Changes in chick liver and brain mevalonate kinase, mevalonate-5-phosphate kinase and mevalonate-5-pyrophosphate decarboxylase during development

Phosphorylation and decarboxylation of mevalonate in chick liver and brain was investigated during early post hatching stages of development. In chick liver, both mevalonate kinase and mevalonate-5-phosphate kinase increased their activity from day 5 of age while pyrophosphate decarboxylase activity remained low during the first days after hatching, increased sharply up to day 9 of age, and remained practically unchanged thereafter. The developmental pattern obtained in brain shows a slight decrease in the phosphorylation and decarboxylation of mevalonate after the first week of postnatal development. Further studies were performed using the specific substrate of mevalonate-5-pyrophosphate decarboxylase, corroborating the results obtained using mevalonate as substrate. Changes in hepatic decarboxylase were more pronounced than those observed in mevalonate-phosphorylating enzymes, thus suggesting an important role for decarboxylase in the control of cholesterogenesis during postnatal development.     

Mevalonate-metabolizing enzymes in Arachis hypogaea

Summary The activities of the mevalonate metabolizing enzymes-HMG-CoA reductase, mevalonate kinase, mevalonate phosphokinase and mevalonate pyrophosphate decarboxylase -were assayed with the respective substrates in green seedlings of Arachis hypogaea. MVAPP decarboxylase is the rate-limiting step among these enzymes and is inhibited by phenolic acids. Its activity in the seedlings was found to decrease in the absence of light and on treatment with abscisic acid. These results suggest that regulation of isoprene pathway in groundnut seedlings may occur at the level of mevalonate decarboxylation.Abbreviations HMG CoA 3-hydroxy-3-methyl-glutaryl coenzyme A - MVA Mevalonate - MVAP Mevalonate-5-phosphate - MVAPP Mevalonate-5-pyrophosphate - DTT Dithiothreitol - ABA Abscisic Acid     

Structural analysis of mevalonate‐3‐kinase provides insight into the mechanisms of isoprenoid pathway decarboxylases

In animals, cholesterol is made from 5‐carbon building blocks produced by the mevalonate pathway. Drugs that inhibit the mevalonate pathway such as atorvastatin (lipitor) have led to successful treatments for high cholesterol in humans. Another potential target for the inhibition of cholesterol synthesis is mevalonate diphosphate decarboxylase (MDD), which catalyzes the phosphorylation of (R)‐mevalonate diphosphate, followed by decarboxylation to yield isopentenyl pyrophosphate. We recently discovered an MDD homolog, mevalonate‐3‐kinase (M3K) from Thermoplasma acidophilum, which catalyzes the identical phosphorylation of (R)‐mevalonate, but without concomitant decarboxylation. Thus, M3K catalyzes half the reaction of the decarboxylase, allowing us to separate features of the active site that are required for decarboxylation from features required for phosphorylation. Here we determine the crystal structure of M3K in the apo form, and with bound substrates, and compare it to MDD structures. Structural and mutagenic analysis reveals modifications that allow M3K to bind mevalonate rather than mevalonate diphosphate. Comparison to homologous MDD structures show that both enzymes employ analogous Arg or Lys residues to catalyze phosphate transfer. However, an invariant active site Asp/Lys pair of MDD previously thought to play a role in phosphorylation is missing in M3K with no functional replacement. Thus, we suggest that the invariant Asp/Lys pair in MDD may be critical for decarboxylation rather than phosphorylation.     

