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PubMed1996

Evidence for farnesol-mediated isoprenoid synthesis regulation in a halophilic archaeon, Haloferax volcanii.

Farnesol strongly inhibited growth of a halophilic archaeon, Haloferax volcanii, with an IC50 value of only 2 microM (0.4 microgram/ml) in rich medium and 50 nM (0.01 microgram/ml) in minimal medium without lysis. Other isoprenoid alcohols such as isopentenol, dimethylallyl alcohol, geraniol, and geranylgeraniol at 500 microM did not affect its growth. Mevalonate, which is the precursor of all isoprenoid membrane lipids in archaea, led to recovery of the growth inhibition of H. volcanii, but acetate had no such effect. Farnesol inhibited incorporation of acetate, but not mevalonate, into the lipid fraction. These results suggest that farnesol inhibited the biosynthetic pathway from acetate (acetyl-CoA) to mevalonate. Farnesol is known to be derived from the important intermediate of isoprenoids, farnesyl diphosphate (FPP), and found in neutral lipid fraction from this archaeon. Moreover, the cell-free extracts from H. volcanii could phosphorylate farnesol with ATP to generate farnesyl monophosphate and FPP. We conclude that farnesol-mediated isoprenoid synthesis regulation system by controlling farnesol concentration is present in H. volcanii.

Acetates↗
PubMed2005

Statins and thrombin.

L-Mevalonic acid is the distant precursor of cholesterol, in contrast to cholesterol, L-mevalonic acid, its distant precursor gives rise to farnesyl and geranylgeranyl pyrophosphates in relatively few metabolic steps. These isoprenyl pyrophophates covalently conjugate with specific G-proteins and serve as membrane anchors enabling them to carry out their function. Although farnesyl-proteins may participate in signal transduction, geranylgeranyl-proteins (e.g., Rho GTP binding proteins) are well known to downregulate signaling pathways by inhibiting L-mevalonic acid synthesis. Such inhibitors include 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, drugs (statins) and isoprenoids of dietary origins, where Rho protein activation appears to be necessary for cellular-mediated thrombin generation. Thrombin and other proteases (e.g., coagulation factor Xa, tryptase) upregulate protease-activated receptor (PAR) synthesis and PAR activation promotes synthesis and expression of other proteins [e.g., tissue factor (TF) and plasminogen activator inhibitor-1 (PAI-1)]. With the PAR-1 activating peptide SSFLRNP, we found that either cerivastatin or atorvastatin mitigated platelet stimulation in a time- and dose-dependent manner, as predicted if a statin-mediated Rho pathway is required. We also found that simvastatin decreased prothrombin fragments F1+2 in plasma from type 2 diabetics, demonstrating that statins downregulate thrombin generation. Thus, independent of cholesterol, statins and dietary isoprenoids behave as inhibitors of TF-dependent thrombin generation. Because thrombin has multiple physiological functions, the 20 pleiotropic effects reported for statins may reflect a common mechanism for downregulation of thrombin-mediated events, in particular at the cellular level.

Down-Regulation↗
PubMed1993

Inhibition of hepatic cholesterol biosynthesis by a 3-hydroxy-3-methylglutaryl coenzyme A synthase inhibitor, 1233A, in mice.

We have studied the in vivo inhibition of hepatic sterol biosynthesis by 1233A, a specific inhibitor of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase. Administration of the compound orally to mice resulted in a dose-dependent inhibition of [14C]acetate incorporation into sterols in liver, but did not exert any significant effect on [14C]mevalonate incorporation. The results indicate that the in vivo inhibition of sterol synthesis occurs only at pre-mevalonate enzymatic steps in the sterol biosynthetic pathway, thus being compatible with the presumed site of inhibition, HMG-CoA synthase. Moreover, owing to irreversible inactivation of the enzyme by 1233A, it was possible to detect in vivo effect on the enzyme by assays of its activity in cell-free extracts from livers; the drug-treatment also resulted in a similarly dose-dependent inhibition of HMG-CoA synthase activity. In contrast, acetoacetyl-CoA thiolase and HMG-CoA reductase, the enzymes also responsible for mevalonate synthesis in the pathway, did not show any significant change in activity. These results clearly demonstrate that the inhibition of hepatic sterol synthesis caused by 1233A is indeed due to selective inhibition of HMG-CoA synthase in the tissues.

Animals↗
PubMedDate not supplied

[Biosynthesis of ergot alkaloids. Some new results on an old problem (Review)].

