Metabolic engineering applications to renewable resource utilization

Lignocellulosic materials containing cellulose, hemicellulose, and lignin are the most abundant renewable organic resource on earth. The utilization of renewable resources for energy and chemicals is expected to increase in the near future. The conversion of both cellulose (glucose) and hemicellulose (hexose and pentose) for the production of fuel ethanol is being studied intensively, with a view to developing a technically and economically viable bioprocess. Whereas the fermentation of glucose can be carried out efficiently, the bioconversion of the pentose fraction (xylose and arabinose, the main pentose sugars obtained on hydrolysis of hemicellulose), presents a challenge. A lot of attention has therefore been focused on genetically engineering strains that can efficiently utilize both glucose and pentoses, and convert them to useful compounds, such as ethanol. Metabolic strategies seek to generate efficient biocatalysts (bacteria and yeast) for the bioconversion of most hemicellulosic sugars to products that can be derived from the primary metabolism, such as ethanol. The metabolic engineering objectives so far have focused on higher yields, productivities and expanding the substrate and product spectra.

Gene expression profiling by DNA microarrays and metabolic fluxes in Escherichia coli

DNA microarray technology was applied to detect differential transcription profiles of a subset of the Escherichia coli genome. A total of 111 E. coli genes, including those in central metabolism, key biosyntheses, and some regulatory functions, were cloned, amplified, and used as probes for detecting the level of transcripts. An E. coli strain was grown in glucose, acetate, and glycerol media, and the transcript levels of the selected genes were detected. Despite extensive studies on E. coli physiology, many new features were found in the regulation of these genes. For example, several genes were unexpectedly up-regulated, such as pps, ilvG, aroF, secA, and dsbA in acetate and asnA and asnB in glycerol, or down-regulated, such as ackA, pta, and adhE in acetate. These genes were regulated with no apparent reasons by unknown mechanisms. Meanwhile, many genes were regulated for apparent purposes but by unknown mechanisms. For example, the glucose transport genes (ptsHI, ptsG, crr) in both acetate and glycerol media were down-regulated, and the ppc, glycolytic, and biosynthetic genes in acetate were also down-regulated because of the reduced fluxes. However, their molecular mechanisms remain to be elucidated. Furthermore, a group of genes were regulated by known mechanisms, but the physiological roles of such regulation remain unclear. This group includes pckA and aspA, which are up-regulated in glycerol, and gnd and aspA, which are down- and up-regulated, respectively, in acetate. The DNA microarray technology demonstrated here is a powerful yet economical tool for characterizing gene regulation and will prove to be useful for strain improvement and bioprocess development.

Arabinose fermentation by Lactobacillus plantarum in sourdough with added pentosans and alphaalpha-L-arabinofuranosidase: a tool to increase the production of acetic acid

Sixty-five strains of obligately and facultatively heterofermentative sourdough lactic acid bacteria were screened for their capacity to grow optimally in the presence of arabinose, ribose and xylose as carbon sources. Lactobacillus alimentarius 15F, Lact. brevis 10A, Lact. fermentum 1F and Lact. plantarum 20B showed higher growth rate, cell yield, acidification rate and production of acetic acid when some pentoses instead of maltose were added to the SDB medium. Lactobacillus plantarum 20B used arabinose also in a synthetic medium where complex growth factors such as yeast extract were omitted. Other Lact. plantarum strains did not show the same property. Pentosan extract was treated with alpha-L-arabinofuranosidase from Aspergillus niger or endo-xylanase from Bacillus subtilis to produce hydrolysates containing mainly arabinose and xylose, respectively. In particular, the hydrolysate containing arabinose substantiated the growth and the production of lactic acid and, especially, of acetic acid by Lact. plantarum 20B. Sourdough fermentation by Lact. plantarum 20B with addition of pentosan extract and alpha-L-arabinofuranosidase increased the acidification rate, titratable acidity and acetic acid content compared with traditional sourdough. A facultatively heterofermentative strain, Lact. plantarum 20B, also produced a sourdough with an optimal fermentation quotient.

A one-step phosphorylation of D-aldohexoses and D-aldopentoses with inorganic cyclo-triphosphate

The phosphorylation reaction by inorganic cyclo-triphosphate (P3m) having a six-membered ring was examined for D-aldohexoses and D-aldopentoses in aqueous solution. Similar to the process for D-glucose, D-galactose, D-xylose or D-allose were phosphorylated with P3m to give stereoselectively beta-D-galactopyranosyl 1-triphosphate, beta-D-xylopyranosyl 1-triphosphate or beta-D-allopyranosyl 1-triphosphate with maximum yields of 31.3, 32.5 or 32.1%, respectively. On the other hand, in the reaction of D-ribose, D-lyxose, D-mannose or D-arabinose with P3m, the yields of beta-D-ribopyranosyl 1-triphosphate, alpha-D-lyxopyranosyl 1-triphosphate, alpha-D-mannopyranosyl 1-triphosphate or alpha-D-arabinopyranosyl 1-triphosphate were 8.0, 16.5, 9.6 or 14.1%, respectively. The phosphorylation mechanism of D-aldopyranoses with P3m was also discussed.