Identification in Haloferax volcanii of Phosphomevalonate Decarboxylase and Isopentenyl Phosphate Kinase as Catalysts of the Terminal Enzyme Reactions in an Archaeal Alternate Mevalonate Pathway

Mevalonate (MVA) metabolism provides the isoprenoids used in archaeal lipid biosynthesis. In synthesis of isopentenyl diphosphate, the classical MVA pathway involves decarboxylation of mevalonate diphosphate, while an alternate pathway has been proposed to involve decarboxylation of mevalonate monophosphate. To identify the enzymes responsible for metabolism of mevalonate 5-phosphate to isopentenyl diphosphate in Haloferax volcanii, two open reading frames (HVO_2762 and HVO_1412) were selected for expression and characterization. Characterization of these proteins indicated that one enzyme is an isopentenyl phosphate kinase that forms isopentenyl diphosphate (in a reaction analogous to that of Methanococcus jannaschii MJ0044). The second enzyme exhibits a decarboxylase activity that has never been directly attributed to this protein or any homologous protein. It catalyzes the synthesis of isopentenyl phosphate from mevalonate monophosphate, a reaction that has been proposed but never demonstrated by direct experimental proof, which is provided in this account. This enzyme, phosphomevalonate decarboxylase (PMD), exhibits strong inhibition by 6-fluoromevalonate monophosphate but negligible inhibition by 6-fluoromevalonate diphosphate (a potent inhibitor of the classical mevalonate pathway), reinforcing its selectivity for monophosphorylated ligands. Inhibition by the fluorinated analog also suggests that the PMD utilizes a reaction mechanism similar to that demonstrated for the classical MVA pathway decarboxylase. These observations represent the first experimental demonstration in H. volcanii of both the phosphomevalonate decarboxylase and isopentenyl phosphate kinase reactions that are required for an alternate mevalonate pathway in an archaeon. These results also represent, to our knowledge, the first identification and characterization of any phosphomevalonate decarboxylase.     

Inhibition of cholesterol biosynthesis by fluorinated mevalonate analogues

The conversion of mevalonate to cholesterol in rat liver homogenates (IC50 = 0.01-1.0 mM) is inhibited by 6- (I), 6,6-di- (II), and 6,6,6-trifluoromevalonate (III), as well as 4,4-difluoromevalonate (IV). Addition of compound I, III, or IV to rat liver homogenates results in the accumulation of 5-phospho- and 5-pyrophosphomevalonate. The conversion of isopentenyl pyrophosphate to cholesterol is not inhibited by the fluorinated analogues. It thus appears likely that the decarboxylation of mevalonate 5-pyrophosphate is inhibited. Rat liver homogenates catalyze the phosphorylation of I and III. The inhibition of the decarboxylation of mevalonate 5-pyrophosphate by I and III was demonstrated directly with partially purified decarboxylase. Compound I is a remarkably effective inhibitor of the decarboxylation (Ki = 10 nM). Similar results were reported by Nave et al. [Nave, J. F., d'Orchymont, H., Ducep, J. B., Piriou F., & Jung, M. J. (1985) Biochem. J. 227, 247]. It is likely that the phosphorylated or pyrophosphorylated forms of all inhibitors tested are responsible for inhibition. We also describe a chemical method for the synthesis of mevalonate 5-pyrophosphate.     

Expression of specific high capacity mevalonate transport in a Chinese hamster cell variant

A variant of a low density lipoprotein receptor-negative Chinese hamster ovary (CHO) cell mutant was isolated using a nutritional selection called MeLoCo. The variant, designated met-18b-2, internalized and metabolized mevalonate at rates 10-40 times greater than the progenitor cells from which they were derived. The extent of incorporation of radioactivity from [3H]mevalonate into steroidal and nonsteroidal mevalonate derivatives, including modified proteins, was much greater in met-18b-2 cells than in their progenitors. Much of the internalized [3H]mevalonate was converted to nonpolar lipids. Unlike wild type CHO cells or the receptor-negative progenitors, met-18b-2 cells were killed by high concentrations of mevalonate (greater than 6 mM) in the culture medium. Regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity and cholesterol esterification was dramatically more sensitive to mevalonate in met-18b-2 cells than in progenitor cells. In cell extracts, both the rates of conversion of [3H]mevalonate to cholesterol and mevalonate kinase activities were similar for met-18b-2 and progenitor cells. In contrast to progenitor cells, met-18b-2 cells internalized [3H]mevalonate with high capacity (Km approximately 0.3 mM) kinetics. The increased uptake of [3H]mevalonate was temperature dependent and highly specific. These results suggest that met-18b-2 cells express a mevalonate transport activity that is not normally expressed by CHO cells. This activity may be due to a specific mevalonate transporter that is differentially expressed in specialized tissues. Because intracellular mevalonate in met-18b-2 cells can be labeled to high specific activity, these cells should prove very useful in further characterizing the structures of mevalonate derivatives and their metabolism.     

