Biosynthesis and Structure of a New Intermediate between Farnesyl Pyrophosphate and Squalene

The Biomedical Importance of the Missing Pathway for Farnesol and Geranylgeraniol Salvage

Isoprenoids are the output of the polymerization of five-carbon, branched isoprenic chains derived from isopentenyl pyrophosphate (IPP) and its isomer, dimethylallyl pyrophosphate (DMAPP). Isoprene units are consecutively condensed to form longer structures such as farnesyl and geranylgeranyl pyrophosphate (FPP and GGPP, respectively), necessary for the biosynthesis of several metabolites. Polyprenyl transferases and synthases use polyprenyl pyrophosphates as their natural substrates; however, it is known that free polyprenols, such as farnesol (FOH), and geranylgeraniol (GGOH) can be incorporated into prenylated proteins, ubiquinone, cholesterol, and dolichols. Furthermore, FOH and GGOH have been shown to block the effects of isoprenoid biosynthesis inhibitors such as fosmidomycin, bisphosphonates, or statins in several organisms. This phenomenon is the consequence of a short pathway, which was observed for the first time more than 25 years ago: the polyprenol salvage pathway, which works via the phosphorylation of FOH and GGOH. Biochemical studies in bacteria, animals, and plants suggest that this pathway can be carried out by two enzymes: a polyprenol kinase and a polyprenyl-phosphate kinase. However, to date, only a few genes have been unequivocally identified to encode these enzymes in photosynthetic organisms. Nevertheless, pieces of evidence for the importance of this pathway abound in studies related to infectious diseases, cancer, dyslipidemias, and nutrition, and to the mitigation of the secondary effects of several drugs. Furthermore, nowadays it is known that both FOH and GGOH can be incorporated via dietary sources that produce various biological effects. This review presents, in a simplified but comprehensive manner, the most important data on the FOH and GGOH salvage pathway, stressing its biomedical importance The main objective of this review is to bring to light the need to discover and characterize the kinases associated with the isoprenoid salvage pathway in animals and pathogens.

Squalene synthase: structure and regulation

Squalene synthase (SQS) catalyzes the first reaction of the branch of the isoprenoid metabolic pathway committed specifically to sterol biosynthesis. Regulation of SQS is thought to direct proximal intermediates in the pathway into either sterol or nonsterol branches in response to changing cellular requirements. The importance of SQS in cholesterol metabolism has stimulated research on the mechanism, structure, and regulation of the enzyme. SQS produces squalene, a C30 isoprenoid, in a two-step reaction in which two molecules of farnesyl diphosphate are condensed head to head. Site-directed mutagenesis of rat SQS has identified conserved Tyr, Phe, and Asp residues that are essential for function. The aromatic rings of Tyr and Phe are postulated to stabilize carbocation intermediates of the first and second half-reactions, respectively; the acidic Asp residues may be required for substrate binding. SQS activity, protein level, and gene transcription are strictly and coordinately regulated by cholesterol status, decreasing with cholesterol surfeit and increasing with cholesterol deficit. The human SQS (hSQS) gene has an unusually complex promoter with multiple binding sites for the sterol regulatory element binding proteins SREBP-1a and SREBP-2, and for accessory transcription factors known to be involved in the control of other sterol-responsive genes. SREBP-1a and SREBP-2 require different subsets of hSQS regulatory DNA elements to achieve maximal promoter activation. Current research is directed at elucidating the precise contribution made by individual SREBPs and accessory transcription factors to hSQS transcriptional control.

Polyisoprenyl phosphates in intracellular signalling

In response to environmental stimuli, leukocyte membrane remodelling generates biologically active lipids that can serve as both intra- and extracellular mediators. There are several classes of lipids that can mediate inflammatory reactions. We report here on a new intracellular lipid signal that regulates oxygen-radical formation in neutrophils, a key response in microbial killing, inflammation and tissue injury. Screening of neutrophil-derived extracts rich in phosphorylated, non-saponifiable lipids revealed a potent inhibitor of superoxide anion (O2-) production. Structural analysis of biologically active fractions gave four major phosphorylated lipids: most abundant was presqualene diphosphate (PSDP). Upon activation of neutrophil receptors, PSDP and its monophosphate form, presqualene monophosphate (PSMP), undergo rapid remodelling. At submicromolar concentrations, PSDP but not PSMP inhibit O2- production by human neutrophil cell-free oxidase preparations. We prepared PSDP and PSMP by total organic synthesis and matched both the physical properties and biological activity of the neutrophil-derived compounds. Our results indicate that PSDP, a recognized intermediate of cholesterol biosynthesis, is present in immune effector cells and is a potent regulator of the cellular response in host defence.

