Vasorelaxant activity of twenty-one physiologically relevant (poly)phenolic metabolites on isolated mouse arteries

Polyphenols are beneficial for health, but are metabolised after consumption.

The pharmacokinetics of anthocyanins and their metabolites in humans

BACKGROUND AND PURPOSE

Anthocyanins are phytochemicals with reported vasoactive bioactivity. However, given their instability at neutral pH, they are presumed to undergo significant degradation and subsequent biotransformation. The aim of the present study was to establish the pharmacokinetics of the metabolites of cyanidin-3-glucoside (C3G), a widely consumed dietary phytochemical with potential cardioprotective properties.

EXPERIMENTAL APPROACH

A 500 mg oral bolus dose of 6,8,10,3′,5′-¹³C₅-C3G was fed to eight healthy male participants, followed by a 48 h collection (0, 0.5, 1, 2, 4, 6, 24, 48 h) of blood, urine and faecal samples. Samples were analysed by HPLC-ESI-MS/MS with elimination kinetics established using non-compartmental pharmacokinetic modelling.

KEY RESULTS

Seventeen ¹³C-labelled compounds were identified in the serum, including ¹³C₅-C3G, its degradation products, protocatechuic acid (PCA) and phloroglucinaldehyde (PGA), 13 metabolites of PCA and 1 metabolite derived from PGA. The maximal concentrations of the phenolic metabolites (C_max) ranged from 10 to 2000 nM, between 2 and 30 h (t_max) post-consumption, with half-lives of elimination observed between 0.5 and 96 h. The major phenolic metabolites identified were hippuric acid and ferulic acid, which peaked in the serum at approximately 16 and 8 h respectively.

CONCLUSIONS AND IMPLICATIONS

Anthocyanins are metabolized to a structurally diverse range of metabolites that exhibit dynamic kinetic profiles. Understanding the elimination kinetics of these metabolites is key to the design of future studies examining their utility in dietary interventions or as therapeutics for disease risk reduction.

Quercetin and its major metabolites selectively modulate cyclic GMP-dependent relaxations and associated tolerance in pig isolated coronary artery

BACKGROUND AND PURPOSE

Quercetin is a major flavonoid that contributes to the reduced risk of cardiovascular disease associated with dietary ingestion of fruits and vegetables. We have pharmacologically characterized the effect of quercetin, and its sulphate and glucuronide metabolites, on vasoconstrictor and vasodilator responses in the porcine isolated coronary artery.

EXPERIMENTAL APPROACH

Segments of the porcine coronary artery were prepared for either isometric tension recording or determination of cyclic GMP content. The effect of quercetin and metabolites on submaximal responses to U46619 was examined in the presence and absence of substance P, bradykinin, forskolin, sodium nitroprusside (SNP) and glyceryl trinitrate (GTN).

KEY RESULTS

Quercetin and quercetin 3'-sulphate inhibited endothelin and U46619-induced contractions with greater potency (three- to fivefold) against the former, while quercetin 3-glucoronide was inactive. Quercetin enhanced both the cyclic GMP content of the artery (threefold) and cyclic GMP-dependent relaxations to GTN and SNP (two to threefold), but forskolin-induced relaxations were unaffected. Although the effect of quercetin was qualitatively similar to that noted for UK-114,542, a selective inhibitor of phosphodiesterase 5, it was still evident against SNP-induced relaxations in the presence of 10 nM UK-114,542. Quercetin and quercetin 3'-sulphate significantly reduced the development of GTN-associated 'tolerance'.

CONCLUSIONS AND IMPLICATIONS

Quercetin and quercetin 3'-sulphate inhibited receptor-mediated contractions of the porcine isolated coronary artery by an endothelium-independent action. Quercetin selectively enhanced cyclic-GMP-dependent relaxations by a mechanism not involving phosphodiesterase 5 inhibition. In addition, quercetin and quercetin 3'-sulphate opposed GTN-induced tolerance in vitro, which may be beneficial for patients treated for angina pectoris.

The procyanidin-mediated induction of apoptosis and cell-cycle arrest in esophageal adenocarcinoma cells is not dependent on p21(Cip1/WAF1)

Previous studies have shown that the proanthocyanidin-mediated induction of apoptosis and arrest of the cell cycle in cancer cells was associated with up-regulation of p21(Cip1/WAF1) (p21), suggesting that p21 may be the molecular mediator of the observed effects. Here we show that procyanidins induce a rapid and sustained arrest of the cell cycle, and increase apoptosis, concomitant with an increase in p21 expression. However, blocking the PA-induced up-regulation of p21 expression with siRNA did not alter PA-mediated changes in apoptosis and cell cycle, demonstrating that p21 is not responsible for the PA-induced effects.

Absorption of kaempferol from endive, a source of kaempferol-3-glucuronide, in humans

OBJECTIVE

To determine the absorption, excretion and metabolism of kaempferol in humans.

DESIGN

A pharmacokinetic study of kaempferol from endive over 24 h.

SUBJECTS

Four healthy males and four healthy females.

RESULTS

Kaempferol, from a relatively low dose (9 mg), was absorbed from endive with a mean maximum plasma concentration of 0.1 microM, at a time of 5.8 h, indicating absorption from the distal section of the small intestine and/or the colon. Although a 7.5-fold interindividual variation between the highest and lowest maximum plasma concentration was observed, most individuals showed remarkably consistent pharmacokinetic profiles. This contrasts with profiles for other flavonoids that are absorbed predominantly from the large intestine (eg rutin). An average of 1.9% of the kaempferol dose was excreted in 24 h. Most subjects also showed an early absorption peak, probably corresponding to kaempferol-3-glucoside, present at a level of 14% in the endive. Kaempferol-3-glucuronide was the major compound detected in plasma and urine. Quercetin was not detected in plasma or urine indicating a lack of phase I hydroxylation of kaempferol.

CONCLUSIONS

Kaempferol is absorbed more efficiently than quercetin in humans even at low oral doses. The predominant form in plasma is a 3-glucuronide conjugate, and interindividual variation in absorption and excretion is low, suggesting that urinary kaempferol could be used as a biomarker for exposure.

Comparison of modular and non-modular xylanases as carrier proteins for the efficient secretion of heterologous proteins from Penicillium funiculosum

Genes encoding three enzymes with xylanase activity from the filamentous fungus Penicillium funiculosum are described. Two of the encoded xylanases are predicted to be modular in structure with catalytic and substrate-binding domains separated by a serine and threonine-rich linker region; the other had none of these properties and was non-modular. In order to develop P. funiculosum as a host for the secreted production of heterologous proteins, each of the xylanases was assessed for use as a carrier protein in a fusion strategy. We show that one of the modular xylanases (encoded by xynA) was an effective carrier protein but the other (encoded by xynB) and the non-modular xylanase (encoded by xynC) were not effective as secretion carriers. We show that the beta-glucuronidase (GUS) protein from Escherichia coli is secreted by P. funiculosum when expressed as an XYNA fusion but that the secreted GUS protein, cleaved in vivo from XYNA, is glycosylated and enzymatically inactive.

