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Selective inhibition of isoprenylation of 21-26-kDa proteins by the anticarcinogen d-limonene and its metabolites.

Limonene has chemotherapeutic activity against chemically induced rat mammary carcinomas, many of which contain activated ras genes. Given the recent discovery of the post-translational modification of p21ras and other cell growth-associated proteins by intermediates in the mevalonic acid pathway, and the common biochemical origins of limonene and these isoprene products, we investigated the effect of limonene on protein isoprenylation. NIH3T3 and human mammary epithelial cells were incubated with lovastatin and [2-14C]mevalonolactone in the absence and presence of limonene. Labeled proteins were then subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Limonene inhibited isoprenylation of a class of cellular proteins of 21-26 kDa, including p21ras and possibly other small GTP-binding proteins, in a dose-dependent manner in both cell lines. In contrast, limonene did not affect the isoprenylation of several other proteins, including nuclear lamins. Limonene is metabolized extensively in vivo but not in cultured cells. The two major rat serum metabolites of limonene, perillic acid and dihydroperillic acid, were more potent than limonene in the inhibition of isoprenylation. These results demonstrate that limonene selectively inhibits isoprenylation of 21-26-kDa proteins at a point in the mevalonic acid pathway distal to 3-hydroxy-3-methylglutaryl coenzyme A reductase, and they provide a plausible explanation for its chemotherapeutic activity. Inhibition of isoprenylation of proteins such as p21ras and other small GTP-binding proteins would alter their intracellular localization and, hence, disrupt their biological activity.

Animals↗

Cloning of the gene encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase from terpenoid antibiotic-producing Streptomyces strains.

We have isolated a mutant lacking 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activity from a terpenoid antibiotic (terpentecin) producer, Streptomyces griseolosporeus MF730-N6, which uses both the mevalonate and nonmevalonate pathways for the formation of isopentenyl diphosphate, by screening terpentecin non-producing mutants. Terpentecin is known to be synthesized via the mevalonate pathway. The gene encoding HMG-CoA reductase (hmgg) was cloned and identified by complementation of the mutant, using a self-cloning system developed in this study for strain MF730-N6. The corresponding hmgs gene for HMG-CoA reductase was also cloned from Streptomyces sp. KO-3988, which produces the terpenoid antibiotic furaquinocin. Sequence analysis of hmgg and hmgs showed that both genes encode polypeptides of 353 amino acids which are 84% identical to each other. A search of protein sequence databases revealed that both gene products were also similar to HMG-CoA reductases from a variety of other organisms, including Streptomyces sp. CL190 (hmgg is 89% and hmgs 85% identical to its CL190 homolog), sea urchin (40.3 and 40.5%), German cockroach (37.6 and 38.4%), and Camptotheca acuminata (39.7 and 40.8%).

Amino Acid Sequence↗

Distribution of the mevalonate and glyceraldehyde phosphate/pyruvate pathways for isoprenoid biosynthesis in unicellular algae and the cyanobacterium Synechocystis PCC 6714.

Isopentenyl diphosphate, the universal isoprenoid precursor, can be produced by two different biosynthetic routes: either via the acetate/mevalonate (MVA) pathway, or via the more recently identified MVA-independent glyceraldehyde phosphate/pyruvate pathway. These two pathways are easily differentiated by incorporation of [1-13C]glucose and analysis of the resulting labelling patterns found in the isoprenoids. This method was successfully applied to several unicellular algae raised under heterotrophic growth conditions and allowed for the identification of the pathways that were utilized for isoprenoid biosynthesis. All isoprenoids examined (sterols, phytol, carotenoids) of the green algae Chlorella fusca and Chlamydomonas reinhardtii were synthesized via the GAP/pyruvate pathway, as in another previously investigated green alga, Scenedesmus obliquus, which was also shown in this study to synthesize ubiquinone by the same MVA-independent route. In the red alga Cyanidium caldarium and in the Chrysophyte Ochromonas danica a clear dichotomy was observed: as in higher plants, sterols were formed via the MVA route, whereas chloroplast isoprenoids (phytol in Cy. caldarium and O. danica and beta-carotene in O. danica) were synthesized via the GAP/pyruvate route. In contrast, the Euglenophyte Euglena gracilis synthesized ergosterol, as well as phytol, via the acetate/MVA route. Similar feeding experiments were performed with the cyanobacterium Synechocystis PCC 6714 using [1-13C]- and [6-13C]-glucose. The two isoprenoids examined, phytol and beta-carotene, were shown to have the typical labelling pattern derived from the GAP/pyruvate route.