Fuel ethanol after 25 years

After 25 years, Brazil and North America are still the only two regions that produce large quantities of fuel ethanol, from sugar cane and maize, respectively. The efficiency of ethanol production has steadily increased and valuable co-products are produced, but only tax credits make fuel ethanol commercially viable because oil prices are at an all-time low. The original motivation for fuel-ethanol production was to become more independent of oil imports; now, the emphasis is on its use as an oxygenated gasoline additive. There will only be sufficient, low-cost ethanol if lignocellulose feedstock is also used.

Genetic engineering for improved xylose fermentation by yeasts

Xylose utilization is essential for the efficient conversion of lignocellulosic materials to fuels and chemicals. A few yeasts are known to ferment xylose directly to ethanol. However, the rates and yields need to be improved for commercialization. Xylose utilization is repressed by glucose which is usually present in lignocellulosic hydrolysates, so glucose regulation should be altered in order to maximize xylose conversion. Xylose utilization also requires low amounts of oxygen for optimal production. Respiration can reduce ethanol yields, so the role of oxygen must be better understood and respiration must be reduced in order to improve ethanol production. This paper reviews the central pathways for glucose and xylose metabolism, the principal respiratory pathways, the factors determining partitioning of pyruvate between respiration and fermentation, the known genetic mechanisms for glucose and oxygen regulation, and progress to date in improving xylose fermentations by yeasts.

Transport and utilization of hexoses and pentoses in the halotolerant yeast Debaryomyces hansenii

Debaryomyces hansenii is a yeast species that is known for its halotolerance. This organism has seldom been mentioned as a pentose consumer. In the present work, a strain of this species was investigated with respect to the utilization of pentoses and hexoses in mixtures and as single carbon sources. Growth parameters were calculated for batch aerobic cultures containing pentoses, hexoses, and mixtures of both types of sugars. Growth on pentoses was slower than growth on hexoses, but the values obtained for biomass yields were very similar with the two types of sugars. Furthermore, when mixtures of two sugars were used, a preference for one carbon source did not inhibit consumption of the other. Glucose and xylose were transported by cells grown on glucose via a specific low-affinity facilitated diffusion system. Cells derepressed by growth on xylose had two distinct high-affinity transport systems for glucose and xylose. The sensitivity of labeled glucose and xylose transport to dissipation of the transmembrane proton gradient by the protonophore carbonyl cyanide m-chlorophenylhydrazone allowed us to consider these transport systems as proton symports, although the cells displayed sugar-associated proton uptake exclusively in the presence of NaCl or KCl. When the V(max) values of transport systems for glucose and xylose were compared with glucose- and xylose-specific consumption rates during growth on either sugar, it appeared that transport did not limit the growth rate.

Oxythiamine and dehydroepiandrosterone induce a G1 phase cycle arrest in Ehrlich's tumor cells through inhibition of the pentose cycle

Transketolase (TK) reactions play a crucial role in tumor cell nucleic acid ribose synthesis utilizing glucose carbons, yet, current cancer treatments do not target this central pathway. Experimentally, a dramatic decrease in tumor cell proliferation after the administration of the TK inhibitor oxythiamine (OT) was observed in several in vitro and in vivo tumor models. Here, we demonstrate that pentose cycle (PC) inhibitors, OT and dehydroepiandrosterone (DHEA), efficiently regulate the cell cycle and tumor proliferation processes. Increasing doses of OT or DHEA were administered by daily intraperitoneal injections to Ehrlich's ascites tumor hosting mice for 4 days. The tumor cell number and their cycle phase distribution profile were determined by DNA flow histograms. Tumors showed a dose dependent increase in their G0-G1 cell populations after both OT and DHEA treatment and a simultaneous decrease in cells advancing to the S and G2-M cell cycle phases. This effect of PC inhibitors was significant, OT was more effective than DHEA, both drugs acted synergistically in combination and no signs of direct cell or host toxicity were observed. Direct inhibition of PC reactions causes a G1 cell cycle arrest similar to that of 2-deoxyglucose treatment. However, no interference with cell energy production and cell toxicity is observed. PC inhibitors, specifically ones targeting TK, introduce a new target site for the development of future cancer therapies to inhibit glucose utilizing pathways selectively for nucleic acid production.

Characterization of the membrane quinoprotein glucose dehydrogenase from Escherichia coli and characterization of a site-directed mutant in which histidine-262 has been changed to tyrosine

The requirements for substrate binding in the quinoprotein glucose dehydrogenase (GDH) in the membranes of Escherichia coli are described, together with the changes in activity in a site-directed mutant in which His262 has been altered to a tyrosine residue (H262Y-GDH). The differences in catalytic efficiency between substrates are mainly related to differences in their affinity for the enzyme. Remarkably, it appears that, if a hexose is able to bind in the active site, then it is also oxidized, whereas some pentoses are able to bind (and act as competitive inhibitors), but are not substrates. The activation energies for the oxidation of hexoses and pentoses are almost identical. In a previously published model of the enzyme, His262 is at the entrance to the active site and appears to be important in holding the prosthetic group pyrroloquinoline quinone (PQQ) in place, and it has been suggested that it might play a role in electron transfer from the reduced PQQ to the ubiquinone in the membrane. The H262Y-GDH has a greatly diminished catalytic efficiency for all substrates, which is mainly due to a marked decrease in their affinities for the enzyme, but the rate of electron transfer to oxygen is unaffected. During the processing of the PQQ into the apoenzyme to give active enzyme, its affinity is markedly dependent on the pH, four groups with pK values between pH7 and pH8 being involved. Identical results were obtained with H262Y-GDH, showing that His262 it is not directly involved in this process.