Esterase activity able to hydrolyze dietary antioxidant hydroxycinnamates is distributed along the intestine of mammals

Hydroxycinnamic acids are effective antioxidants and are abundant components of plant cell walls, especially in cereal bran. For example, wheat and rye brans are rich sources of the hydroxycinnamates ferulic acid, sinapic acid, and p-coumaric acid. These phenolics are part of human and animal diets and may contribute to the beneficial effects derived from consumption of cereal bran. However, these compounds are ester linked to the main polymers in the plant cell wall and cannot be absorbed in this complex form. The present work shows that esterases with activity toward esters of the major dietary hydroxycinnamates are distributed throughout the intestinal tract of mammals. In rats, the cinnamoyl esterase activity in the small intestine is derived mainly from the mucosa, whereas in the large intestine the esterase activity was found predominantly in the luminal microflora. Mucosa cell-free extracts obtained from human duodenum, jejunum, and ileum efficiently hydrolyzed various hydroxycinnamoyl esters, providing the first evidence of human cinnamoyl esterase(s). This study first demonstrates the release by human colonic esterase(s) (mostly of microbial origin) of sinapic acid and p-coumaric acid from rye and wheat brans. Hydrolysis by intestinal esterase(s) is very likely the major route for release of antioxidant hydroxycinnamic acids in vivo.

NMR structure and backbone dynamics of a concatemer of epidermal growth factor homology modules of the human low-density lipoprotein receptor

The ligand-binding region of the low-density lipoprotein (LDL) receptor is formed by seven N-terminal, imperfect, cysteine-rich (LB) modules. This segment is followed by an epidermal growth factor precursor homology domain with two N-terminal, tandem, EGF-like modules that are thought to participate in LDL binding and recycling of the endocytosed receptor to the cell surface. EGF-A and the concatemer, EGF-AB, of these modules were expressed in Escherichia coli. Correct protein folding of EGF-A and the concatemer EGF-AB was achieved in the presence or absence of calcium ions, in contrast to the LB modules, which require them for correct folding. Homonuclear and heteronuclear 1H-15N NMR spectroscopy at 17.6 T was used to determine the three-dimensional structure of the concatemer. Both modules are formed by two pairs of short, anti-parallel beta-strands. In the concatemer, these modules have a fixed relative orientation, stabilized by calcium ion-binding and hydrophobic interactions at the interface. 15N longitudinal and transverse relaxation rates, and [1H]-15N heteronuclear NOEs were used to derive a model-free description of the backbone dynamics of the molecule. The concatemer appears relatively rigid, particularly near the calcium ion-binding site at the module interface, with an average generalized order parameter of 0.85+/-0.11. Some mutations causing familial hypercholesterolemia may now be rationalized. Mutations of D41, D43 and E44 in the EGF-B calcium ion-binding region may affect the stability of the linker and thus the orientation of the tandem modules. The diminutive core also provides little structural stabilization, necessitating the presence of disulfide bonds. The structure and dynamics of EGF-AB contrast with the N-terminal LB modules, which require calcium ions both for folding to form the correct disulfide connectivities and for maintenance of the folded structure, and are connected by highly mobile linking peptides.

Intestinal release and uptake of phenolic antioxidant diferulic acids

Diferulic acids are potent antioxidants and are abundant structural components of plant cell walls, especially in cereal brans. As such, they are part of many human and animal diets and may contribute to the beneficial effect of cereal brans on health. However, these phenolics are ester-linked to cell wall polysaccharides and cannot be absorbed in this form. This study provides the first evidence that diferulic acids can be absorbed via the gastrointestinal tract. The 5-5-, 8-O-4-, and 8-5-diferulic acids were identified in the plasma of rats after oral dosing with a mixture of the three acids in oil. Our study also reveals that human and rat colonic microflora contain esterase activity able to release 5-5-, 8-O-4-, and 8-5-diferulic acids from model compounds and dietary cereal brans, hence providing a mechanism for release of dietary diferulates prior to absorption of the free acids. In addition, cell-free extracts from human and rat small intestine mucosa exhibited esterase activity towards diferulate esters. Hence, we have shown that esterified diferulates can be released from cereal brans by intestinal enzymes, and that free diferulic acids can be absorbed and enter the circulatory system. Our results suggest that the phenolic antioxidant diferulic acids are bioavailable.

A modular esterase from Penicillium funiculosum which releases ferulic acid from plant cell walls and binds crystalline cellulose contains a carbohydrate binding module

An esterase was isolated from cultures of the filamentous fungus Penicillium funiculosum grown on sugar beet pulp as the sole carbon source. The enzyme (ferulic acid esterase B, FAEB) was shown to be a cinnamoyl esterase (CE), efficiently releasing hydroxycinnamic acids from synthetic ester substrates and plant cell walls, and bound strongly to microcrystalline cellulose. A gene fragment was obtained by PCR using partial amino-acid sequences obtained from the pure enzyme and used to a probe a P. funiculosum genomic DNA library. A clone containing a 1120-bp ORF, faeB, was obtained which encoded a putative 353-residue preprotein including an 18-residue signal peptide, which when expressed in Eschericia coli produced CE activity. Northern analysis showed that transcription of faeB was tightly regulated, being stimulated by growth of the fungus on sugar beet pulp but inhibited by free glucose. The faeB promoter sequence contains putative motifs for binding an activator protein, XLNR, and a carbon catabolite repressor protein, CREA. FAEB was comprised of two distinct domains separated by a 20 residue Thr/Ser/Pro linker region. The N-terminal domain comprised 276 amino acids, contained a G-X-S-X-G motif typical of serine esterases, and was shown to be a member of a family comprising serine esterases, including microbial acetyl xylan esterases, poly (3-hydroxyalkanoate) depolymerases and CEs, and proteins of unknown function from Mycobacterium spp. and plants. The C-terminal domain comprised 39 amino acids and closely resembled the family 1 cellulose binding carbohydrate-binding modules (CBM) of fungal glycosyl hydrolases. This is the first report of a fungal CE with a CBM.

Three-dimensional NMR structure of the sixth ligand-binding module of the human LDL receptor: comparison of two adjacent modules with different ligand binding specificities

The sixth ligand-binding module of the low-density lipoprotein receptor contributes to the binding of apolipoprotein B100-containing lipoproteins. 1H NMR spectroscopy, DYANA and X-PLOR structure calculations were used to determine that this module has a well defined structure with a backbone conformation similar to other modules. Structures from calculations that simulated the presence of a calcium ion showed increased resolution without large increases in energy, increased deviations from idealised geometry or violations of experimental constraints. Investigation of the surface properties of this module indicates there are significant differences from the fifth module, which binds apolipoprotein E-containing lipoproteins in addition to apolipoprotein B100-containing lipoproteins.