The biosynthetic pathway leading from L-tryptophan, mevalonic acid and methionine to the tetracyclic ergoline ring system of the ergot alkaloids in Claviceps species is reviewed. This pathway entails many mechanistically intriguing features. Recent studies are also discussed which reveal the stereochemical course of the isoprenylation of tryptophan and of the N-methylation of dimethylallyltryptophan (DMAT) and which shed some light on the likely steps leading from the open-chain precursors, N-methyl-DMAT to the tricyclic intermediate, chanoclavine-1. Finally, some plans are outlined to probe the evolutionary relationship of ergot alkaloid biosynthesis in fungi to that in higher plants of the family Convolvulaceae.

Biological Evolution↗
PubMed2005

Supernatant protein factor stimulates HMG-CoA reductase in cell culture and in vitro.

Supernatant protein factor (SPF) is a 46-kDa cytosolic protein that stimulates squalene monooxygenase in vitro and, unexpectedly, cholesterol synthesis in cell culture. Because squalene monooxygenase is not thought to be rate-limiting with regard to cholesterol synthesis, we investigated the possibility that SPF might stimulate other enzymes in the cholesterol biosynthetic pathway. Substitution of [(14)C]mevalonate for [(14)C]acetate in McARH7777 hepatoma cells expressing SPF reduced the 1.8-fold increase in cholesterol synthesis by half, suggesting that SPF acted on or prior to mevalonate synthesis. This conclusion was supported by the finding that substitution with [(14)C]mevalonate completely blocked an SPF-induced increase in squalene synthesis. Evaluation of 2,3-oxidosqualene synthesis from [(14)C]mevalonate demonstrated that SPF also stimulated squalene monooxygenase (1.3-fold) in hepatoma cells. Immunoblot analysis showed that SPF did not increase HMG-CoA reductase or squalene monooxygenase enzyme levels, indicating a direct effect on enzyme activity. Addition of purified recombinant SPF to rat liver microsomes stimulated HMG-CoA reductase by about 1.5-fold, and the SPF-concentration/activation curve paralleled that for the SPF-mediated stimulation of squalene monooxygenase. These results reveal that SPF directly stimulates HMG-CoA reductase, the rate-limiting step of the cholesterol biosynthetic pathway, as well as squalene monooxygenase, and suggest a new means by which cholesterol synthesis can be rapidly modulated in response to hormonal and environmental signals.

Acetic Acid↗
PubMed2003

Effects of peroxisome proliferators gemfibrozil and clofibrate on syntheses of dolichol and cholesterol in rat liver.

The effects of two peroxisome proliferators, gemfibrozil and clofibrate, on syntheses of dolichol and cholesterol in rat liver were investigated. Gemfibrozil did not affect the overall content of dolichyl phosphate, but it changed the chain-length distribution of dolichyl phosphate, increasing the levels of species with shorter isoprene units. Gemfibrozil suppressed synthesis of dolichyl phosphate from [(3)H]mevalonate and [(3)H]farnesyl pyrophosphate in rat liver. In contrast, clofibrate increased the content of dolichol (free and acyl ester forms). It remarkably enhanced dolichol synthesis from mevalonate, but did not affect dolichol synthesis from farnesyl pyrophosphate. Gemfibrozil elevated cholesterol synthesis from [(14)C]acetate, but did not affect the synthesis from mevalonate. Clofibrate suppressed cholesterol synthesis from acetate, but did not affect cholesterol synthesis from mevalonate. These results suggest that gemfibrozil suppresses synthesis of dolichyl phosphate by inhibiting, at the least, the pathway from farnesyl pyrophosphate to dolichyl phosphate. As a result, the chain-length pattern of dolichyl phosphate may show an increase in shorter isoprene units. Clofibrate may increase the content of dolichol by enhancing dolichol synthesis from mevalonate. Gemfibrozil may increase cholesterol synthesis by activating the pathway from acetate to mevalonate. Unlike gemfibrozil, clofibrate may decrease cholesterol synthesis by inhibiting the pathway from acetate to mevalonate.

Acetates↗
PubMed1980

Postnatal development of mevalonate phosphorylation in the chick brain and kidney.

The quantitative role of the brain and the kidney of the chick the phosphorylation of mevalonic acid was studied in the early steps of development. In both tissues no significant differences were found in the amount of phosphomevalonic acid formed until 4 days, but a sharp decrease between 4 and 6 days was observed. The amount of pyrophosphomevalonic acid formed showed some change with age, but this change was relatively small when compared with the changes in the phosphomevalonic acid formed. According to the function of the brain and the kidney in the sterol and nonsterol pathways in the metabolism of mevalonic acid during neonatal development of chick, an important role for the mevalonate kinase during myelination can be suggested.