3-Hydroxy-3-methylglutaryl CoA reductase and mevalonate kinase of Neurospora crassa

Two enzymes of polyisoprenoid synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase (mevalonate:NADP oxidoreductase [acylating CoA], EC 1.1.1.34) and mevalonate kinase (ATP:mevalonate 5-phosphotransferase, EC 2.7.1.36), are present in the microsomal and soluble fractions of Neurospora crassa, respectively. HMG CoA reductase specifically uses NADPH as reductant and has a K(m) for dl-HMG CoA of 30 micro M. The activities of HMG CoA reductase and mevalonate kinase are low in conidia and increase threefold during the first 12 hr of stationary growth. Maximum specific activities of both enzymes occur when aerial hyphae and conidia first appear (2 days), but total activities peak later (3-4 days). Addition to the growth media of ergosterol or beta-carotene, alone or in combination, does not affect the specific or total activity of either enzyme. The mevalonate kinase of N. crassa, purified 200-fold to a specific activity of 5 micro moles/min/mg, is free from HMG CoA reductase, phosphomevalonate kinase, ATPase, adenylate kinase, and NADH oxidase activities. Mevalonate kinase specifically requires ATP as cosubstrate and exhibits a marked preference for Mg(2+) over Mn(2+), especially at high ratios of divalent metal ion to ATP. Kinase activity is inhibited by p-hydroxymercuribenzoate, and this inhibition is partially prevented by mevalonate or MgATP. Optimum activity occurs at pH 8.0-8.5 and at about 55 degrees C. The Neurospora kinase, like that of hog liver, has a sequential mechanism for substrate addition. The Michaelis constants obtained were 2.8 mM for dl-mevalonate and 1.8 mM for MgATP(-2). Geranyl pyrophosphate is an inhibitor competitive with MgATP (K(i) = 0.11 mM).     

Hypolipidemic activity of dipyridamole: effects on the main regulatory enzyme of cholesterogenesis.

The in vivo dipyridamole treatment for 16 days produced a significant decrease in chick plasma cholesterol, mainly due to the esterified form. This effect was especially patent in the VLDL + LDL fraction. Similar results were observed in triglyceride content. To our knowledge, this is the first report on this hypolipidemic effects of dipyridamole. Total and esterified cholesterol increased after the same treatment in chick liver, while brain cholesterol content was not affected. Hepatic 3-hydroxy-3- methylglutaryl-CoA reductase activity was drastically reduced, while other secondary regulatory enzymes such as mevalonate kinase, mevalonate 5-phosphate kinase and mevalonate 5-pyrophosphate decarboxylase did not change significantly. No significant differences were found in cholesterol and lipidic phosphorus from liver microsomes, so that the effect of dipyridamole on reductase activity cannot be due to modifications in cholesterol/lipidic phosphorus molar ratio. Neither of these enzyme activities was affected in vitro by dipyridamole.     