A male gametophyte-specific monosaccharide transporter in Arabidopsis

The AtSTP2 gene (sugar transport protein 2) of Arabidopsis thaliana encodes a high affinity, low specificity monosaccharide carrier that can transport a number of hexoses and pentoses at similar rates. AtSTP2 has 12 putative transmembrane helices and a molecular mass of 55.0 kDa. AtSTP2 expression was localized in AtSTP2 promoter-beta-glucuronidase (GUS) Arabidopsis plants showing AtSTP2-driven GUS activity during pollen maturation and also in germinating pollen. Immunohistochemical studies with anti-AtSTP2 antiserum as well as RNA in situ hybridization analyses modified these results and showed that AtSTP2 expression is confined to the early stages of gametophyte development. Both AtSTP2 mRNA and AtSTP2 protein are first seen at the time of beginning callose degradation and microspore release from the tetrades. AtSTP2 mRNA and AtSTP2 protein are no longer detected after the mitotic divisions and the formation of the trinucleate gametophyte. No AtSTP2 mRNA or AtSTP2 protein is seen in fully developed or germinating pollen. The putative role of AtSTP2 in the uptake of glucose units resulting from callose degradation during pollen maturation is discussed.

Priority of pentose utilization at the level of transcription: arabinose, xylose, and ribose operons

When E. coli cells were grown in minimal medium supplemented with D-ribose and D-xylose, a diauxic growth preferring D-xylose was observed. Transcription of the ribose (rbs) operon was repressed in the presence of D-xylose, phenotypically similar to catabolite repression by D-glucose, although D-ribose did not affect transcription of the xylose (xyl) operon. Complementation analysis with xylR revealed that the repression of the rbs operon by D-xylose is exerted at the transcriptional level through XylR, suggesting a novel mechanism for catabolite repression. Furthermore, it was shown that L-arabinose reduced transcriptions of both xyl and rbs operons, whereas the arabinose operon was not affected by D-xylose or D-ribose, suggesting a priority mechanism for pentose utilization.

Mass isotopomer study of the nonoxidative pathways of the pentose cycle with [1,2-13C2]glucose

We present a single-tracer method for the study of the pentose phosphate pathway (PPP) using [1,2-13C2]glucose and mass isotopomer analysis. The metabolism of [1,2-13C2]glucose by the glucose-6-phosphate dehydrogenase, transketolase (TK), and transaldolase (TA) reactions results in unique pentose and lactate isotopomers with either one or two 13C substitutions. The distribution of these isotopomers was used to estimate parameters of the PPP using the model of Katz and Rognstad (J. Katz and R. Rognstad. Biochemistry 6: 2227-2247, 1967). Mass and position isotopomers of ribose, and lactate and palmitate (products from triose phosphate) from human hepatoma cells (Hep G2) incubated with 30% enriched [1,2-13C2]glucose were determined using gas chromatography-mass spectrometry. After 24-72 h incubation, 1.9% of lactate molecules in the medium contained one 13C substitution (m1) and 10% contained two 13C substitutions (m2). A similar m1-to-m2 ratio was found in palmitate as expected. Pentose cycle (PC) activity determined from incubation with [1,2-13C2]glucose was 5.73 +/- 0.52% of the glucose flux, which was identical to the value of PC (5.55 +/- 0.73%) determined by separate incubations with [1-13C] and [6-13C]glucose, 13C was found to be distributed in four ribose isotopomers ([1-13C]-, [5-13C]-, [1,2-13C2]-, and [4,5-13C2]ribose). The observed ribose isotopomer distribution was best matched with that provided from simulation by substituting 0.032 for TK and 0.85 for TA activity relative to glucose uptake into the model of Katz and Rognstad. The use of [1,2-13C2]glucose not only permits the determination of PC but also allows estimation of relative rates through the TK and TA reactions.

Noncovalent enzyme-substrate interactions in the catalytic mechanism of yeast aldose reductase

The role of noncovalent interactions in the catalytic mechanism of aldose reductase from the yeast Candida tenuis was determined by steady-state kinetic analysis of the NADH-dependent reduction of various aldehydes, differing in hydrophobicity and the hydrogen bonding capability with the binary enzyme-NADH complex. In a series of aliphatic aldehydes, substrate hydrophobicity contributes up to 13.7 kJ/mol of binding energy. The aldehyde binding site of aldose reductase appears to be 1.4 times more hydrophobic than n-octanol and can accommodate a linear alkyl chain with at least seven methylene groups (approximately 14 A in length). Binding energy resulting from interactions at positions 3-6 of the aldehyde is distributed between increasing the catalytic constant 2.6-fold and decreasing the apparent dissociation constant 59-fold. Hydrogen bonding interactions of the enzyme nucleotide complex with the C-2(R) hydroxyl group of the aldehyde are crucial to transition state binding and contribute up to 17 kJ/mol of binding energy. A comparison of the kinetic data of yeast aldose reductase, a key enzyme in the metabolism of D-xylose, and human aldose reductase, a presumably perfect detoxification catalyst [Grimshaw, C. E. (1992) Biochemistry 31, 10139], clearly reflects these differences in physiological function.