Animals↗
PubMed1999

Lovastatin prevents angiotensin II-induced cardiac hypertrophy in cultured neonatal rat heart cells.

Angiotensin II activates p21ras, and mediates cardiac hypertrophic growth through the type 1 angiotensin II receptor in cardiac myocytes. An inhibitor of 3-hydroxy-3-methyglutaryl-coenzyme A (HMG-CoA) reductase has been shown to block the post-translational farnesylation of p21ras and inhibit protein synthesis in several cell types. Primary cultures of neonatal cardiac myocytes were used to determine whether HMG-CoA reductase inhibitors, lovastatin, simvastatin and pravastatin inhibit the angiotensin II-induced hypertrophic growth. Angiotensin II (10(-6) M) significantly increased protein-DNA ratio, RNA-DNA ratio, ratios of protein synthesis and mitogen-activated protein (MAP) kinase activity. Lipid-soluble HMG-CoA reductase inhibitors, lovastatin (10(-6) M) and simvastatin (10(-6) M) partially and significantly inhibited the angiotensin II-induced increases in these parameters, but a water-soluble HMG-CoA reductase inhibitor, pravastatin (10(-6) M) did not. Mevalonate (10(-4) M) overcame the inhibitory effects of lovastatin and simvastatin on angiotensin II-induced increases in these parameters. A selective protein kinase C inhibitor, calphostin C (10(-6) M) partially and significantly prevented angiotensin II-induced increases in these parameters, and treatment with both lovastatin and calphostin C inhibited completely. Angiotensin II increased p21ras activity and membrane association, and lovastatin inhibited them. These studies demonstrate that a lipid-soluble HMG-CoA reductase inhibitor, lovastatin, may prevent angiotensin II-induced cardiac hypertrophy, at least in part, through p21ras/MAP kinase pathway, which is linked to mevalonate metabolism.

Angiotensin II↗
PubMed2000

Deuterium-labelled isotopomers of 2-C-methyl-D-erythritol as tools for the elucidation of the 2-C-methyl-D-erythritol 4-phosphate pathway for isoprenoid biosynthesis.

Escherichia coli synthesizes its isoprenoids via the mevalonate-independent 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. The MC4100dxs::CAT strain, defective in deoxyxylulose-5-phosphate synthase, which is the first enzyme in this metabolic route, exclusively synthesizes its isoprenoids from exogenous 2-C-methyl-D-erythritol (ME) added to the culture medium. The fate of the hydrogen atoms in the MEP pathway was followed by the incorporation of [1,1-(2)H(2)]ME and [3,5,5,5-(2)H(4)]ME. The two C-1 hydrogen atoms of ME were found without any loss in the prenyl chain of menaquinone and/or ubiquinone on the carbon atoms derived from C-4 of isopentenyl diphosphate (IPP) and on the E-methyl group of dimethylallyl diphosphate (DMAPP), the C-5 hydrogen atoms on the methyl groups derived from IPP C-5 methyl group and the Z-methyl group of DMAPP. This showed that no changes in the oxidation state of these carbon atoms occurred in the reaction sequence between MEP and IPP. Furthermore, no deuterium scrambling was observed between the carbon atoms derived from C-4 and C-5 of IPP or DMAPP, suggesting a completely stereoselective IPP isomerase or no significant activity of this enzyme. The C-3 deuterium atom of [3,5,5,5-(2)H(4)]ME was preserved only in the DMAPP starter unit and was completely missing from all those derived from IPP. This finding, aided by the non-essential role of the IPP isomerase gene, suggests the presence in E. coli of two different routes towards IPP and DMAPP, starting from a common intermediate derived from MEP.

Base Sequence↗
PubMed1991

Evidence for regulation of juvenile hormone biosynthesis operating before mevalonate in locust corpora allata.

Exogenous mevalonate poorly stimulated juvenile hormone III (JH-3) biosynthesis by corpora allata from Locusta migratoria. However, mevalonolactone strongly stimulated glands from different physiological states and fully restored JH synthesis in mevinolin-treated glands. Asymmetry in spontaneous rate of JH release was abolished by exogenous mevalonolactone. After transection of nervus corporis allati 1 (NCA-1), the rate of mevalonolactone-stimulated JH synthesis was maintained at the preoperative levels although the spontaneous rate of JH biosynthesis decreased rapidly. These results suggest that the spontaneous asymmetry of JH biosynthesis and the low rate of JH biosynthesis by denervated corpora allata both result from non-stimulation or inhibition acting on the JH pathway before the utilisation of mevalonate.