Development of a radiometric spot-wash assay for squalene synthase

The principle of selective elution from a solid phase has been exploited to develop an assay for the determination of squalene biosynthesis in rat liver homogenates. Using either [1-14C]isopentenyl diphosphate as a precursor for squalene or [2-14C]farnesyl diphosphate as a direct substrate of squalene synthase, the production of radiolabeled squalene is determined after adsorption of assay mixtures onto silica gel thin-layer chromatography sheets and selective elution of the diphosphate precursors into a solution of sodium dodecyl sulfate at alkaline pH. The use of [2-14C]farnesyl diphosphate, and of an endogenous oxygen consumption system (ascorbate/ascorbate oxidase) to prevent further metabolism of squalene, allows the method to be applied as a dedicated assay for squalene synthase activity. The assay has been developed in microtiter plate format and may be deployed either in a quantitative, low-throughout mode or in a qualitative, high-through-put mode. The latter is suitable for screening to aid in the discovery of new inhibitors of squalene synthase.

Isolation and primary structure of the ERG9 gene of Saccharomyces cerevisiae encoding squalene synthetase

The ERG9 gene of Saccharomyces cerevisiae has been cloned by complementation of the erg9-1 mutation which affects squalene synthetase. From the 5 kb insert isolated, the functional gene has been localized on a DNA fragment of 2.5 kb. The presence of squalene synthetase activity in E. coli bearing the yeast DNA fragment isolated, indicates that the structural gene encoding squalene synthetase has been cloned. The sequence of the 2.5 kb fragment contains an open reading frame which could encode a protein of 444 amino acids with a deduced relative molecular mass of 51,600. The amino acid sequence reveals one to four potential transmembrane domains with a hydrophobic segment in the C-terminal region. The N-terminus of the deduced protein strongly resembles the signal sequence of yeast invertase suggesting a specific mechanism of integration into the membranes of the endoplasmic reticulum.

Regulation of squalene synthetase in human hepatoma cell line Hep G2 by sterols, and not by mevalonate-derived non-sterols

Incubations of Hep G2 cells for 18 h with human low-density lipoprotein (LDL) resulted in a decrease of squalene synthetase activity, whereas heavy high-density lipoprotein (hHDL) stimulated the activity. Simultaneous addition of LDL abolished the hHDL-induced stimulation, indicating that manipulating the regulatory sterol pool within the cells influenced the enzyme activity. Blocking the endogenous cholesterol synthesis either at the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase site with compactin or at the 2,3-oxidosqualene cyclase site with the inhibitor U18666A gave rise to an elevation of the squalene synthetase activity. Simultaneous addition of mevalonate abolished the compactin-induced increase. However, at total blockade of sterol synthesis by 30 microM U18666A, added compactin and/or mevalonate did not change the enzyme activity further. It was concluded that sterols regulate the squalene synthetase activity, whereas, in contrast with the regulation of the HMG-CoA reductase activity in Hep G2 cells, mevalonate-derived non-sterols did not influence this enzyme.

Regulation of squalene synthetase activity in rat liver: elevation by cholestyramine, but no diurnal variation

Squalene synthetase activity in liver microsomes from rats sacrificed at three different times of the diurnal cycle showed no significant differences. Addition of 4% cholestyramine to the food resulted in a marked increase in activity (280% of control), independent of the time of killing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase activity, determined as positive controls, were also found to be elevated by cholestyramine and additionally showed a diurnal variation. On the other hand, five control enzyme activities, not directly related to cholesterol metabolism, i.e. glutamate dehydrogenase, NADPH cytochrome-c reductase, beta-hexosaminidase, catalase and acyl coenzyme A oxidase, showed neither an influence of cholestyramine feeding nor a time of sacrifice dependent variation.