Induction of aldose reductase and xylitol dehydrogenase activities in Candida tenuis CBS 4435

In this study the ability of various sugars and sugar alcohols to induce aldose reductase (xylose reductase) and xylitol dehydrogenase (xylulose reductase) activities in the yeast Candida tenuis was investigated. Both enzyme activities were induced when the organism was grown on D-xylose or L-arabinose as well as on the structurally related sugars D-arabinose or D-lyxose. Mixtures of D-xylose with the more rapidly metabolizable sugar D-glucose resulted in a decrease in the levels of both enzymes formed. These results show that the utilization of D-xylose by C. tenuis is regulated by induction and catabolite repression. Furthermore, the different patterns of induction on distinct sugars suggest that the synthesis of both enzymes is not under coordinate control.

Aspects of the mechanisms of action of biguanides on trophozoites and cysts of Acanthamoeba castellanii

A non-radioactive method was used to investigate the uptake by Acanthamoeba castellanii of chlorhexidine diacetate (CHA) and polyhexamethylene biguanide (PHMD). Based on the Giles et al. (1974) hypothesis, the uptake of CHA by trophozoites appeared to be of the L3 pattern whereas that of cysts was C2. Unlike CHA, trophozoites took up PHMB with an L2 pattern at low concentrations followed by a C-type pattern at higher concentrations, the uptake by cysts was found to be of the C2 pattern with a plateau effect at high concentrations. A diphasic leakage effect was found in trophozoites whereas a relatively low peak of maximal leakage occurred from cysts treated with high biocide concentrations. The amount of pentose release depended on the formulation ingredients. No correlation between pentose leakage and trophozoicidal or cysticidal activity was found.

Taxonomic study of aromatic-degrading bacteria from deep-terrestrial-subsurface sediments and description of Sphingomonas aromaticivorans sp. nov., Sphingomonas subterranea sp. nov., and Sphingomonas stygia sp. nov

Phylogenetic analyses of 16S rRNA gene sequences by distance matrix and parsimony methods indicated that six strains of bacteria isolated from deep saturated Atlantic coastal plain sediments were closely related to the genus Sphingomonas. Five of the strains clustered with, but were distinct from, Sphingomonas capsulata, whereas the sixth strain was most closely related to Blastobacter natatorius. The five strains that clustered with S. capsulata, all of which could degrade aromatic compounds, were gram-negative, non-spore-forming, non-motile, rod-shaped organisms that produced small, yellow colonies on complex media. Their G + C contents ranged from 60.0 to 65.4 mol%, and the predominant isoprenoid quinone was ubiquinone Q-10. All of the strains were aerobic and catalase positive. Indole, urease, and arginine dihydrolase were not produced. Gelatin was not liquified, and glucose was not fermented. Sphingolipids were present in all strains; 2OH14:0 was the major hydroxy fatty acid, and 18:1 was a major constituent of cellular lipids. Acid was produced oxidatively from pentoses, hexoses, and disaccharides, but not from polyalcohols and indole. All of these characteristics indicate that the five aromatic-degrading strains should be placed in the genus Sphingomonas as currently defined. Phylogenetic analysis of 16S rRNA gene sequences, DNA-DNA reassociation values, BOX-PCR genomic fingerprinting, differences in cellular lipid composition, and differences in physiological traits all indicated that the five strains represent three previously undescribed Sphingomonas species. Therefore, we propose the following new species: Sphingomonas aromaticivorans (type strain, SMCC F199), Sphingomonas subterranea (type strain, SMCC B0478), and Sphingomonas stygia (type strain, SMCC B0712).

An important role for pentose cycle in the synthesis of citrulline and proline from glutamine in porcine enterocytes

This study was designed to determine a role of pentose cycle in the provision of NADPH for the synthesis of citrulline and proline from glutamine in porcine enterocytes. Enterocytes from 4-day-old pigs were incubated at 37 degrees C for 0 to 30 min in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 2 mM glutamine and 5 mM glucose in the presence of 0, 0.1, or 0.25 mM dehydroepiandrosterone (DHEA), a potent inhibitor of glucose-6-phosphate dehydrogenase which is the key regulatory enzyme of pentose cycle. The activity of this cycle was estimated with the use of [1-14C]glucose and [6-14C]glucose. In some experiments, the medium included 2 mM ornithine and 2 mM NH4Cl (no glutamine). About 14% of glucose taken up by enterocytes was metabolized via pentose cycle. The flux from glucose into this cycle was decreased by 70 and 86%, respectively, in the presence of 0.1 and 0.25 mM DHEA compared with its absence. DHEA inhibited the synthesis of ornithine, citrulline, arginine, and proline from glutamine in a concentration-dependent manner, but had no effect on the formation of citrulline and arginine from ornithine. However, DHEA decreased the synthesis of proline from ornithine by 79%. DHEA had no effect on cellular ATP concentrations. These results provide the first line of evidence suggesting that glucose metabolism via pentose cycle plays an important role in providing NADPH for the conversion of glutamine into pyrroline-5-carboxylate in porcine enterocytes, which may explain the glucose-dependent synthesis of citrulline and proline from glutamine in these cells.