Animals↗

Biosynthesis of mono- and sesquiterpenes in carrot roots and leaves (Daucus carota L.): metabolic cross talk of cytosolic mevalonate and plastidial methylerythritol phosphate pathways.

The biosynthesis of the monoterpenes terpinolene and myrcene and the sesquiterpene beta-caryophyllene in roots and leaves of two carrot varieties (Daucus carota L. cultivars Bolero and Kazan) were investigated by in vivo feeding experiments with [5,5-2H2]-mevalonic acid lactone (d2-MVL) and [5,5-2H2]-1-deoxy-D-xylulose (d2-DOX). The volatiles of the tissues were extracted by stir bar sorptive extraction and analyzed using thermal desorption-multidimensional gas chromatography-mass spectrometry. The experiments demonstrate independent de novo-biosynthesis of terpenoids in carrot roots and in carrot leaves. In both plant tissues monoterpenes are biosynthesized exclusively via the 1-deoxy-D-xylulose/2-C-methyl-D-erythritol-4-phosphate (DOXP/MEP) pathway, whereas sesquiterpenes are generated by the classical mevalonic acid pathway as well as by the DOXP/MEP route. A more detailed investigation of carrot root tissues revealed that the biosynthesis of terpenes is mainly localized in the phloem. Nevertheless, in xylem a de novo-biosynthesis of terpenes was detectable as well, even in the absence of oil ducts in this tissue.

Cytosol↗

Metabolic engineering of the mevalonate and non-mevalonate isopentenyl diphosphate-forming pathways for the production of health-promoting isoprenoids in tomato.

The genetic manipulation of both the mevalonic acid (MVA) and methylerythritol-4-phosphate (MEP) pathways, leading to the formation of isopentenyl diphosphate (IPP), has been achieved in tomato using 3-hydroxymethylglutaryl CoA (hmgr-1) and 1-deoxy-d-xylulose-5-phosphate synthase (dxs) genes, respectively. Transgenic plants containing an additional hmgr-1 from Arabidopsis thaliana, under the control of the cauliflower mosaic virus (CaMV) 35S constitutive promoter, contained elevated phytosterols (up to 2.4-fold), but IPP-derived isoprenoids in the plastid were unaltered. Transgenic lines containing a bacterial dxs targeted to the plastid with the tomato dxs transit sequence resulted in an increased carotenoid content (1.6-fold), which was inherited in the next generation. Phytoene and beta-carotene exhibited the greatest increases (2.4- and 2.2-fold, respectively). Extra-plastidic isoprenoids were unaffected in these lines. These data are discussed with respect to the regulation, compartmentalization and manipulation of isoprenoid biosynthetic pathways and their relevance to plant biotechnology.

Journal Article↗

Genetic evidence of branching in the isoprenoid pathway for the production of isopentenyl diphosphate and dimethylallyl diphosphate in Escherichia coli.

An alternative mevalonate-independent pathway for isoprenoid biosynthesis has been recently discovered in eubacteria (including Escherichia coli) and plant plastids, although it is not fully elucidated yet. In this work, E. coli cells were engineered to utilize exogenously provided mevalonate and used to demonstrate by a genetic approach that branching of the endogenous pathway results in separate synthesis of the isoprenoid building units isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In addition, the IPP isomerase encoded by the idi gene was shown to be functional in vivo and to represent the only possibility for interconverting IPP and DMAPP in this bacterium.

Arabinose↗

The methylerythritol phosphate pathway and its significance as a novel drug target.

Isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) are the precursors for all isoprenoid compounds. Two pathways are found in Nature for their biosynthesis. The mevalonate (MVA) pathway is found in eukaryotes, algae, archae and some gram-positive bacteria. Gram-negative bacteria, plants and some gram-positive bacteria utilize the methyl erythritol phosphate (MEP) pathway. The distribution and the orthogonal nature of the pathways make the MEP pathway an attractive new target for antibiotics and herbicides. The MEP pathway is essential for bacterial viability. Inhibitors to the MEP pathway represent a "dual-use technology" because potential targets include potential biological warfare agents in addition to common human pathogens. The CDC has three categories designated for Biological Diseases/Agents. Three of the six entities designated as the highest priority (Category A) are organisms that utilize, or appear to utilize, the MEP pathway. Among the 12 second highest priority agents (Category B) listed, 8 are organisms that appear to utilize the MEP pathway. Common human pathogens that can be targeted include the organisms responsible for peptic ulcers, tuberculosis, malaria, food safety threats, and sexually transmitted diseases. There is so far only one inhibitor reported that specifically blocks the MEP pathway and is being investigated clinically. This compound, fosmidomycin, has been shown to be somewhat effective in treating Plasmodium falciparum, the parasite responsible for malaria. We foresee that new MEP pathway inhibitors will open up an entirely new class of antibiotics. An MEP pathway intermediate has also been shown to be the most potent gammadelta T cell activator.

Animals↗

The identification of isoprenoids that bind in the intersubunit cavity of Escherichia coli 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase by complementary biophysical methods.

The discovery of a distinct metabolic pathway, the non-mevalonate or 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway for isoprenoid precursor biosynthesis, in eubacteria and apicomplexan parasites has revealed a new set of potential drug targets. The emphasis of research on this pathway has been on delineating the intermediates and the biochemical and structural characterization of component enzymes. Two new monoclinic crystal forms of recombinant Escherichia coli 2C-methyl-D-erythritol-2,4-cyclodiphosphate (MECP) synthase cocrystallized with (i) CMP and (ii) CMP and MECP show well defined electron density at the subunit interface suggestive of an isoprenoid-like ligand. 31P NMR analysis of the recombinant protein sample indicates the presence of bound diphosphate species and electrospray mass spectrometry identifies a mixture of isopentenyl diphosphate (and/or dimethylallyl diphosphate), geranyl diphosphate and farnesyl diphosphate in an approximate ratio of 1:4:2. The most prevalent species, geranyl diphosphate, was successfully modelled into the electron density, revealing the important protein-ligand interactions that stabilize binding of the isoprenoid. The observation that MECP synthase binds three metabolites that are produced by enzymes two, three and four stages downstream in isoprenoid biosynthesis suggests that feedback regulation of the non-mevalonate pathway is possible.

Binding Sites↗

Neutrophils are required for the DNA synthetic response of human lymphocytes to mevalonic acid: evidence suggesting that a nonsterol product of mevalonate is involved.

Human peripheral blood lymphocytes, in the presence of human neutrophils, initiate DNA synthesis and cell cycling when exposed to mevalonic acid. The ability of lymphocytes to respond in this manner is a radiosensitive property of cells, whereas the help provided by neutrophils is maintained despite their exposure to x-irradiation. Other organic acid anions, including precursors of mevalonic acid biosynthesis and a variety of products of mevalonate metabolism, fail to initiate DNA synthesis when added to human lymphocytes. Because only the metabolically active R(--) enantiomer of mevalonic acid initiates lymphocyte DNA synthesis, we presume that physiological pathways of mevalonate metabolism are involved. The response to mevalonic acid of ML-236B (compactin)-inhibited lymphocytes is increased, and the threshold concentration of meyalonate at which lymphocyte DNA synthesis first appears is decreased, when the cells are cultured in lipoprotein-containing (as opposed to lipoprotein-depleted) medium. The response to mevalonic acid of lymphocytes cultured in lipoprotein-depleted medium can be enhanced by addition to the cultures of low density lipoprotein but not by addition of high density lipoprotein. Based upon the flux diversion hypothesis of mevalonate metabolism, these observations suggest that a nonsterol product of mevalonate metabolism may be responsible for the initiation of lymphocyte DNA synthesis by mevalonic acid.