NMR structure of a concatemer of the first and second ligand-binding modules of the human low-density lipoprotein receptor

The ligand-binding domain of the human low-density lipoprotein receptor consists of seven modules, each of 40-45 residues. In the presence of calcium, these modules adopt a common polypeptide fold with three conserved disulfide bonds. A concatemer of the first and second modules (LB(1-2)) folds efficiently in the presence of calcium ions, forming the same disulfide connectivities as in the isolated modules. The three-dimensional structure of LB(1-2) has now been solved using two-dimensional 1H NMR spectroscopy and restrained molecular dynamics calculations. No intermodule nuclear Overhauser effects were observed, indicating the absence of persistent interaction between them. The near random-coil NH and H alpha chemical shifts and the low phi and psi angle order parameters of the four-residue linker suggest that it has considerable flexibility. The family of LB(1-2) structures superimposed well over LB1 or LB2, but not over both modules simultaneously. LB1 and LB2 have a similar pattern of calcium ligands, but the orientations of the indole rings of the tryptophan residues W23 and W66 differ, with the latter limiting solvent access to the calcium ion. From these studies, it appears that although most of the modules in the ligand-binding region of the receptor are joined by short segments, these linkers may impart considerable flexibility on this region.

Dietary flavonoid and isoflavone glycosides are hydrolysed by the lactase site of lactase phlorizin hydrolase

Lactase phlorizin hydrolase (LPH; EC 3.2.1.62) is a membrane-bound, family 1 beta-glycosidase found on the brush border of the mammalian small intestine. LPH, purified from sheep small intestine, was capable of hydrolysing a range of flavonol and isoflavone glycosides. The catalytic efficiency (k(cat)/K(m)) for the hydrolysis of quercetin-4'-glucoside, quercetin-3-glucoside, genistein-7-glucoside and daidzein-7-glucoside was 170, 137, 77 and 14 (mM(-1) s(-1)) respectively. The majority of the activity occurred at the lactase and not phlorizin hydrolase site. The ability of LPH to deglycosylate dietary (iso)flavonoid glycosides suggests a possible role for this enzyme in the metabolism of these biologically active compounds.

A cinnamoyl esterase from Aspergillus niger can break plant cell wall cross-links without release of free diferulic acids

A cinnamoyl esterase, ferulic acid esterase A, from Aspergillus niger releases ferulic acid and 5-5- and 8-O-4-dehydrodiferulic acids from plant cell walls. The breakage of one or both ester bonds from dehydrodimer cross-links between plant cell wall polymers is essential for optimal action of carbohydrases on these substrates, but it is not known if cinnamoyl esterases can break these cross-links by cleaving one of the ester linkages which would not release the free dimer. It is difficult to determine the mechanism of the reaction on complex substrates, and so we have examined the catalytic properties of ferulic acid esterase A from Aspergillus niger using a range of synthetic ethyl esterified dehydrodimers (5-5-, 8-5-benzofuran and 8-O-4-) and two 5-5-diferulate oligosaccharides. Our results show that the esterase is able to cleave the three major dehydrodiferulate cross-links present in plant cell walls. The enzyme is highly specific at hydrolysing the 5-5- and the 8-5-benzofuran diferulates but the 8-O-4-is a poorer substrate. The hydrolysis of dehydrodiferulates to free acids occurs in two discrete steps, one involving dissociation of a monoesterified intermediate which is negatively charged at the pH of the reaction. Although ferulic acid esterase A was able to release monoesters as products of reactions with all three forms of diesters, only the 5-5- and the 8-O-4-monoesters were substrates for the enzyme, forming the corresponding free diferulic acids. The esterase cannot hydrolyse the second ester bond from the 8-5-benzofuran monoester and therefore, ferulic acid esterase A does not form 8-5-benzofuran diferulic acid. Therefore, ferulic acid esterase A from Aspergillus niger contributes to total plant cell wall degradation by cleaving at least one ester bond from the diferulate cross-links that exist between wall polymers but does not always release the free acid product.

Purification of cytosolic beta-glucosidase from pig liver and its reactivity towards flavonoid glycosides

Flavonoid glycosides are common dietary components which may have health-promoting activities. The metabolism of these compounds is thought to influence their bioactivity and uptake from the small intestine. It has been suggested that the enzyme cytosolic beta-glucosidase could deglycosylate certain flavonoid glycosides. To test this hypothesis, the enzyme was purified to homogeneity from pig liver for the first time. It was found to have a molecular weight (55 kDa) and specific activity (with p-nitrophenol glucoside) consistent with other mammalian cytosolic beta-glucosidases. The pure enzyme was indeed found to deglycosylate various flavonoid glycosides. Genistein 7-glucoside, daidzein 7-glucoside, apigenin 7-glucoside and naringenin 7-glucoside all acted as substrates, but we were unable to detect activity with naringenin 7-rhamnoglucoside. Quercetin 4'-glucoside was a substrate, but neither quercetin 3, 4'-diglucoside, quercetin 3-glucoside nor quercetin 3-rhamnoglucoside were deglycosylated. Estimates of K(m) ranged from 25 to 90 microM while those for V(max) were about 10% of that found with the standard artificial substrate p-nitrophenol glucoside. The non-substrate quercetin 3-glucoside was found to partially inhibit deglycosylation of quercetin 4'-glucoside, but it had no effect upon activity with p-nitrophenol glucoside. This study confirms that mammalian cytosolic beta-glucosidase can deglycosylate some, but not all, common dietary flavonoid glycosides. This enzyme may, therefore, be important in the metabolism of these compounds.

A modular cinnamoyl ester hydrolase from the anaerobic fungus Piromyces equi acts synergistically with xylanase and is part of a multiprotein cellulose-binding cellulase-hemicellulase complex

A collection of clones, isolated from a Piromyces equi cDNA expression library by immunoscreening with antibodies raised against affinity purified multienzyme fungal cellulase-hemicellulase complex, included one which expressed cinnamoyl ester hydrolase activity. The P. equi cinnamoyl ester hydrolase gene (estA) comprised an open reading frame of 1608 nt encoding a protein (EstA) of 536 amino acids and 55540 Da. EstA was modular in structure and comprised three distinct domains. The N-terminal domain was closely similar to a highly conserved non-catalytic 40-residue docking domain which is prevalent in cellulases and hemicellulases from three species of anaerobic fungi and binds to a putative scaffolding protein during assembly of the fungal cellulase complex. The second domain was also not required for esterase activity and appeared to be an atypically large linker comprising multiple tandem repeats of a 13-residue motif. The C-terminal 270 residues of EstA contained an esterase catalytic domain that exhibited overall homology with a small family of esterases, including acetylxylan esterase D (XYLD) from Pseudomonas fluorescens subsp. cellulosa and acetylxylan esterase from Aspergillus niger. This region also contained several smaller blocks of residues that displayed homology with domains tentatively identified as containing the essential catalytic residues of a larger group of serine hydrolases. A truncated variant of EstA, comprising the catalytic domain alone (EstA'), was expressed in Escherichia coli as a thioredoxin fusion protein and was purified to homogeneity. EstA' was active against synthetic and plant cell-wall-derived substrates, showed a marked preference for cleaving 1-->5 ester linkages between ferulic acid and arabinose in feruloylated arabino-xylo-oligosaccharides and was inhibited by the serine-specific protease inhibitor aminoethylbenzene-sulphonylfluoride. EstA' acted synergistically with xylanase to release more than 60% of the esterified ferulic acid from the arabinoxylan component of plant cell walls. Western analysis confirmed that EstA is produced by P. equi and is a component of the aggregated multienzyme cellulase-hemicellulase complex. Hybrid proteins, harbouring one, two or three iterations of the conserved 40-residue fungal docking domain fused to the reporter protein glutathione S-transferase, were produced. Western blot analysis of immobilized P. equi cellulase-hemicellulase complex demonstrated that each of the hybrid proteins bound to a 97 kDa polypeptide in the extracellular complex.