Transition metal saccharide chemistry and biology: synthesis, characterization, electrochemistry and EPR studies of oxovanadium (IV) complexes of saccharides and their derivatives and in vitro interaction of some of these with ribonuclease and deoxyribonuclease

Low molecular weight, water-soluble saccharide complexes of oxovanadium(IV) have been synthesized and characterized by analytical, spectroscopic and electrochemical techniques. All the complexes were found to be mononuclear, possessing the VO2+ moiety. These are shown to be hydrolytically and oxidatively stable over a wide range of pH (1-12) and have been extensively characterized by absorption and EPR spectroscopy and by electrochemistry. Several correlations have been drawn from the data generated. Some of these complexes have been demonstrated to possess in vitro RNase inhibition activity with no effect on DNase. This suggests that these molecules closely mimic the substrate portion of the RNase-catalysed RNA hydrolysis and can act as transition-state analogues to RNase.

Cation and sugar selectivity determinants in a novel family of transport proteins

A new family of homologous membrane proteins that transport galactosides-pentoses-hexuronides (GPH) is described. By analysing the aligned amino acid sequences of the GPH family, and by exploiting their different specificities for cations and sugars, we have designed mutations that yield novel insights into the nature of ligand binding sites in membrane proteins. Mutants have been isolated/constructed in the melibiose transport proteins of Escherichia coli, Klebsiella pneumoniae and Salmonella typhimurium, and the lactose transport protein of Streptococcus thermophilus which facilitate uncoupled transport or have an altered cation and/or substrate specificity. Most of the mutations map in the amino-terminal region, in or near amphipathic alpha-helices II and IV, or in interhelix-loop 10-11 of the transport proteins. On the basis of the kinetic properties of these mutants, and the primary and secondary structure analyses presented here, we speculate on the cation binding pocket of this family of transporters. The regulation of the transporters through interaction with, or phosphorylation by, components of the phosphoenolpyruvate:sugar phosphotransferase system is also discussed.

The evolution of sugar isomerases

L-Arabinose isomerase (EC 5.3.1.4) catalyzes the isomerization of L-arabinose to L-ribulose. Here we report on the purification, kinetic mechanism and chemical mechanism of L-arabinose isomerase from Escherichia coli. The enzyme catalyzes the isomerization of L-arabinose to L-ribulose by a proton transfer mechanism, in contrast to xylose isomerase which uses a hydride transfer mechanism to perform a similar isomerization. Arabinose isomerase activity is metal dependent, although the enzyme can catalyze the exchange of the proton attached to carbon 2 of arabinose with the solvent in the absence of metal ion. Manganese(II) is the only metal ion which renders the enzyme active for the isomerization reaction. Arabinose isomerase has high substrate specificity for L-arabinose. The difference in chemical mechanism between xylose isomerase and arabinose isomerase suggests that these enzymes are not related by convergent evolution. This work also suggests that unless convergent evolution has been demonstrated, the mechanism of one enzyme may not give any insight into the mechanism of a second enzyme catalyzing the same reaction.

Aldose reductase as a target for drug design: molecular modeling calculations on the binding of acyclic sugar substrates to the enzyme

In an attempt to obtain a picture of the binding conformation of aldehyde substrates to human aldose reductase (hAR), modeling calculations have been performed on the binding of three substrates, D-xylose, L-xylose, and D-lyxose, to wild-type human aldose reductase and two of its site-directed mutants. It was found that the average geometry of D-xylose in the active site of wild-type aldose reductase is characterized by strong hydrogen bonds involving the reactive carbonyl oxygen of the substrate and both Tyr48 and His110. The calculations also suggest the importance of Trp111 in the binding of 2'-hydroxyl-containing aldehyde substrates. A good correlation between calculated interaction enthalpies and experimental log(Km) or log(kcat/Km) values was obtained when His110 was modeled with its N epsilon 2 atom protonated and N delta 1 unprotonated. No correlation was found for the other two configurations of His110. On the basis of comparisons of the calculated substrate binding conformations for the three possible His110 configurations, and on the correlations between measured log(Km) or log(kcat/Km) and calculated parameters, it is proposed that His110 is neutral and protonated at N epsilon 2 when an aldehyde substrate is bound to the hAR/NADPH complex. A chain of three hydrogen-bonded water molecules has been identified in all available crystal structures and is located in an enzyme channel which links the N delta 1 atom of His110 to the solvent-accessible surface of the enzyme. A possible role of this channel in the mechanism of catalysis of aldose reductase is suggested.