Formation of 3-hexaprenyl-4-hydroxybenzoate by matrix-free mitochondrial membrane-rich preparations of yeast

It has been shown that a 10 000 x g matrix-free mitochondrial membrane-rich preparation from commercial bakers' yeast is able to synthesize 3-all-transhexaprenyl-4-hydroxybenzoate from 4-hydroxybenzoate and isopentenyl pyrophosphate. The synthesis is Mg2+ dependent and is stimulated markedly by the primer for polyprenylpyrophosphate synthesis of 3-hexaprenyl-4-hydroxybenzoate from 4-hydroxybenzoate, isopentenyl pyrophosphate and 3,3-dimethylallyl pyrophosphate the priming function of 3,3-dimethylallyl pyrophosphate can be performed by either geranyl pyrophosphate (most efficient) or farnesyl pyrophosphate. At high Mg2+ concentrations, however, geranyl pyrophosphate and farnesyl pyrophosphate act mainly as sources of preformed side chains and 3-diprenyl- and 3-tripenyl-4-hydroxybenzoate, respectively, are produced. In the presence of a source of preformed polyprenyl pyrophosphates the membrane preparations catalysed the polyprenylation of methyl-4-hydroxybenzoate, 4-hydroxybenzaldehyde, 4-hydroxybenzylalcohol and 4-hydroxycinnamate. No evidence was obtained for the involvement of either 4-hydroxybenzoyl CoA or 4-hydroxybenzoyl-S-protein in the formation of 3-polyprenyl-4-hydroxybenzoates.

Inhibition of liver prenyltransferase by alkyl phosphonates and phosphonophosphates

n-Pentyl and n-decyl phosphonate and the corresponding phosphonophosphates were found to inhibit cholesterol synthesis from mevalonate in the 10000 X g supernatants of liver homogenates and the synthesis of farnesyl pyrophosphate from geranyl and isopentenyl pyrophosphate by purified liver prenyltransferase. Kinetic analysis of the inhibition of prenyltransferase showed that the phosphonates and the phosphonophosphates interacted with two forms, or two sites, of the enzyme. The order of increasing potency was C5-phosphonate less than C10-phosphonate less than C5-phosphonophosphate less than C10-phosphonophosphate. The phosphonophosphates were at least ten times stronger inhibitors than the phosphonates.

Gas-liquid chromatography of carotenoids and other terpenoids

The retention behaviour of over seventy terpenoids on three silicone polymer liquid phases under both isothermal and temperature-programmed conditions is reported. Terpenoids with conjugated unsaturation (e.g., carotenoids) were hydrogenated prior to analysis in order to prevent thermal decomposition. Analyses of the acetates and TMS ethers of both the natural hydroxycarotenoids and their perhydroderivatives are also reported. In addition, a system is described for the routine analysis of terpenols, including those whose pyrophosphates are intermediates in sterol and carotenoid biosynthesis.

Die chemische struktur der biosynthetischen vorstufe des squalens

Presqualenyl-pyrophosphate (PSPP) was synthesized from farnesyl-pyrophosphate (FPP). The purified product could be transformed into a squalene in 90% yield. The structure of PSPP was established by the following experiments:1) PSPP displays a remarkably acid-labile phosphate-ester bond. Analogous to FPP it was possible to stabilize this bond by the addition of bromine. This indicates a structure of the allylester type.2) On acid hydrolysis PSPP splits off not only the pyrophosphate but also water. This may be best explained by the presence of a tertiary hydroxyl group in PSPP.3) The pyrophosphate group of PSPP could be split by prostatic phosphatase. In gas-liquid-chromatography the trimethyl-silylated product gave a single peak. From the mass spectra of this compound and of the corresponding alcohol the expected molecular weight of 426 could be verified for the latter.4) Determining the 3H/(14)C ratio in 1-(3)H(2)-4(14)C-FPP and in PSPP and squalene, synthesized from it, only three 3H-atoms could be detected in PSPP and in squalene.5) Ozonisation of PSPP, synthesized from 1-(14)C-FPP, gave radioactive malondialdehyde, which was characterised as azobenzene-malondialdehyde and 4-azobenzene-1-phenyl-pyrazole. The experimental results are in agreement with the following structure of 2, 6, 10, 15, 19, 23-hexamethyl-n-tetracosa-2, 6, 11, 14, 18, 22-hexaen-10-yl-pyrophosphate for PSPP.