NMR studies of the low-density lipoprotein receptor-binding peptide of apolipoprotein E bound to dodecylphosphocholine micelles

Circular dichroism and NMR spectroscopy have been used to determine the structure of the low-density lipoprotein (LDL) receptor-binding peptide, comprising residues 130-152, of the human apolipoprotein E. This peptide has little persistent three-dimensional structure in solution, but when bound to micelles of dodecylphosphocholine (DPC) it adopts a predominantly alpha-helical structure. The three-dimensional structure of the DPC-bound peptide has been determined by using 1H-NMR spectroscopy: the structure derived from NOE-based distance constraints and restrained molecular dynamics is largely helical. The derived phi and psi angle order parameters show that the helical structure is well defined but with some flexibility that causes the structures not to be superimposable over the full peptide length. Deuterium exchange experiments suggest that many peptide amide groups are readily accessible to the solvent, but those associated with hydrophobic residues exchange more slowly, and this helix is thus likely to be positioned on the surface of the DPC micelles. In this conformation the peptide has one hydrophobic face and two that are rich in basic amino acid side chains. The solvent-exposed face of the peptide contains residues previously shown to be involved in binding to the LDL receptor.

Activity of an Aspergillus terreus alpha-arabinofuranosidase on phenolic-substituted oligosaccharides

The effect of phenolic substitutions on the activity of an alpha-arabinofuranosidase from Aspergillus terreus was investigated using feruloylated oligosaccharides isolated from plant cell walls, equivalent oligosaccharides obtained through treatment with specific ferulic acid esterases, and a synthetic lignin-carbohydrate complex (LCC). Feruloyl substituents limited the hydrolysis of arabinoxylan and arabinan oligosaccharides but only if the feruloyl group was esterified to the terminal non-reducing arabinose. Somewhat surprisingly, the LCC-model compound, in which the arabinose residue is substituted with a bulky dilignol group, was degraded by the enzyme. This indicated that the enzyme is able to approach this linkage from the xylose side.

Folding, calcium binding, and structural characterization of a concatemer of the first and second ligand-binding modules of the low-density lipoprotein receptor

The ligand-binding domain of the low-density lipoprotein (LDL) receptor is comprised of seven tandemly repeated ligand-binding modules, each being approximately 40 amino acids long and containing six conserved cysteine residues. We have expressed and characterized a concatemer of the first two modules (LB1 and LB2) of the human LDL receptor. Oxidative folding of the recombinant concatemer (rLB1-2), in the presence of calcium ions, gave a single dominant isomer with six disulfide bonds. Peptic cleavage of the short linker region that connects the last cysteine residue of LB1 and the first cysteine residue of LB2 yielded two discrete fragments, thus excluding the presence of intermodule disulfide bonds. The N-terminal module, LB1, reacted with a conformation-specific monoclonal antibody (IgG-C7) made to LB1 in the native LDL receptor. From this, we concluded that the first module was correctly folded, with the same set of disulfide bonds as LB1 of the LDL receptor. The disulfide bond connections of LB2 were identified from mass spectral analysis of fragments formed by digestion of the C-terminal peptic fragment with elastase. These data showed that the disulfide bonds of LB2 connected Cys(I) and Cys(III), Cys(II) and Cys(V), and Cys(IV) and Cys(VI). This pattern is identical to that found for recombinant LB1 and LB2. The concatemer has two high-affinity calcium-binding sites, one per module. An analysis of the secondary chemical shifts of Calpha protons shows that the conformations of LB1 and LB2 in the concatemer are very similar to those of the individual modules, with no evidence for strong interactions between the two modules.

Calcium is essential for the structural integrity of the cysteine-rich, ligand-binding repeat of the low-density lipoprotein receptor

Seven cysteine-rich repeats form the ligand-binding region of the low-density lipoprotein (LDL) receptor. Each of these repeats is assumed to bind a calcium ion, which is needed for association of the receptor with its ligands, LDL and beta-VLDL. The effects of metal ions on the folding of the reduced N-terminal cysteine-rich repeat have been examined by using reverse-phase high-performance liquid chromatography to follow the formation of fully oxidized isomers with different disulfide connectivities. In the absence of calcium many of the 15 possible isomers formed on oxidation, whereas in its presence the predominant product at equilibrium had the native disulfide bond connectivities. Other metals were far less effective at directing disulfide bond formation: Mn2+ partly mimicked the action of Ca2+, but Ba2+, Sr2+, and Mg2+ had little effect. This metal-ion specificity was also observed in two-dimensional 1H NMR spectral studies; only Ca2+ induced the native three-dimensional fold. The two paramagnetic ions, Gd3+ and Mn2+, and Cd2+ did not promote adoption of a well-defined structure, and the two paramagnetic ions did not displace calcium ions. The location of calcium ion binding sites in the repeat was also explored by NMR spectroscopy. The absence of chemical shift changes for the side chain proton resonances of Asp26, Asp36, and Glu37 from pH 3.9 to 6.8 in the presence of calcium ions and their proximal location in the NMR structures implicated these side chains as calcium ligands. Deuterium exchange NMR experiments also revealed a network of hydrogen bonds that stabilizes the putative calcium-binding loop.

Influence of ferulic acid on the production of feruloyl esterases by Aspergillus niger

Extracellular feruloyl esterases from the filamentous fungus Aspergillus niger are induced by growth on oat spelt xylan (OSX), which contains no detectable esterified ferulic acid. FAE-III accounted for most of the feruloyl esterase activity. Addition of free ferulic acid to OSX at the start of the culture induced FAE-III secretion a further 2.3-fold, and also induced other feruloyl esterases which could not be ascribed to FAE-III. Wheat bran-(WB)-grown cultures, containing 1% (m/v) esterlinked ferulic acid, gave almost identical FAE-III and total feruloyl esterase activities as the cultures grown on OSX plus ferulic acid. De-esterification of WB yielded less total feruloyl esterase, and 2.4-fold less FAE-III, compared to untreated WB. A slightly modified form of FAE-III was produced on de-esterified WB. These results show that production of FAE-III does not absolutely require ferulic acid. However, production is stimulated by the presence of free ferulic acid through increased expression, and is reduced by the removal of esterified ferulic acid from the growth substrate.