Quantitative role of different embryonic tissues in mevalonate metabolism by sterol and nonsterol pathways. Relationship with enzyme activities of cholesterogenesis

3-Hydroxy-3-methylglutaryl-CoA reductase, mevalonate kinase, mevalonate-5-phosphate kinase and mevalonate-5-pyrophosphate decarboxylase activities have been determined in brain, liver, intestine and kidneys from 19-day-old chick embryo. Levels of brain reductase and decarboxylase were clearly higher than those found in the other tissues assayed. However, only small differences were observed in the activity of both kinases among the different tissues. Mevalonate metabolism by sterol and nonsterol pathways has been investigated in chick embryo at the same developmental stage. Mevalonate incorporation into total nonsaponifiable lipids was maximal in liver, followed by intestine, brain and kidneys. The shunt pathway of mevalonate not leading to sterols was negligible in both brain and liver, while a clear CO2 production was observed in intestine and kidneys. Sterols running in TLC as lanosterol and cholesterol were the major sterols formed from mevalonate by brain and kidney slices, while squalene and squalene oxide(s) were found to be mainly synthesized by liver slices. Minor differences in the percentage of different sterols were observed in chick embryo intestine. The importance of free and esterified cholesterol accumulation in the different tissues on the inhibition of cholesterogenic activity is discussed.     

Posthatching evolution of in vivo mevalonate metabolism in chick liver

The in vivo mevalonate incorporation into total nonsaponifiable lipids by chick liver was minimal after hatching and drastically increased between 1-5 days. The hepatic synthesis of different cholesterol precursors emerged sequentially after hatching. Between 1-5 days increased strongly the conversion of mevalonate into squalene and also the formation of oxygenated lanosterol derivatives from squalene. The conversion of squalene became completely active at day 8. Cholesterol formation from lanosterol derivatives was completely activated between 8-11 days. Results in this paper demonstrate for the first time the accumulation of a fraction of nonsaponifiable lipids identified as lanosterol derivatives and cholesterol precursors formed from [5-14C]mevalonate in experiments carried out in vivo. Postnatal evolution of these oxysterols may explain the great increase of 3-hydroxy-3-methylglutaryl-CoA reductase activity found in chick liver between 5-11 days, simultaneous or posterior to the diminution of the oxygenated cholesterol precursors.     

Evidence That IAA Conjugates Are Slow-Release Sources of Free IAA in Plant Tissues

Evidence that indoleacetic acid (IAA) conjugates are metabolized via enzyme-catalyzed hydrolysis to free IAA and that their biological activities are related to the rates at which they are hydrolyzed by the tissue is presented. These conclusions are based on the following observations. Slow but continuous decarboxylation of the IAA moiety of IAA-l-alanine and IAA-glycine occurs when these conjugates are applied to pea (Pisum sativum L. cv. Alaska) stem segments. Inasmuch as IAA conjugates are protected from peroxidase-catalyzed oxidative decarboxylation, the conjugates are probably hydrolyzed and the freed IAA then further metabolized. Free IAA and IAA-l-alanine are converted, by pea stem tissue, into the same metabolites. The metabolism is enzymic, since conjugates of IAA with the d-isomers of the amino acids are inactive. Ethylene production induced by IAA-l-alanine and by IAA-glycine is correlated with their hydrolysis, as indicated by their decarboxylation and with the appearance or nonappearance of IAA metabolites in the tissues.     

Crystal structure of mevalonate 3,5-bisphosphate decarboxylase reveals insight into the evolution of decarboxylases in the mevalonate metabolic pathways