Animals↗
PubMed1990

Purification and regulation of mevalonate kinase from rat liver.

Mevalonate kinase may play a key role in regulating cholesterol biosynthesis because its activity may be regulated via feedback inhibition by intermediates in the cholesterol biosynthetic pathway. To study the regulation of mevalonate kinase, the enzyme was purified to homogeneity from rat liver, and monospecific antibody against mevalonate kinase was prepared. The purified mevalonate kinase had a dimeric structure composed of identical subunits, and the Mr of the enzyme determined by gel chromatography was 86,000. Based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the subunit Mr was 39,900. The pI for mevalonate kinate was 6.2. The levels of mevalonate kinase protein and enzyme activity were determined in the livers of rats treated with either cholesterol-lowering agents (cholestyramine, pravastatin, and lovastatin) or with dietary modifications. Diets containing cholestyramine alone or cholestyramine and either pravastatin or lovastatin increased mevalonate kinase activity 3-6-fold. Mevalonate kinase activity decreased approximately 50% in rats treated with diets containing either 5% cholesterol or 5% cholesterol and 0.5% cholic acid. Fasting did not significantly change mevalonate kinase activity. The amount of mevalonate kinase protein in the liver was quantitated using immunoblots, and the changes in the levels of kinase activity induced by either drug treatment or by cholesterol feeding were correlated with similar changes in the levels of mevalonate kinase protein. Therefore, under these experimental conditions, mevalonate kinase activity in the liver was regulated principally by changes in the rates of enzyme synthesis and degradation.

Amino Acids↗
PubMed1992

Synergism between a novel amphibian oocyte ribonuclease and lovastatin in inducing cytostatic and cytotoxic effects in human lung and pancreatic carcinoma cell lines.

A novel anti-tumour amphibian oocyte RNase, ONCONASER (ONC), previously known as P-30 Protein, is in the clinical trials. The effect of ONC alone and in combination with lovastatin (LVT), an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme of mevalonate (MVA) and cholesterol synthesis pathway, in three human tumour cell lines ASPC-1 pancreatic, A-549 lung, and HT-520 lung carcinomas, has been presently studied. A synergism between ONC and LVT in inducing the cytostatic and cytotoxic effects was observed. The cytostatic effect, seen during the early phase of the treatment with this combination of drugs was manifested as prolongation of the cell cycle duration, especially of the G1 phase; cell death was apparent after 72 h of treatment. The synergistic effect of ONC and LVT was also evident in the clonogenicity assays. Both LVT lactone and its in vitro activated beta-hydroxy acid form, alone and in respective combinations with ONC, exerted similar degree of growth suppression. The effects of both forms of LVT (used alone or in combination with ONC) were reversed by MVA, which suggests that HMG-CoA reductase inhibition is a primary mechanism of LVT action. The data indicate that the LVT lactone can be activated intracellularly by tumour cells studied, and that the combination of ONC with LVT can produce significantly enhanced anti-tumour activities.

Adenocarcinoma↗
PubMed2007

Statins induce differentiation and cell death in neurons and astroglia.

Statins are potent inhibitors of the hydroxy-methyl-glutaryl-coenzyme A reductase, the rate limiting enzyme for cholesterol biosynthesis. Experimental and clinical studies with statins suggest that they have beneficial effects on neurodegenerative disorders. Thus, it was of interest to characterize the direct effects of statins on CNS neurons and glial cells. We have treated defined cultures of neurons and astrocytes of newborn rats with two lipophilic statins, atorvastatin and simvastatin, and analyzed their effects on morphology and survival. Treatment of astrocytes with statins induced a time- and dose-dependent stellation, followed by apoptosis. Similarly, statins elicited programmed cell death of cerebellar granule neurons but with a higher sensitivity. Analysis of different signaling cascades revealed that statins fail to influence classical pathways such as Akt or MAP kinases, known to be activated in CNS cells. In addition, astrocyte stellation triggered by statins resembled dibutryl-cyclic AMP (db-cAMP) induced morphological differentiation. However, in contrast to db-cAMP, statins induced upregulation of low-density lipoprotein receptors, without affecting GFAP expression, indicating separate underlying mechanisms. Analysis of the cholesterol biosynthetic pathway revealed that lack of mevalonate and of its downstream metabolites, mainly geranylgeranyl-pyrophosphate (GGPP), is responsible for the statin-induced apoptosis of neurons and astrocytes. Moreover, astrocytic stellation triggered by statins was inhibited by mevalonate and GGPP. Interestingly, neuronal cell death was significantly reduced in astrocyte/neuron co-cultures treated with statins. We postulate that under these conditions signals provided by astrocytes, e.g., isoprenoids play a key role in neuronal survival.