Models of the liver pentose cycle

Simple and complete models of the classical liver pentose cycle, and a model of Williams' proposed "L-type" pentose cycle, are compared. All extant experimental data on well-oxygenated whole cell systems can be fitted to the predicted output of the complete classical pentose cycle model; however, there are gross discrepancies between key experimental data and Williams' proposed scheme. The complete classical model allows isotopic reversibility in the non-oxidative segment of the cycle, but none of the reversible enzymes are extremely close to isotopic equilibrium. General approaches are presented to estimate the isotopic reversibility of most enzymic steps, without requiring isolation of the intermediates, present in some cases at very low concentrations. The isotopic reversibility of the non-oxidative pathway causes only minor errors in the equations used to estimate liver pentose cycle flux, which were based on simple unidirectional models.

Activation of a cryptic gene encoding a kinase for L-xylulose opens a new pathway for the utilization of L-lyxose by Escherichia coli

A silent gene encoding a kinase that specifically phosphorylates L-xylulose was activated and rendered constitutive in mutant cells of Escherichia coli. L-Xylulose kinase was purified to homogeneity and found to be a dimer of two subunits of 55 kDa, highly specific for L-xylulose with a Km of 0.8 mM, a Vmax of 33 mumol/min/mg, and an optimum pH of 8.4. Physical (thin layer chromatography) and spectroscopic (nuclear magnetic resonance and optical rotation) characterization of the product of L-xylulose kinase indicated that the enzyme phosphorylated the sugar at position 5. The gene encoding L-xylulose kinase was mapped in the 80.2 min region of the chromosome by conjugation and transduction. Cloning and comparison of the restriction map with the Kohara map (Kohara, Y., Akiyame, K., and Isono, K. (1987) Cell 50, 495-501) located the gene between positions 3963 and 3965 kilobases. The molecular and functional features of L-xylulose kinase together with the location of the corresponding gene indicate that this enzyme did not derive from mutation of any other known kinase. The new kinase opens a route for the utilization of L-lyxose through the action of rhamnose permease, rhamnose isomerase, and the phosphorylation of the L-xylulose formed to L-xylulose 5-phosphate, which is then introduced into the pentose phosphate pathway for subsequent metabolism.

Pentose transport by the ruminal bacterium Butyrivibrio fibrisolvens

Butyrivibrio fibrisolvens is a fibrolytic ruminal bacterium that degrades hemicellulose and ferments the resulting pentose sugars. Washed cells of strain D1 accumulated radiolabelled xylose (Km = 1.5 microM) and arabinose (Km = 0.2 microM) when the organism was grown on xylose, arabinose, or glucose, but cultures grown on sucrose or cellobiose had little capacity to transport pentose. Glucose and xylose inhibited transport of each other non-competitively. Both sugars were utilized preferentially over arabinose, but since they did not inhibit transport of arabinose, it appeared that the preference was related to an internal metabolic step. Although the protonmotive force was completely abolished by ionophores, cells retained some ability to transport pentose. In contrast, the metabolic inhibitors iodoacetate, arsenate, and fluoride had little effect on protonmotive force but caused a large decrease in intracellular ATP and xylose and arabinose uptake. These results suggested that high-affinity, ATP-dependent mechanisms were responsible for pentose transport and hexose sugars affected the utilization of xylose and arabinose.

Cost analysis of ethanol production from willow using recombinant Escherichia coli

This study comprises a technical and economic analysis of the production of fuel ethanol by fermentation of a pentose-rich hydrolysate with recombinant Escherichia coli, strain KO11. Hydrolysate from steam-pretreated willow was used as raw material in calculations regarding the fermentation. The calculations were based on a feed capacity of 10 metric tons of dry willow per hour to the pretreatment stage, providing 35 metric tons of hydrolysate per hour, consisting of 45 g of sugars/L, to the pentose fermentation plant. A detoxification step was included, since the hydrolysate has been shown to have an inhibitory effect on the E. coli KO11. The technical data used in the calculations were based on a kinetic fermentation model, which was developed from laboratory-scale experiments in a previous study. The economic analysis predicted an ethanol production cost of 48/L in the pentose fermentation plant, indicating potentially good economy. The detoxification cost constitutes 22% of this cost. Sensitivity analyses revealed that if the concentration of sugars in the feed to the fermentation was decreased by 40% to 27 g/L, the ethanol production cost was increased to 54/L. The production cost was increased to 50/L ethanol if the cell mass was recirculated to the fermentation stage 5 times instead of 20.

Insulin stimulates glycolysis and pentose cycle activity in bovine microvascular endothelial cells

Glucose metabolism via the pentose cycle, glycolysis and the Krebs cycle was quantified in bovine microvascular endothelial cells. The major measured end-product of glucose was L-lactate, with relatively small amounts of glucose carbons converted to CO2 and pyruvate. The pentose cycle accounted for less than 4% of the glucose utilized. About 60-70% of the metabolized glucose carbons could not be accounted for by lactate, pyruvate and CO2. Insulin stimulated glycolysis and pentose cycle activity, but had no effect on glucose oxidation via the Krebs cycle. As the pentose cycle is a major source of NADPH which is required for the synthesis of nitric oxide (the endothelium relaxing factor), insulin may play a role in regulating NO generation in endothelial cells by modulating the pentose cycle activity.