Mevalonate 3,5-bisphosphate decarboxylase is involved in the recently discovered Thermoplasma-type mevalonate pathway. The enzyme catalyzes the elimination of the 3-phosphate group from mevalonate 3,5-bisphosphate as well as concomitant decarboxylation of the substrate. This entire reaction of the enzyme resembles the latter half-reactions of its homologs, diphosphomevalonate decarboxylase and phosphomevalonate decarboxylase, which also catalyze ATP-dependent phosphorylation of the 3-hydroxyl group of their substrates. However, the crystal structure of mevalonate 3,5-bisphosphate decarboxylase and the structural reasons of the difference between reactions catalyzed by the enzyme and its homologs are unknown. In this study, we determined the X-ray crystal structure of mevalonate 3,5-bisphosphate decarboxylase from Picrophilus torridus, a thermoacidophilic archaeon of the order Thermoplasmatales. Structural and mutational analysis demonstrated the importance of a conserved aspartate residue for enzyme activity. In addition, although crystallization was performed in the absence of substrate or ligands, residual electron density having the shape of a fatty acid was observed at a position overlapping the ATP-binding site of the homologous enzyme, diphosphomevalonate decarboxylase. This finding is in agreement with the expected evolutionary route from phosphomevalonate decarboxylase (ATP-dependent) to mevalonate 3,5-bisphosphate decarboxylase (ATP-independent) through the loss of kinase activity. We found that the binding of geranylgeranyl diphosphate, an intermediate of the archeal isoprenoid biosynthesis pathway, evoked significant activation of mevalonate 3,5-bisphosphate decarboxylase, and several mutations at the putative geranylgeranyl diphosphate–binding site impaired this activation, suggesting the physiological importance of ligand binding as well as a possible novel regulatory system employed by the Thermoplasma-type mevalonate pathway.     

2,3-Oxidosqualene cyclase and cycloartenol-s-adenosylmethionine methyltransferase activities in vivo in the cotyledon and axis tissues of germinating pea seeds.

Axis tissues, root and shoot, of germinating pea seedlings actively synthesize sterol from [2-14C]mevalonate during the first 3 days of germination. In addition to the intermediates of sterol synthesis, cycloartenol and 24-methylenecycloartanol, these tissues also form the triterpene beta-amyrin. The cyclase catalysing the formation of cycloartenol from oxidosqualene is about four times as active as that for beta-amyrin synthesis. 2. Sterol synthesis in the cotyledon is negligible, but cycloartenol and 24-methylenecycloartanol, as well as beta-amyrin, are synthesized there. Oxidosqualene cyclase activity in this tissue is 2.6 times as active for beta-amyrin synthesis as for cycloartenol synthesis. 3. Comparison of the relative amounts of 14C in cycloartenol and 24-methylenecycloartanol in the axis tissues and cotyledons of 3-day-old seedlings point to relatively active cycloartenol-S-adenosylmethionine methyltransferase systems in both axis tissues and a poorly active system in the cotyledon. 4. The role of beta-amyrin synthesis in the germinating pea seedling is discussed.     

Age-related changes in the mevalonate metabolism in vivo in chick kidneys

The mevalonate incorporation in vivo into total nonsaponifiable lipids by chick kidneys drastically increased after hatching, reaching similar levels to those previously observed in liver. Cholesterol was the major sterol formed from mevalonate from 11 days onward, while a fraction of polar nonsaponifiable lipid(s) was observed as the major compound(s) synthesized at 5-8 days. Relative percentages of squalene, squalene oxide(s) and lanosterol synthesized from mevalonate also increased between 11-18 days after hatching. Results in this paper demonstrate for the first time the accumulation of a fraction of nonsaponifiable lipid(s) identified as lanosterol derivatives and cholesterol precursors formed by kidneys from [5-14C]mevalonate in experiments carried out in vivo, as well as their evolution during postnatal period.     

Biosynthesis of the Macrocyclic Diterpene Casbene in Castor Bean (Ricinus communis L.) Seedlings : Changes in Enzyme Levels Induced by Fungal Infection and Intracellular Localization of the Pathway