Animals↗
PubMed1994

Exogenous CoQ10 preserves plasma ubiquinone levels in patients treated with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors.

Ubiquinone is a carrier of the mitochondrial respiratory chain which regulates oxidative phosphorylation: it also acts as a membrane stabilizer preventing lipid peroxidation. In man the quinone ring originates from tyrosine, while the formation of the polyisoprenoid lateral chain starts from acetyl CoA and proceeds through mevalonate and isopentenylpyrophosphate; this biosynthetic pathway is the same as the cholesterol one. We therefore performed this study to evaluate whether statins (hypocholesterolemic drugs that inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase) modify blood levels of ubiquinone. Thirty unrelated outpatients with primary hypercholesterolemia (IIa phenotype) were treated with 20 mg of simvastatin for a 3-month period (group S) or with 20 mg of simvastatin plus 100 mg CoQ10 (group US). The following parameters were evaluated at time 0, and at 45 and 90 days: total plasma cholesterol, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, triglycerides, Apo A1, Apo B and CoQ10 in plasma and in platelets. In the S group, there was a marked decrease in total cholesterol low-density lipoprotein-cholesterol and in plasma CoQ10 levels from 1.08 mg/dl to 0.80 mg/dl. In contrast, in the US group we observed a significant increase of plasma CoQ10 (from 1.20 to 1.48 mg/dl) while the hypocholesterolemic effect was similar to that observed in the S group. Platelet CoQ10 also decreased in the S group (from 104 to 90 ng/mg) and increased in the US group (from 95 to 145 ng/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗
PubMed2005

Brassica juncea HMG-CoA synthase: localization of mRNA and protein.

3-Hydroxy-3-methylglutaryl-coenzyme-A (HMG-CoA) synthase (HMGS; EC 2.3.3.10) synthesizes HMG-CoA, a substrate for mevalonate biosynthesis in the isoprenoid pathway. It catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA (AcAc-CoA) to yield S-HMG-CoA and HS-CoA. In Brassica juncea (Indian mustard), HMGS is encoded by four isogenes (BjHMGS1-BjHMGS4). We have already enzymatically characterized recombinant BjHMGS1 expressed in Escherichia coli, and have identified its residues that are significant in catalysis. To further study HMGS mRNA expression that is developmentally regulated in flowers and seedlings, we have examined its mRNA distribution by in situ hybridization and reverse transcriptase-polymerase chain reaction (RT-PCR). We observed predominant localization of HMGS mRNA in the stigmas and ovules of flower buds and in the piths of seedling hypocotyls. RT-PCR analysis revealed that BjHMGS1 and BjHMGS2 but not BjHMGS3 and BjHMGS4were expressed in floral buds. To investigate the subcellular localization of BjHMGS1, we fused BjHMGS1 translationally in-frame either to the N- or C-terminus of green fluorescent protein (GFP). BjHMGS1-GFP and GFP-BjHMGS1 fusions were used in particle gun bombardment of onion epidermal cells and tobacco BY-2 cells. The GFP-BjHMGS1 construct was also used in agroinfiltration of tobacco leaves. Both GFP-fusion proteins were observed transiently expressed in the cytosol on confocal microscopy of onion epidermal cells, tobacco BY-2 cells, and agroinfiltrated tobacco leaves. Further, subcellular fractionation of total proteins from transgenic plants expressing GFP-BjHMGS1 derived from Agrobacterium-mediated transformation confirmed that BjHMGS1 is a cytosolic enzyme. We suggest that the presence of BjHMGS isoforms is likely related to the specialization of each in different cellular and metabolic processes rather than to a different intracellular compartmentation of the enzyme.

Amino Acid Sequence↗
PubMed2006

HMG-CoA reductase inhibitors induce apoptosis in pericytes.