The faeA genes from Aspergillus niger and Aspergillus tubingensis encode ferulic acid esterases involved in degradation of complex cell wall polysaccharides

We report the cloning and characterization of a gene encoding a ferulic acid esterase, faeA, from Aspergillus niger and Aspergillus tubingensis. The A. niger and A. tubingensis genes have a high degree of sequence identity and contain one conserved intron. The gene product, FAEA, was overexpressed in wild-type A. tubingensis and a protease-deficient A. niger mutant. Overexpression of both genes in wild-type A. tubingensis and an A. niger protease-deficient mutant showed that the A. tubingensis gene product is more sensitive to degradation than the equivalent gene product from A. niger. FAEA from A. niger was identical to A. niger FAE-III (C. B. Faulds and G. Williamson, Microbiology 140:779-787, 1994), as assessed by molecular mass, pH and temperature optima, pI, N-terminal sequence, and activity on methyl ferulate. The faeA gene was induced by growth on wheat arabinoxylan and sugar beet pectin, and its gene product (FAEA) released ferulic acid from wheat arabinoxylan. The rate of release was enhanced by the presence of a xylanase. FAEA also hydrolyzed smaller amounts of ferulic acid from sugar beet pectin, but the rate was hardly affected by addition of an endo-pectin lyase.

Methyl phenylalkanoates as substrates to probe the active sites of esterases

We have used methyl esters of phenylalkanoic acids to probe the active site of two esterases (FAE-III and CinnAE) from Aspergillus niger. Only methyl 4-hydroxy-3-methoxycinnamate and 4-hydroxy-3-methoxyphenylpropionate out of 19 substrates tested were significant substrates for both enzymes (k(cat) values about 10(2) s(-1) and 10(3) s(-1), respectively). Lengthening or shortening the aliphatic side chain while maintaining the same aromatic substitutions completely abolished activity for both enzymes, which demonstrates the importance of the correct distance between the aromatic group and the ester bond. Differences in Km values for FAE-III were small (0.45-2.08 mM) but there were two orders of magnitude difference in k(cat) values (12.1-1063 s(-1)), whereas for CinnAE, there were large differences in values for both Km (0.014-1.32 mM) and k(cat) (41.3-1410 s(-1)). Lability of the ester bonds, as estimated from second-order rate constants (k2) for chemical reaction with sodium hydroxide, did not correlate to k(cat) for CinnAE (r = 0.33) or for FAE-III (r = 0.43). Maintaining the phenylpropenoate structure but altering the substitutions on the aromatic ring demonstrated the following: a 3-methoxy group is essential for FAE-III activity, whereas a 3-methoxy group precluded activity of CinnAE, with the exception of methyl 4-hydroxy-3-methoxycinnamate which was a relatively poor substrate for CinnAE; (b) increasing the number of methoxy substitutions increased the activity of FAE-III, and decreased the activity of CinnAE; (c) 4-hydroxy substituents, and additional hydroxy substituents, increased the activity of CinnAE, but decreased that of FAE-III; (d) the rate of hydrolysis with sodium hydroxide of the methyl esters in general is decreased by hydroxy substitutions on the aromatic ring but increased by methoxy substitutions. Analysis of kinetic data obtained in the presence of inhibitors indicated that substrate analogs were able to bind to both free CinnAE and to a CinnAE-substrate complex, but conversely, were only able to bind to free FAE-III. The results show that the specificities of the two A. niger esterases are complementary. The rate of hydrolysis by this class of carboxylic ester hydrolase does not depend on the intrinsic lability of the ester bond, but depends on both the distance between the aromatic ring and the ester bond, and the substitutions on the aromatic ring.

Cholesterol and atherosclerosis

Cholesterol is an essential component of cellular membranes, but when present in excess in the circulation, can be deposited in the arterial wall, leading to the formation of atherosclerotic lesions. The principal plasma carrier of cholesterol, low density lipoprotein (LDL), is removed from the circulation by LDL receptors in the liver. This process plays a critical role in regulating plasma LDL levels. The unique structure and composition of the LDL particle makes it particularly susceptible to modification by oxidative reactions. Current evidence suggests that the production of oxidised LDL in the intima, and the removal of oxidised LDL particles by scavenger receptors on macrophages, play a central role in the development and progression of atherosclerotic lesions.

An Aspergillus niger esterase (ferulic acid esterase III) and a recombinant Pseudomonas fluorescens subsp. cellulosa esterase (Xy1D) release a 5-5' ferulic dehydrodimer (diferulic acid) from barley and wheat cell walls

Diferulate esters strengthen and cross-link primary plant cell walls and help to defend the plant from invading microbes. Phenolics also limit the degradation of plant cell walls by saprophytic microbes and by anaerobic microorganisms in the rumen. We show that incubation of wheat and barley cell walls with ferulic acid esterase from Aspergillus niger (FAE-III) or Pseudomonas fluorescens (Xy1D), together with either xylanase I from Aspergillus niger, Trichoderma viride xylanase, or xylanase from Pseudomonas fluorescens (XylA), leads to release of the ferulate dimer 5-5' diFA [(E,E)-4,4'-dihydroxy-5,5'-dimethoxy-3,3'-bicinnamic acid]. Direct saponification of the cell walls without enzyme treatment released the following five identifiable ferulate dimers (in order of abundance): (Z)-beta-(4-[(E)-2-carboxyvinyl]-2-methoxyphenoxy)-4-hydroxy-3-methoxycinnamic acid, trans-5-[(E)-2-carboxyvinyl]-2-(4-hydroxy-3-methoxy-phenyl) -7-methoxy-2, 3-dihydrobenzofuran-3-carboxylic acid, 5-5' diFA, (E,E)-4, 4'-dihydroxy-3, 5'-dimethoxy-beta, 3'-bicinnamic acid, and trans-7-hydroxy-1-(4-hydroxy-3-methoxyphenyl) -6-methoxy-1, 2-dihydronaphthalene-2, 3-dicarboxylic acid. Incubation of the wheat or barley cell walls with xylanase, followed by saponification of the solubilized fraction, yielded 5-5'diFA and, in some cases, certain of the above dimers, depending on the xylanase used. These experiments demonstrate that FAE-III and XYLD specifically release only esters of 5-5'diFA from either xylanase-treated or insoluble fractions of cell walls, even though other esterified dimers were solubilized by preincubation with xylanase. It is also concluded that the esterified dimer content of the xylanase-solubilized fraction depends on the source of the xylanase.