Casbene is a macrocyclic diterpene hydrocarbon that is produced in young castor bean (Ricinus communis L.) seedlings after they are exposed to Rhizopus stolonifer or other fungi. The activities of enzymes that participate in casbene biosynthesis were measured in cell-free extracts of 67-hour castor bean seedlings (a) that had been exposed to R. stolonifer spores 18 hours prior to the preparation of extracts, and (b) that were maintained under aseptic conditions throughout. Activity for the conversion of mevalonate to isopentenyl pyrophosphate does not change significantly after infection. On the other hand, the activities of farnesyl pyrophosphate synthetase (geranyl transferase), geranylgeranyl pyrophosphate synthetase (farnesyl transferase), and casbene synthetase are all substantially greater in infected tissues in comparison with control seedlings maintained under sterile conditions. The subcellular localization of these enzymes of casbene biosynthesis was investigated in preparations of microsomes, mitochondria, glyoxysomes, and proplastids that were resolved by centrifugation in linear and step sucrose density gradients of homogenates of castor bean endosperm tissue from both infected and sterile castor bean seedlings. Isopentenyl pyrophosphate isomerase and geranyl transferase activities are associated with proplastids from both infected and sterile seedlings. Significant levels of farnesyl transferase and casbene synthetase are found only in association with the proplastids of infected tissues and not in the proplastids of sterile tissues. From these results, it appears that at least the last two steps of casbene biosynthesis, geranylgeranyl pyrophosphate synthetase and casbene synthetase, are induced during the process of infection, and that the enzymes responsible for the conversion of isopentenyl pyrophosphate to casbene are localized in proplastids.     

Microplate enzyme-coupled assays of mevalonate and phosphomevalonate kinase from Catharanthus roseus suspension cultured cells

A microplate assay for mevalonate and 5-phosphomevalonate kinase activities has been developed using an enzyme-coupled system of pyruvate kinase and lactate dehydrogenase. Mevalonate and 5-phosphomevalonate kinase activities were measured in crude and partially purified enzyme preparations from Catharanthus roseus suspension-cultured plant cells. The assay was validated with respect to protein and substrate concentration. Mevalonate and 5-phosphomevalonate kinase showed Michaelis-Menten kinetics with respect to ATP and their specific substrates; the apparent Km values of mevalonate kinase for ATP and mevalonate were 210 and 65 microM, respectively, and of 5-phosphomevalonate kinase for ATP and 5-phosphomevalonate were 0.41 and 0.4 mM, respectively. Considering mevalonate kinase, the relative standard deviation of enzyme activity within a determination (n = 3) is always less than 2.5% and in between determinations (n = 9) is less than 2%. The method can be used in a continuous assay as well as in a discontinuous assay.     

Formation of isopentenyl diphosphate via mevalonate does not occur within etioplasts and etiochloroplasts of mustard (Sinapis alba L.) seedlings

Seedlings from the white mustard, Sinapis alba, grown under continuous far-red light exhibit enhanced plastid enzyme activities when compared with dark-grown seedlings (for review, see Mohr 1981). These activities are even more pronounced upon illumination with white light during the etioplast/chloroplast transformation. Etioplasts and etiochloroplasts from the cotyledons of such seedlings show high prenyl-lipid-synthesizing activities when [1-¹⁴C]isopentenyl diphosphate is used as the precursor. They lack, however, any enzymatic activities for the formation of isopentenyl diphosphate via the mevalonate pathway, i.e. hydroxymethylglutaryl-CoA reductase, mevalonate kinase, phosphomevalonate kinase and diphosphomevalonate decarboxylase, which are present and easily detectable within the endoplasmic reticulum and cytoplasm. These results corroborate the view that the cytoplasm of the plant cell is the only site of isopentenyl-diphosphate formation via the mevalonate pathway.     

Cell Cycling of Astrocytes and Their Precursors in Primary Cultures: A Mevalonate Requirement Identified in Late G1, but Before the G1/S Transition, Involves Polypeptides