Pericytes, which surround endothelial cells in precapillary arterioles, capillaries, and postcapillary venules, are important for the development, maturation, and maintenance of the vascular system. Pericytes are also pluripotent cells that can differentiate into a variety of mesenchymal cells including smooth muscle cells and osteoblasts. Possibly because of their vasculature regulating activities and ability to differentiate in situ, pericytes are implicated in several diseases with vascular complications, including diabetic retinopathy, as well as Reynaud's Syndrome, central nervous system dementias, and vascular calcification among others. Statin drugs, which block the conversion of HMG-CoA to mevalonate in the cholesterol synthesis pathway, are known to have apoptotic and growth inhibitory effects on cells in vitro and complex pleiotropic effects on cells and tissues in vivo. Recently, evidence has emerged that statin drug use in human patients results in a significant 20% reduction in cancer incidence. It is not known whether these results are due to direct statin action on normal tissue, growth inhibitory/pro-apoptotic effects on tumor cells, and/or effects on angiogenesis. Because of the role of pericytes in angiogenesis and the effects of statins on cancer incidence, we tested the direct effects of statins on pericytes. Specifically, we demonstrate that 3 statins, simvastatin, lovastatin, and mevastatin induce dose-dependent apoptosis in the TR-PCT1 pericyte cell line, that simvastatin (empirically shown to be the most potent of the 3 statins) induces similar levels of apoptosis in freshly isolated pericytes, and that simvastatin-induced apoptosis in pericytes is cholesterol, caspase-3, and caspase-7 mediated.

Animals↗
PubMed1980

Changes in sterol biosynthesis accompanying cessation of glial cell growth in serum-free medium.

C-6 glioma cells, grown in medium supplemented with 5% delipidated foetal calf serum, were induced to enter a quiescent state by removing serum from the medium. Within 24h there was a 75-80% decline in the rate of incorporation of [(14)C]acetate or (3)H(2)O into digitonin-precipitable sterols. Experiments with [(3)H]mevalonolactone as a labelled sterol precursor suggested that the decline in sterol synthesis was regulated primarily at a point in the pathway before the formation of mevalonate. The specific activities of 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA reductase decreased sharply in conjunction with the decline in sterol synthesis in the serum-free cultures; however, the activity of acetoacetyl-CoA thiolase was altered only slightly. The magnitude of the initial decline in reductase activity was not affected when 50-mm-NaF was included in the preincubation and assay buffers to prevent activation of physiologically inactive enzyme. However, after 6h of serum deprivation the decline in 3-hydroxy-3-methylglutaryl-CoA reductase activity was due to a decrease in the amount of latent activity. The sterol concentration in C-6 cells was unchanged after 24h in serum-free medium, although a 20% decrease in the sterol/fatty acid molar ratio occurred as a result of a small increase in the fatty-acid concentration. Incorporation of (3)H(2)O into fatty acids was inhibited in the serum-deprived glial cells; however, this inhibition developed more slowly and was not as pronounced as the diminution in sterol synthesis. The results suggest that in C-6 glia, which resemble the glial stem cells of the developing brain, the decreased demand for membrane sterols in the quiescent state results in a decline in sterol synthesis, mediated primarily through co-ordinate changes in the activities of 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA reductase.

Acetates↗
PubMed2004

Isoprenoid biosynthesis in the diatoms Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen).

Isoprenoid biosynthesis in the widespread diatomaceous algae, Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen), results not only in the production of diterpenoids, triterpenoids, and sterols but, unusually for diatoms, also in the production of sesterterpenoids. By using 13C and 2H isotopic labeling techniques followed by NMR and mass spectrometry, specific inhibition of mevalonate (MVA) and methylerythritol (MEP) pathways, and comparison with the natural 13C/12C isotope ratios of the lipids, the different biosynthetic pathways of the sesterterpenes and other isoprenoids have now been determined. Surprisingly, whereas the sesterterpenes (Delta(7(20))-haslenes) in R. setigera were made by the MVA pathway, as were the related triterpenoid rhizenes and desmosterol, in H. ostrearia the structurally similar Delta(6(17))-haslenes and the major sterol, 24-ethylcholest-5-en-3beta-ol, were instead biosynthesized by the MEP route. Phytol was biosynthesized in both diatoms by the MEP route. Subfractionation of R. setigera cells revealed that although phytol was located in the chloroplasts, the haslenes, rhizenes, and sterols were present in the cytoplasm. The observations described here for R. setigera and H. ostrearia show that terpenoid biosynthesis in diatoms is species-dependent and cannot simply be grouped according to structural type. Triterpenes appear to be made by the MVA route as in higher plants, whereas sesterterpenes and sterols can be made by either the MVA or MEP routes. In neither organism were the isoprenoids biosynthesized by leucine metabolism. Sesterterpene and triterpene biosynthesis in diatoms has not been investigated previously.

Alkenes↗