Cell Cycle↗

Inhibition of ubiquinone and cholesterol synthesis by the monoterpene perillyl alcohol.

Monoterpenes such as perillyl alcohol have been shown to cause the complete regression of advanced mammary carcinomas. Given the common origin of plant monoterpenes and many important molecules in mammalian mevalonate metabolism pathway, we investigated the effects of the monoterpene perillyl alcohol on mevalonate metabolism. NIH3T3 cells were labeled with [14C]mevalonolactone and lipids were analyzed. Perillyl alcohol inhibited ubiquinone synthesis and blocked the conversion of lathosterol to cholesterol. These two cellular effects of perillyl alcohol may contribute to the antitumor activity of the monoterpenes.

3T3 Cells↗

Cloning and expression of cDNAs encoding two enzymes of the MEP pathway in Catharanthus roseus.

Two periwinkle cDNAs (crdxr and crmecs) encoding enzymes of the non-mevalonate terpenoid pathway were characterized using reverse transcription-PCR strategy based on the design of degenerated oligonucleotides. The deduced amino acid sequence of crdxr is homologue to 1-deoxy-D-xylulose 5-phosphate reductoisomerases. Crmecs represents the first plant cDNA encoding a protein similar to the 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase from Escherichia coli. Expression of crdxr and crmecs genes was up-regulated in periwinkle cells producing monoterpenoid indole alkaloids. Involvement of the 2C-methyl-D-erythritol 4-phosphate pathway in alkaloid biosynthesis is discussed.

Aldose-Ketose Isomerases↗

Staphylococcus aureus mevalonate kinase: isolation and characterization of an enzyme of the isoprenoid biosynthetic pathway.

It has been proposed that isoprenoid biosynthesis in several gram-positive cocci depends on the mevalonate pathway for conversion of acetyl coenzyme A to isopentenyl diphosphate. Mevalonate kinase catalyzes a key reaction in this pathway. In this study the enzyme from Staphylococcus aureus was expressed in Escherichia coli, isolated in a highly purified form, and characterized. The overall amino acid sequence of this enzyme was very heterologous compared with the sequences of eukaryotic mevalonate kinases. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and analytical gel filtration chromatography suggested that the native enzyme is a monomer with a molecular mass of approximately 33 kDa. The specific activity was 12 U/mg, and the pH optimum was 7.0 to 8.5. The apparent K(m) values for R,S-mevalonate and ATP were 41 and 339 micro M, respectively. There was substantial substrate inhibition at millimolar levels of mevalonate. The sensitivity to feedback inhibition by farnesyl diphosphate and its sulfur-containing analog, farnesyl thiodiphosphate, was characterized. These compounds were competitive inhibitors with respect to ATP; the K(i) values were 46 and 45 micro M for farnesyl diphosphate and its thio analog, respectively. Parallel measurements with heterologous eukaryotic mevalonate kinases indicated that S. aureus mevalonate kinase is much less sensitive to feedback inhibition (K(i) difference, 3 orders of magnitude) than the human enzyme. In contrast, both enzymes tightly bound trinitrophenyl-ATP, a fluorescent substrate analog, suggesting that there are similarities in structural features that are important for catalytic function.

Amino Acid Sequence↗

Inhibition of paclitaxel and baccatin III accumulation by mevinolin and fosmidomycin in suspension cultures of Taxus baccata.

