activity. Mevalonic acid is a common metabolite used by virtually all eukaryotic cells. It serves as a precursor to a large number of important molecules including farnesyl pyrophosphate, geranylgeranyl pyrophosphate, cholesterol, and oxysterols. Inhibition of LXRalpha could be reversed by addition of mevalonic acid and certain oxysterols but not by other products of mevalonic acid metabolism. Surprisingly, the constitutive activity of LXRalpha was inhibited by geranylgeraniol, a metabolite of mevalonic acid. These findings suggest that LXRalpha may represent a central component of a signaling pathway that is both positively and negatively regulated by multiple products of mevalonate metabolism.

Cell Line↗
PubMed1980

Protection of cell viability and respiratory quinone levels by carotenoid in Micrococcus lysodeikticus (M. luteus).

Viability and respiratory activity of post-exponential phase cultures of Micrococcus lysodeikticus (M. luteus) decreased with time more rapidly in carotenoidless mutants than in a parent pigmented strain. The concentration of menaquinone, the respiratory quinone, was found to be low in carotenoidless mutants and in cultures of the pigmented strain where carotenoid synthesis had been partially blocked by diphenylamine. Cell suspensions incorporated [2-14C]mevalonate into menaquinone. Carotenoidless strains incorporated label at substantially higher rates than did the pigmented wild-type strain. Gently prepared membranes of M. lysodeikticus also incorporated mevalonate into menaquinone suggesting that the enzymes for the isoprenoid pathway are bound (loosely) to the membrane. Carotenoidless membranes with low concentrations of menaquinone incorporated radioactivity from [2-14C]mevalonate into quinone more rapidly than did membranes from the wild-type. Azide inhibited the incorporation but n-heptyl-4-hydroxyquinolone-N-oxide did not. It is concluded that the low concentrations of menaquinone in carotenoidless strains are due to rapid breakdown of the quinone. Carotenoid is therefore seen as protecting menaquinone from breakdown by factors as yet unidentified.

Carotenoids↗