A deletion in the first cysteine-rich repeat of the low-density-lipoprotein receptor leads to the formation of multiple misfolded isomers

The ligand-binding domain of the low-density-lipoprotein (LDL) receptor comprises seven cysteine-rich repeats, each approximately 40 amino acids long. The deletion of two amino acids (Asp26 and Gly27) from the first of these repeats (LB1), leads to a defective LDL receptor, and the clinical syndrome of familial hypercholesterolemia [Leitersdorf, E., Hobbs, H. H., Fourie, A. M., Jacobs, M., van der Westhuyzen, D.R. & Coetzee, G.A. (1988) Proc. Natl Acad. Sci. USA 85, 7912-7916]. Receptors which reach the cell surface fail to bind IgG-C7, a conformation-specific monoclonal antibody directed to LB1. To determine the effects of the two-amino-acid deletion on the folding of the LB1 of the LDL receptor, we have expressed LB1 and the mutant repeat, des-Asp26, Gly27-LB1, as recombinant (rLB1 and des-Asp26, Gly27-rLB1) peptides, and have determined their ability to fold in vitro. Unlike rLB1, which folded into a single isomer that was recognized by IgG-C7 and had three disulfide bonds, des-Asp26, Gly27-rLB1 folded into an equilibrium mixture of four isomers. Each of these isomers contained three disulfide bonds, but none were recognized by IgG-C7. We suggest that LDL receptors in the endoplasmic reticulum (ER) of the cell also fold into an equilibrium mixture of distinct receptor molecules, each with an abnormally folded isomer of des-Asp26, Gly27-LB1, and that the retarded transport of receptors to the cell surface arises because only a subset of the isomers reaches the cell surface.

Release of ferulic acid from sugar-beet pulp by using arabinanase, arabinofuranosidase and an esterase from Aspergillus niger

Aspergillus niger cinnamoyl esterase (CinnAE) is shown to be active towards a wide range of feruloylated oligosaccharides derived from sugar-beet pulp (SBP). The esterase hydrolysed ferulic acid ester-linked to either C-2 of arabinose or C-6 of galactose residues, and demonstrated the highest activity towards the feruloylated arabinose trisaccharide. However, CinnAE was able to release only 0.88% of total alkali-extractable ferulic acid from SBP in 24 h when acting alone. To determine whether cell-wall-degrading enzymes could increase the release of ferulic acid by CinnAE, SBP was incubated with various carbohydrases [cellulase, polygalacturonase, endo-arabinanase, alpha-L-arabinofuranosidase, endo-(1,4-beta-D-galactanase, beta-D-galactosidase]. These were added alone and in pairs, both in the presence and absence of CinnAE. We showed that all the carbohydrases tested were free of esterase activity. When individual carbohydrases were incubated with SBP, whether in the presence or absence of CinnAE, less than 1% of the feruloyl groups were released. When incubated with a mixture of endo-arabinanase and alpha-L-arabinofuranosidase, the esterase was able to release 14 times more of the alkali-extractable ferulic acid present in the whole pulp as free acid than CinnAE alone. Ferulic acid is linked either to L-arabinose or D-galactose in SBP, but no corresponding increase in ferulic acid release was detected when SBP was incubated with CinnAE plus endo-(1,4)-beta-D-galactanase and beta-D-galactosidase (both from A. niger). Hence feruloylated arabinans in SBP are readily available for hydrolysis by arabinan-degrading enzymes, whereas feruloylated galactans are not available for hydrolysis by galactan-degrading enzymes.

Purification and characterization of a novel esterase induced by growth of Aspergillus niger on sugar-beet pulp

An inducible esterase has been isolated from a liquid culture of Aspergillus niger grown on sugar-beet pulp. The enzyme was active on methyl esters of cinnamic acids, caffeic > p-coumaric > ferulic, and is therefore termed a cinnamoyl esterase. The enzyme was not active on methyl sinapinate, a good substrate for ferulic acid esterase III, which was purified previously from A. niger [Faulds and Williamson (1994) Microbiology 140, 779-787]. With methyl caffeate as substrate the enzyme had temperature and pH optima of 50 degrees C and 6.0 respectively, and a specific activity of 96.9 units per mg of protein. The purified protein (native molecular mass 145 000 Da) gave a single heavily stained band on SDS/PAGE, suggesting the protein was a dimer, and seemed to be heavily glycosylated. Isoelectric focusing gave a single band corresponding to a pl of 4.80. The pure enzyme was free of other carbohydrase activities. The activity of the pure enzyme was inhibited by more than 99% after treatment with the serine-specific protease inhibitor aminoethylbenzenesulphonylfluoride (1 mM) for 12 h. The enzyme was capable of releasing ferulic acid from sugar beet pulp.

Three-dimensional structure of the second cysteine-rich repeat from the human low-density lipoprotein receptor

The ligand-binding domain of the low-density lipoprotein receptor comprises seven cysteine-rich repeats, which have been highly conserved through evolution. This domain mediates interactions of the receptor with two lipoprotein apoproteins, apo E and apo B-100, putatively through a calcium-dependent association of the ligands with a cluster of acidic residues on the receptor. The second repeat (rLB2) of the receptor binding domain has been expressed as a thrombin-cleavable GST fusion protein, cleaved, and purified. On oxidation the protein refolded to give a single peak on reverse-phase HPLC. The aqueous solution structure of rLB2 has been determined using two-dimensional 1H NMR spectroscopy. In contrast to the amino-terminal repeat, rLB1, rLB2 has a very flexible structure in water. However, the conformation of rLB2 is markedly more ordered in the presence of a 4-fold molar excess of calcium chloride; the proton resonance dispersion and the number of NOESY cross-peaks are greatly enhanced. The three-dimensional structure of rLB2, obtained from the NMR data by molecular geometry and restrained molecular dynamics methods, parallels that of rLB1, with an amino-terminal hairpin structure followed by a succession of turns. However, there are clear differences in the backbone topology and structural flexibility. As for rLB1, the acidic residues are clustered on one face of the module. The side chain of Asp 37, which is part of a completely conserved SDE sequence thought to be involved in ligand binding, is buried, as is its counterpart (Asp 36) in rLB1. These results provide the first experimental support for the hypothesis that each of the repeats in the ligand-binding domain has a similar global fold but also highlight significant differences in structure and internal dynamics.

Disulfide bridges of a cysteine-rich repeat of the LDL receptor ligand-binding domain

The low density lipoprotein (LDL) receptor is the prototype of a family of structurally related cell surface receptors that mediate the endocytosis of multiple ligands in mammalian cells. Its ligand-binding domain consists of seven cysteine-rich ligand-binding repeats, each approximately 40 amino acid residues long. Ligand-binding repeats occur in other members of the LDL receptor (LDLR) gene family and in a number of functionally unrelated proteins. As a first step toward an understanding of the structure and function of LB repeats, we have expressed the amino-terminal ligand-binding repeat (LB1) of the human LDLR as a recombinant peptide (rLB1) and have determined its disulfide-pairing scheme. Oxidative folding of rLB1 yielded a single isomer which contained three disulfide bonds. This isomer reacted with a conformation-specific monoclonal antibody (IgG-C7) made to LB1 in the native LDLR, suggesting that rLB1 was correctly folded. rLB1 was resistant to digestion with trypsin, chymotrypsin, and V8 protease, consistent with a tightly folded structure. Disulfide bond connections were established using two separate approaches. Digestion with the nonspecific proteolytic enzyme proteinase K yielded an 8 amino acid peptide with a single disulfide bond which connected Cys(IV) and Cys(VI). In the second approach, disulfide bonds were sequentially reduced with tris(2-carboxyethyl)phosphine and the resulting cysteine residues alkylated with iodoacetamide. An analysis of peptides which contained two cysteinylacetamide residues, derived from a single reduced disulfide bond, showed that Cys(I) and Cys(III) were disulfide-bonded and confirmed the presence of a disulfide bond between Cys(IV) and Cys(VI). We infer that the remaining disulfide bond bridges Cys(II) and Cys(V).(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and disulfide-bond connectivity of the second ligand-binding repeat of the human LDL receptor

The human LDL receptor (LDLR) has a binding domain which consists of seven contiguous ligand-binding (LB) repeats, each approximately 40 amino acids long with three disulfide bonds. The second LB repeat, which is required for full binding of LDL, has been expressed, purified and folded to yield a single, fully oxidized isomer. By selective reduction and alkylation, we have shown that the cysteine residues have a I-III, II-V, IV-VI connectivity, matching that recently determined for the amino-terminal repeat. We suggest that the first two LB repeats of the LDLR, with their unique disulfide-bonding pattern, serve as a structural paradigm for other LB repeats.