The relationship between mevalonate and cell cycling was investigated in developing glial cells. Primary cultures of newborn rat brains were serum-depleted (0.1%, vol/vol) for 48 h on days 4-6 in vitro, then returned to 10% calf serum (time 0). After 48 h, 70-80% of the cells were glial fibrillary acidic protein (GFAP)-negative by indirect immunofluorescence; 79 +/- 7% were GFAP-positive after an additional 3 days. Serum shift-up resulted in 12 h of quiescence, and then by 20 h (S phase) in increased proportions of cells synthesizing DNA (from 15 +/- 6% to 75 +/- 4% by bromodeoxyuridine immunofluorescence at 12 h and 20 h, respectively) and rates of DNA synthesis (42 +/- 6 versus 380 +/- 32 cpm/micrograms of protein/h of [3H]thymidine uptake). Additional mevalonate (25 mM) for 30 min at 10 h reversed the inhibition of DNA synthesis apparent with mevinolin (150 microM), an inhibitor of mevalonate synthesis, present from time 0. Cycloheximide added simultaneously with mevalonate prevented this reversal of inhibition. To cause arrest at G1/S, cultures were exposed to hydroxyurea between 10 and 22 h. By 3 h after hydroxyurea removal, bromodeoxyuridine-labeled nuclei increased from 0% to 75 +/- 9%, and DNA synthesis increased 10-fold. Mevinolin failed to inhibit these increases. Thus, primary astroglial precursors stimulated to progress through the cell cycle express a mevalonate requirement in late G1, but before the G1/S transition. The effect of mevalonate was characterized further as being brief (30 min) and as requiring polypeptides.     

In vitro and in vivo synthesis of dolichol and other main mevalonate products in various organs of the rat

The relative rate of biosynthesis of dolichol from [3H]mevalonate in nine rat organs was studied in slices and in the whole animal. This biosynthesis was also compared to that of cholesterol and ubiquinone. All tissues examined are able to synthesize dolichol, as well as ubiquinone and cholesterol. Comparison of the data from slices in vitro with the in vivo studies demonstrated relatively good agreement for dolichol and ubiquinone synthesis. Although dolichol of high specific radioactivity was recovered in the blood, redistribution between organs, such as occurs with cholesterol, appears to be insignificant. The highest rates of dolichol biosynthesis were found in kidney, spleen and liver. On the other hand, muscle makes the largest contribution to total body dolichol synthesis. Newly synthesized dolichol also appears in the bile, but excretion by this route is far from sufficient to account for dolichol turnover. Incorporation of mevalonate into the final products is mainly dependent on biosynthetic activity. For comparison of the biosynthetic rates in different organs, possible sources of errors (such as variations in the size of the precursor pool, limitation by the rate of precursor uptake or non-linear incorporation) were investigated the size of the mevalonate pool in various organs. Equilibration of this pool with exogenous mevalonate is a rapid and passive process. The size of the mevalonate pool does not determine the rates of cholesterol and dolichol biosynthesis, indicating the presence of regulatory steps in the terminal portion of these biosynthetic pathways.     

Identification, evolution, and essentiality of the mevalonate pathway for isopentenyl diphosphate biosynthesis in gram-positive cocci

The mevalonate pathway and the glyceraldehyde 3-phosphate (GAP)-pyruvate pathway are alternative routes for the biosynthesis of the central isoprenoid precursor, isopentenyl diphosphate. Genomic analysis revealed that the staphylococci, streptococci, and enterococci possess genes predicted to encode all of the enzymes of the mevalonate pathway and not the GAP-pyruvate pathway, unlike Bacillus subtilis and most gram-negative bacteria studied, which possess only components of the latter pathway. Phylogenetic and comparative genome analyses suggest that the genes for mevalonate biosynthesis in gram-positive cocci, which are highly divergent from those of mammals, were horizontally transferred from a primitive eukaryotic cell. Enterococci uniquely encode a bifunctional protein predicted to possess both 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and acetyl-CoA acetyltransferase activities. Genetic disruption experiments have shown that five genes encoding proteins involved in this pathway (HMG-CoA synthase, HMG-CoA reductase, mevalonate kinase, phosphomevalonate kinase, and mevalonate diphosphate decarboxylase) are essential for the in vitro growth of Streptococcus pneumoniae under standard conditions. Allelic replacement of the HMG-CoA synthase gene rendered the organism auxotrophic for mevalonate and severely attenuated in a murine respiratory tract infection model. The mevalonate pathway thus represents a potential antibacterial target in the low-G+C gram-positive cocci.