To achieve a better understanding of the metabolism and accumulation of paclitaxel and baccatin III in cell cultures of Taxus, inhibitors of the early steps in the terpenoid pathway were applied to a cell suspension culture of Taxus baccata: fosmidomycin as an inhibitor of the non-mevalonate branch of the pathway, and mevinolin as an inhibitor of the mevalonate branch. Synthesis of both taxanes in the cell suspension was first increased when cultured in the product formation medium supplemented with methyljasmonate (100 microM). The product formation medium was selected after assaying 24 different culture media. When fosmidomycin (200 microM) was added to the product formation medium together with the elicitor, the accumulation of paclitaxel and baccatin III was reduced by up to 3.0 and 1.5 times, respectively, whereas the inhibitory effect of mevinolin (1 microM) was only clearly exerted in the case of paclitaxel. Under the conditions of our experiment, we conclude that in the synthesis of both taxanes, the non-mevalonate pathway is the main source of the universal terpenoid precursor isopentenyl diphosphate (IPP).

Acetates↗

Biosynthesis of the hemi- and monoterpene moieties of isoprenyl phenyl ethers from the liverwort Trichocolea tomentella.

The incorporation of 13C labelled glucose into trichocolein, deoxytomentellin, trans-phytol and stigmasterol has been studied in axenic cultures of the liverwort Trichocolea tomentella. Quantitative 13C NMR spectroscopic analysis of the resulting labelling patterns showed that the isoprene units of the hemi- and monoterpenoid moieties and the diterpene phytol are derived from the methylerythritol phosphate pathway, whereas the isoprene units of stigmasterol are built up via the mevalonic acid pathway. These results indicate the involvement of both IPP biosynthetic pathways in different cellular compartments. A new, hydroperoxy geranyl phenyl ether derivative is also described.

Ethers↗

COMPARATIVE BIOSYNTHESIS OF MEVALONIC ACID BY MYCOPLASMA.

Smith, Paul F. (University of South Dakota, Vermillion), and C. V. Henrikson. Comparative biosynthesis of mevalonic acid by Mycoplasma. J. Bacteriol. 89:146-153. 1965.-Three representative Mycoplasma, M. laidlawii strain B, M. gallisepticum strain J, and M. hominis strain 07, were examined for the presence or absence of enzymes associated with the biosynthetic pathway to mevalonic acid. M. laidlawii served as a control, because it synthesizes carotenoids from acetate. M. laidlawii was shown to contain a specific acetokinase and phosphotransacetylase for the synthesis of acetyl coenzyme A, and a beta-ketothiolase and coenzyme A transferase for the synthesis of acetoacetyl coenzyme A. M. gallisepticum contained a specific acetokinase, phosphotransacetylase, and possibly an aceto coenzyme A kinase forming acetyl coenzyme A; it also contained a beta-ketothiolase, a coenzyme A transferase, and a coenzyme A transphorase forming acetoacetyl coenzyme A directly or indirectly. The beta-ketothiolase of M. gallisepticum was not affected by iodoacetamide, in contrast to the other two strains. M. laidlawii exhibited beta-hydroxy-beta-methylglutaryl coenzyme A condensing enzyme, and M. hominis did not. This activity of M. gallisepticum was masked by thiolase activity. M. laidlawii and M. gallisepticum contained a nicotinamide adenine dinucleotide phosphate-linked beta-hydroxy-beta-methylglutaryl coenzyme A reductase, and M. hominis did not. C(14)-labeled acetate was incorporated into mevalonic acid only by M. laidlawii and M. gallisepticum. The lack of beta-hydroxy-beta-methylglutaryl coenzyme A condensing enzyme and reductase activities in M. hominis explains its growth requirement for sterol. The enzymatic block in M. gallisepticum must occur after mevalonic acid in the biosynthetic pathway to terpenoids.

Acetates↗

Nine 3-ketoacyl-CoA thiolases (KATs) and acetoacetyl-CoA thiolases (ACATs) encoded by five genes in Arabidopsis thaliana are targeted either to peroxisomes or cytosol but not to mitochondria.