Three-dimensional structure of a cysteine-rich repeat from the low-density lipoprotein receptor

The low-density lipoprotein (LDL) receptor plays a central role in mammalian cholesterol metabolism, clearing lipoproteins which bear apolipoproteins E and B-100 from plasma. Mutations in this molecule are associated with familial hypercholesterolemia, a condition which leads to an elevated plasma cholesterol concentration and accelerated atherosclerosis. The N-terminal segment of the LDL receptor contains a heptad of cysteine-rich repeats that bind the lipoproteins. Similar repeats are present in related receptors, including the very low-density lipoprotein receptor and the LDL receptor-related protein/alpha 2-macroglobulin receptor, and in proteins which are functionally unrelated, such as the C9 component of complement. The first repeat of the human LDL receptor has been expressed in Escherichia coli as a glutathione S-transferase fusion protein, and the cleaved and purified receptor module has been shown to fold to a single, fully oxidized form that is recognized by the monoclonal antibody IgG-C7 in the presence of calcium ions. The three-dimensional structure of this module has been determined by two-dimensional NMR spectroscopy and shown to consist of a beta-hairpin structure, followed by a series of beta turns. Many of the side chains of the acidic residues, including the highly conserved Ser-Asp-Glu triad, are clustered on one face of the module. To our knowledge, this structure has not previously been described in any other protein and may represent a structural paradigm both for the other modules in the LDL receptor and for the homologous domains of several other proteins. Calcium ions had only minor effects on the CD spectrum and no effect on the 1H NMR spectrum of the repeat, suggesting that they induce no significant conformational change.

Antibodies made against a formaldehyde-protein adduct cross react with an acetaldehyde-protein adduct. Implications for the origin of antibodies in human serum which recognize acetaldehyde-protein adducts

Acetaldehyde, the major metabolite of ethanol, reacts with lysine and other free amino groups on proteins to form acetaldehyde-protein adducts. The presence of antibodies which recognize such acetaldehyde-protein adducts in sera from alcoholics has been attributed to an immune response to such adducts. Complicating this conclusion is the finding that sera from non-alcoholic control subjects also contain antibodies which recognize acetaldehyde-protein adducts. In the current research we sought to determine whether antibodies which recognize epitopes formed by the reaction of a protein with acetaldehyde can be formed in response to a protein modified with a structurally related protein adduct. We modified lysine residues on apolipoprotein (apo) B-100 with acetaldehyde and formaldehyde under reducing conditions, to form epsilon-N-methyl- and epsilon-N-ethyl-lysine residues, and with acetic anhydride to form epsilon-N-acetyl-lysine residues, and made antibodies against these modified proteins in guinea-pigs. In ELISA assays antibodies made against methylated apoB-100 (Me-apoB) cross-reacted effectively with ethylated apoB-100 (Et-apoB), while antibodies made against acetic anhydride-modified apoB-100 did not cross-react. We conclude that methyl-lysine shares one or more immunoreactive epitopes with ethyl-lysine, and that antibodies which recognize acetaldehyde-modified proteins can be formed in response to formaldehyde-modified proteins. We demonstrate that sera from both alcoholics and non-drinkers contain antibodies which recognize Me-apoB and Et-apoB and that the titres of these antibodies are comparable. These data raise the possibility that some human serum antibodies which recognize acetaldehyde-modified protein epitopes may have been made against formaldehyde-modified protein epitopes.(ABSTRACT TRUNCATED AT 250 WORDS)

Low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase gene expression in human mononuclear leukocytes is regulated coordinately and parallels gene expression in human liver

The liver plays a key regulatory role in cholesterol metabolism. Two proteins are central in this role; the LDL receptor and 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), the rate-limiting enzyme in cholesterol biosynthesis. In the current investigation, we have used a sensitive nonradioactive method to study the regulation of LDL receptor and HMG CoA reductase mRNA levels in liver biopsy samples and freshly isolated mononuclear leukocytes from 13 patients who underwent cholecystectomy for gallstones. mRNA copy numbers were determined by PCR amplification of reverse-transcribed RNA using synthetic RNA as an internal standard. Incorporation of digoxigenin-11-dUTP during amplification allowed direct detection and quantitation of mRNA levels by chemiluminescence. These experiments showed that the average number of LDL receptor mRNA molecules in liver (21 +/- 3 x 10(4)/micrograms of RNA) and mononuclear leukocytes (24 +/- 3 x 10(4)/micrograms of RNA) are indistinguishable, whereas the number of HMG CoA reductase molecules in liver (107 +/- 15 x 10(4)/micrograms of RNA) is smaller than that in mononuclear leukocytes (158 +/- 21 x 10(4)/micrograms of RNA, P < 0.05). These numbers correspond to an average of 1-6 copies of LDL receptor mRNA and 5-42 copies of HMG CoA reductase mRNA per cell. There was a significant correlation between the numbers of LDL receptor (P = 0.0005) and HMG CoA reductase (P = 0.003) mRNA molecules in liver and mononuclear leukocytes. Furthermore, the numbers of copies of HMG CoA reductase and LDL receptor mRNA were correlated with each other in both liver (P = 0.02) and mononuclear leukocytes (P = 0.01), consistent with coordinate regulation. These data demonstrate that the mechanisms which regulate mRNA levels in liver and mononuclear cells are similar and suggest that freshly isolated mononuclear cells can be used to predict HMG CoA reductase and LDL receptor mRNA levels in liver.