Pentose utilization by the ruminal bacterium Ruminococcus albus

Ruminococcus albus is an important fibrolytic ruminal bacteria which degrades hemicellulose and ferments the resulting pentose sugars. However, little information is available on the utilization of pentoses by this organism or the effect of hexose sugars on pentose metabolism. Enzymatic studies indicated that R. albus metabolized pentoses via the pentose phosphate pathway and possessed constitutive transketolase activity. Cellobiose was preferred over xylose and arabinose, and it appeared that the disaccharide decreased pentose metabolism by repression of transport activity and catabolic enzymes (isomerases and kinases). Glucose and xylose were co-utilized, and transport studies suggested that there was a common transport system for both sugars. In contrast, glucose was preferred over arabinose and the hexose noncompetitively inhibited the transport of arabinose. Since R. albus lacks a glucose phosphotransferase system, the inhibition of arabinose uptake could not be explained by previously described models of inducer exclusion involving such a system. Because accumulation of radiolabeled xylose, arabinose, and glucose proceeded in the absence of a proton motive force and since transport was correlated with the intracellular ATP concentration, it appeared that monosaccharide uptake was driven by ATP hydrolysis.

Primed pentose cycle activity supports production and elimination of superoxide anion in Kupffer cells from rats treated with endotoxin in vivo

Glucose use and pentose cycle activity were determined in freshly isolated rat Kupffer cells 3 h after an i.v. injection of Escherichia coli endotoxin (0.1 mg/kg body weight), by using [1-14C], [6-14C] and [2-3H]glucose. Endotoxin treatment in vivo caused a 5-fold increase in the basal glucose uptake in Kupffer cells. Pentose cycle activity was elevated from 8.7 to 13.6 nmol/h per 10(7) cells after endotoxin. In vitro treatment of the cells from saline- and endotoxin-treated animals with phorbol ester (10(-6) M) increased pentose cycle activity 2-fold and 8-fold, respectively. Phorbol ester caused a 50% increase in glucose uptake in both groups. t-Butyl hydroperoxide (0.5 mM) caused a similar increase in pentose cycle activity as phorbol ester. Glucose oxidation in the Krebs cycle was also doubled after endotoxin. KC from endotoxin-treated animals produced O2- spontaneously, and were primed to produce additional large amounts of O2- upon phorbol ester treatment. Addition of t-butyl hydroperoxide inhibited O2- production by Kupffer cells. Depletion of glutathione by N-ethylmaleimide (0.1 mM), or inhibition of NADPH oxidase by diphenyliodonium (0.1 mM) inhibited both the pentose cycle activity and the O2- production. Increasing the concentration of exogenous glucose in the cell medium elevated the glycolytic rate, while pentose cycle flux was not affected either under basal conditions or following subsequent challenges by phorbol ester or t-butyl hydroperoxide. Our data suggest that the endotoxin-induced elevated glucose use in Kupffer cells is accompanied by a primed state of the pentose cycle. This condition supports superoxide and macromolecule synthesis and could also represent a potentiated protective mechanism against oxidative cellular injury during bacterial infections.

Pentose-phosphate pathway in Saccharomyces cerevisiae: analysis of deletion mutants for transketolase, transaldolase, and glucose 6-phosphate dehydrogenase

Deletion mutants for the yeast transketolase gene TKL1 were constructed by gene replacement. Transketolase activity was below the level of detection in mutant crude extracts. Transketolase protein could be detected as a single protein band of the expected size by Western-blot analysis in wild-type strains but not in the deletion mutant. Deletion of TKL1 led to a reduced but distinct growth in synthetic medium without an aromatic amino-acid supplement. We also isolated double and triple mutants for transketolase (tkl1), transaldolase (tal1), and glucose 6-phosphate dehydrogenase (zwf1) by crossing the different mutants. A tal1 tkl1 double mutant grew nearly like wild-type in rich medium. Only the tkl1 zwf1 double and the tal1 tkl1 zwf1 triple mutant grew more slowly than the wild-type in rich medium. This growth defect could be partly alleviated by the addition of xylulose but not ribose. The triple mutant still grew slowly on a synthetic mineral salts medium without a supplement of aromatic amino acids. This suggests the existence of an alternative but limited source of pentose phosphates and erythrose 4-phosphate in the tkl1 zwf1 double mutants. Hybridization with low stringency showed the existence of a sequence with homology to transketolase, possibly a second gene.

The structure of the MnIIADP-nitrate-lyxose complex at the active site of hexokinase

The coordination scheme of Mn2+ in the hexokinase-MnIIADP-nitrate-lyxose complex has been determined by electron paramagnetic resonance (EPR) spectroscopy with 17O-enriched ligands. Nitrate binds to the active site of hexokinase when MnIIADP and a sugar substrate or analogue are present. The binding of nitrate enhances inhibition by glucose when ADP is present and narrows the EPR signals of the enzyme-bound MnIIADP complex in the presence of sugar substrates or analogues. Experiments using regiospecifically 17O-enriched ADP, 17O-enriched nitrate, and 17O-enriched water establish the coordination scheme of Mn2+. The EPR experiments show that ADP is a beta-monodentate ligand and that nitrate binds directly to Mn2+. Four water molecules complete the coordination sphere of the enzyme-bound Mn2+. The dissociation constant (Kd approximately 8 mM) of nitrate for the complex with enzyme, MnIIADP, and lyxose was obtained from titration experiments. These results suggest that nitrate-stabilized, dead-end complexes of hexokinase may be useful in stabilizing the closed conformation of this "hinge-bending" enzyme for crystallographic experiments.