The sub-cellular location of enzymes of fatty acid beta-oxidation in plants is controversial. In the current debate the role and location of particular thiolases in fatty acid degradation, fatty acid synthesis and isoleucine degradation are important. The aim of this research was to determine the sub-cellular location and hence provide information about possible functions of all the putative 3-ketoacyl-CoA thiolases (KAT) and acetoacetyl-CoA thiolases (ACAT) in Arabidopsis. Arabidopsis has three genes predicted to encode KATs, one of which encodes two polypeptides that differ at the N-terminal end. Expression in Arabidopsis cells of cDNAs encoding each of these KATs fused to green fluorescent protein (GFP) at their C-termini showed that three are targeted to peroxisomes while the fourth is apparently cytosolic. The four KATs are also predicted to have mitochondrial targeting sequences, but purified mitochondria were unable to import any of the proteins in vitro. Arabidopsis also has two genes encoding a total of five different putative ACATs. One isoform is targeted to peroxisomes as a fusion with GFP, while the others display no targeting in vivo as GFP fusions, or import into isolated mitochondria. Analysis of gene co-expression clusters in Arabidopsis suggests a role for peroxisomal KAT2 in beta-oxidation, while KAT5 co-expresses with genes of the flavonoid biosynthesis pathway and cytosolic ACAT2 clearly co-expresses with genes of the cytosolic mevalonate biosynthesis pathway. We conclude that KATs and ACATs are present in the cytosol and peroxisome, but are not found in mitochondria. The implications for fatty acid beta-oxidation and for isoleucine degradation in mitochondria are discussed.

Acetyl-CoA C-Acetyltransferase↗

Cloning and characterization of a gene from Escherichia coli encoding a transketolase-like enzyme that catalyzes the synthesis of D-1-deoxyxylulose 5-phosphate, a common precursor for isoprenoid, thiamin, and pyridoxol biosynthesis.

For many years it was accepted that isopentenyl diphosphate, the common precursor of all isoprenoids, was synthesized through the well known acetate/mevalonate pathway. However, recent studies have shown that some bacteria, including Escherichia coli, use a mevalonate-independent pathway for the synthesis of isopentenyl diphosphate. The occurrence of this alternative pathway has also been reported in green algae and higher plants. The first reaction of this pathway consists of the condensation of (hydroxyethyl)thiamin derived from pyruvate with the C1 aldehyde group of D-glyceraldehyde 3-phosphate to yield D-1-deoxyxylulose 5-phosphate. In E. coli, D-1-deoxyxylulose 5-phosphate is also a precursor for the biosynthesis of thiamin and pyridoxol. Here we report the molecular cloning and characterization of a gene from E. coli, designated dxs, that encodes D-1-deoxyxylulose-5-phosphate synthase. The dxs gene was identified as part of an operon that also contains ispA, the gene that encodes farnesyl-diphosphate synthase. D-1-Deoxyxylulose-5-phosphate synthase belongs to a family of transketolase-like proteins that are highly conserved in evolution.

Amino Acid Sequence↗

Leaf Ests from Stevia rebaudiana: a resource for gene discovery in diterpene synthesis.

Expressed sequence tags (ESTs) are providing a new approach to gene discovery in plant secondary metabolism. Stevia rebaudiana Bert. leaves produce high concentrations of diterpene steviol glycosides and should be a rich source of transcripts involved in diterpene synthesis. In order to create a resource for gene discovery and increase our understanding of steviol glycoside biosynthesis, we sequenced 5,548 ESTs from a S. rebaudiana leaf cDNA library. The EST collection was fully annotated based on database search results. ESTs involved in diterpene synthesis were identified using published sequences as electronic probes, by keyword searches of search results, and by differential representation. A significant portion of the ESTs were specific for standard leaf metabolic pathways; energy and primary metabolism represented 17.6% and 13.1% of total transcripts respectively. Diterpene metabolism in S. rebaudiana represented 1.1% of total transcripts. This study identified candidate genes for 70% of the known steps in the steviol glycoside pathway. One candidate, kaurene oxidase, was the 8th most abundant EST in the collection. Identification of many candidate genes specific to the I -deoxyxylulose 5-phosphate pathway suggests that the primary source of isopentenyl diphosphate, a precursor of geranylgeranyl diphosphate, is via the non-mevalonic acid pathway. The use of ESTs has greatly facilitated the identification of candidate genes and increased our understanding of diterpene metabolism.

DNA, Complementary↗