Fluorescence study of the motional states of core and surface lipids in native and reconstituted low density lipoproteins

Low density lipoproteins (LDL) consist of an apolar core of cholesterol esters and triglycerides surrounded by a monolayer of phospholipid, cholesterol, and a single molecule of apolipoprotein B (apoB-100). To determine the influence of core and surface constituents on the surface of LDL, we have measured core and surface order parameters for native LDL, and reconstituted LDLs (rLDL) whose apolar core lipids were extracted and replaced with either cholesterol oleate (CO) or triolein (TO). Order parameters were measured by fluorescence depolarization of diphenylhexatriene (DPH), which is located primarily in the core, and of trimethylammoniumdiphenylhexatriene (TMA-DPH), which is anchored at the water-phospholipid interface. DPH order parameters for LDL reconstituted with TO (r-[TO]LDL) were much lower than those for LDL reconstituted with CO (r-[CO]LDL), consistent with the physical properties of TO, a nonviscous liquid at all temperatures studied, and CO, which exists in a liquid crystalline or viscous liquid state at the temperatures studied. Although core cholesterol esters in r[CO]LDL and native LDL undergo distinct order-disorder transitions, these transitions were not detected by DPH. This is most likely due to the difference between the time scale for end-over-end tumbling of cholesterol esters and the fluorescence lifetime of DPH. Despite the fact that the core lipids of r-[CO]LDL were much more ordered than those of r-[TO]LDL, surface order parameters for both lipoproteins were similar. We conclude that the motional states of the core and surface lipids are relatively independent. Surface order parameters for native LDL were higher than those for reconstituted LDLs.(ABSTRACT TRUNCATED AT 250 WORDS)

Isoprenyl diphosphate synthases: protein sequence comparisons, a phylogenetic tree, and predictions of secondary structure

Isoprenyl diphosphate synthases are ubiquitous enzymes that catalyze the basic chain-elongation reaction in the isoprene biosynthetic pathway. Pairwise sequence comparisons were made for 6 farnesyl diphosphate synthases, 6 geranylgeranyl diphosphate synthases, and a hexaprenyl diphosphate synthase. Five regions with highly conserved residues, two of which contain aspartate-rich DDXX(XX)D motifs found in many prenyltransferases, were identified. A consensus secondary structure for the group, consisting mostly of alpha-helices, was predicted for the multiply aligned sequences from amino acid compositions, computer assignments of local structure, and hydropathy indices. Progressive sequence alignments suggest that the 13 isoprenyl diphosphate synthases evolved from a common ancestor into 3 distinct clusters. The most distant separation is between yeast hexaprenyl diphosphate synthetase and the other enzymes. Except for the chromoplastic geranylgeranyl diphosphate synthase from Capsicum annuum, the remaining farnesyl and geranylgeranyl diphosphate synthases segregate into prokaryotic/archaebacterial and eukaryotic families.

Measurement of mRNA by quantitative PCR with a nonradioactive label

This report describes the development of a method to measure mRNA in small samples of human tissue by the polymerase chain reaction with a nonradioactive label. In this method RNA is reverse-transcribed in the presence of a control RNA, and subsequently amplified by the polymerase chain reaction during which a nonradioactive label (digoxigenin-11-dUTP) is incorporated. Gel blotting and immunological detection of digoxigenin followed by a chemiluminescent reaction provide an intense signal on film. This allows the detection and quantitation of 3-hydroxy-3-methylglutaryl (HMG) CoA reductase mRNA in 12 ng of RNA. We demonstrate that this is a sensitive and reproducible method, and that quantitation is linear with respect to the amount of mRNA present. The application of this method to the measurement of low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA levels in circulating peripheral blood mononuclear cells and human liver biopsy samples is discussed. The use of chemiluminescent reagents instead of radioactive labels allows this procedure to be performed safely in laboratories not equipped for radioactivity.

Liver, lipoproteins and disease: I. Biochemistry of lipoprotein metabolism

Cholesterol is a structural component of biological membranes and an immediate precursor for steroid hormones and bile acids. The liver is central to the production and removal of cholesterol-rich lipoproteins and bile acids. The basic biochemical aspects of hepatic lipoprotein and cholesterol metabolism and how abnormalities in liver function impair these metabolic pathways are reviewed.

Liver, lipoproteins and disease: II. Clinical relevance of disordered cholesterol metabolism in liver disease

The alterations in the concentration and composition of lipoproteins that occur in liver disease indicate the central role of the liver in lipoprotein metabolism. A number of studies have characterized plasma lipoproteins in patients with liver disease, although in most cases the underlying molecular defects responsible for the changes are still undetermined.

Atherogenic diet increases cholesteryl ester transfer protein messenger RNA levels in rabbit liver

Cholesteryl ester transfer activity is increased in plasma of cholesterol-fed rabbits. To investigate the mechanisms leading to changes in activity, we measured cholesteryl ester transfer protein (CETP) mass by RIA and CETP mRNA abundance by Northern and slot blot analysis using a human CETP cDNA probe in control (n = 8) and cholesterol-fed rabbits (n = 10). Cholesterol feeding (chow plus 0.5% cholesterol, 10% corn oil) for 30 d increased CETP mass in plasma 3.2-fold in the cholesterol-fed rabbits (12.45 +/- 0.82 micrograms/ml) compared with controls (3.86 +/- 0.38 micrograms/ml). In the hypercholesterolemic rabbit, liver CETP mRNA levels were increased 2.8 times control mRNA levels. Actin, apo E, lecithin-cholesterol acyltransferase, and albumin mRNA abundances were unchanged. In contrast to the widespread tissue distribution in humans, CETP mRNA was not detected in extrahepatic tissues of either control or cholesterol-fed animals. Using a sensitive RNase protection assay, the increase in liver CETP mRNA was detectable within 3 d of beginning the high cholesterol diet. Thus, in response to the atherogenic diet there is an early increase in liver CETP mRNA, probably causing increased CETP synthesis and secretion, and increased plasma CETP. The results indicate that the CETP gene may be regulated by diet-induced changes in lipid metabolism.

Cloning, analysis, and bacterial expression of human farnesyl pyrophosphate synthetase and its regulation in Hep G2 cells

A partial length cDNA encoding farnesyl pyrophosphate synthetase (hpt807) has been isolated from a human fetal liver cDNA library in lambda gt11. DNA sequence analysis reveals hpt807 is 1115 bp in length and contains an open reading frame coding for 346 amino acids before reaching a stop codon, a polyadenylation addition sequence, and the first 14 residues of a poly(A+) tail. Considerable nucleotide and deduced amino acid sequence homology is observed between hpt807 and previously isolated rat liver cDNAs for farnesyl pyrophosphate synthetase. Comparison with rat cDNAs suggests that hpt807 is about 20 bp short of encoding the initiator methionine of farnesyl pyrophosphate synthetase. The human cDNA was cloned into a prokaryotic expression vector and Escherichia coli strain DH5 alpha F'IQ was transformed. Clones were isolated that express an active fusion protein which can be readily observed on protein gels and specifically stained on immunoblots with an antibody raised against purified chicken farnesyl pyrophosphate phosphate synthetase. These data confirm the identity of hpt807 as encoding farnesyl pyrophosphate synthetase. Slot blot analyses of RNA isolated from Hep G2 cells show that the expression of farnesyl pyrophosphate synthetase mRNA is regulated. Lovastatin increases mRNA levels for farnesyl pyrophosphate synthetase 2.5-fold while mevalonic acid, low-density lipoprotein, and 25-hydroxycholesterol decrease mRNA levels to 40-50% of control values.