Metabolism of freshly isolated human hair follicles capable of hair elongation: a glutaminolytic, aerobic glycolytic tissue

The metabolism of the human hair follicle was investigated in vitro under conditions that maintained glycogen and adenosine triphosphate (ATP) content and the growth rate of the follicle at values observed in vivo. We have shown that only 10% of the total glucose utilized was oxidized to CO2 and 40% of this was oxidized via the pentose phosphate shunt. Although fatty acids and ketone bodies were oxidized by the hair follicle, they are poor energetic substitutes for glucose. Nor will fatty acids or ketone bodies sustain hair growth in vitro. Glutamine, however, was shown, both biochemically and by comparing growth rates, to be an important fuel with 23% of uptake being oxidized, generating a possible 2.16 +/- 0.32 nmoles ATP/follicle/h (mean +/- SEM) (glucose metabolism generates 4.54 +/- 0.61 nmoles ATP/follicle/h). Sixty-four percent of the glutamine taken up was calculated to be metabolized to lactate, showing that the hair follicle engages in both glycolysis and glutaminolysis. The glucose-fatty acid cycle appears to be unimportant in the hair follicle but our data indicates that a glucose-glutamine cycle does operate.

Exchange reactions catalyzed by group-transferring enzymes oppose the quantitation and the unravelling of the identify of the pentose pathway

1. The distributions and rates of transfer of carbon isotopes from a selection of specifically labelled ketosugar-phosphate substrates by exchange reactions catalyzed by the pentose and photosynthetic carbon-reduction-pathway group-transferring enzymes transketolase, transaldolase and aldolase have been measured using 13C-NMR spectroscopy. 2. The rates of these exchange reactions were 5, 4 and 1.5 mumol min-1 mg-1 for transketolase exchange, transaldolase exchange and aldolase exchange, respectively. 3. A comparison of the exchange capacities contributed by the activities of these enzymes in three in vitro liver preparations with the maximum non-oxidative pentose pathway flux rates of the preparations shows that transketolase and aldolase exchanges exceeded flux by 9-19 times in liver cytosol and acetone powder enzyme preparations and by 5 times in hepatocytes. Transaldolase was less effective in the comparison of exchange versus flux rates: transaldolase exchange exceeded flux by 1.6 and 5 in catalysis by liver cytosol and acetone powder preparations, respectively, but was only 0.6 times the flux in hepatocytes. 4. Values of group enzyme exchange and pathway flux rates in the above three preparations are important because of the feature role of liver and of these particular preparations in the establishment, elucidation and measurement of a proposed reaction scheme for the fat-cell-type pentose pathway in biochemistry. 5. It is the claim of this paper that the excess of exchange rate activity (particularly transketolase exchange) over pathway flux will overturn attempts to unravel, using isotopically labelled sugar substrates, the identity, reaction sequence and quantitative contribution of the pentose pathway to glucose metabolism. 6. The transketolase exchange reactions relative to the pentose pathway flux rates in normal, regenerating and foetal liver, Morris hepatomas, mammary carcinoma, melanoma, colonic epithelium, spinach chloroplasts and epididymal fat tissue show that transketolase exchange may exceed flux in these tissues by factors ranging over 5-600 times. 7. The confusion of pentose pathway theory by the effects of transketolase exchange action is illustrated by the 13C-NMR spectrum of the hexose 6-phosphate products of ribose 5-phosphate dissimilation, formed after 30 min of liver enzyme action, and shows 13C-labelling in carbons 1 and 3 of glucose 6-phosphate with ratios which range over 2.1-6.4 rather than the mandatory value of 2 which is imposed by the theoretical mechanism of the pathway.

Proton-linked L-rhamnose transport, and its comparison with L-fucose transport in Enterobacteriaceae

1. An alkaline pH change occurred when L-rhamnose, L-mannose or L-lyxose was added to L-rhamnose-grown energy-depleted suspensions of strains of Escherichia coli. This is diagnostic of sugar-H+ symport activity. 2. L-Rhamnose, L-mannose and L-lyxose were inducers of the sugar-H+ symport and of L-[14C]rhamnose transport activity. L-Rhamnose also induced the biochemically and genetically distinct L-fucose-H+ symport activity in strains competent for L-rhamnose metabolism. 3. Steady-state kinetic measurements showed that L-mannose and L-lyxose were competitive inhibitors (alternative substrates) for the L-rhamnose transport system, and that L-galactose and D-arabinose were competitive inhibitors (alternative substrates) for the L-fucose transport system. Additional measurements with other sugars of related structure defined the different substrate specificities of the two transport systems. 4. The relative rates of H+ symport and of sugar metabolism, and the relative values of their kinetic parameters, suggested that the physiological role of the transport activity was primarily for utilization of L-rhamnose, not for L-mannose or L-lyxose. 5. L-Rhamnose transport into subcellular vesicles of E. coli was dependent on respiration, was optimal at pH 7, and was inhibited by protonophores and ionophores. It was insensitive to N-ethylmaleimide or cytochalasin B. 6. L-Rhamnose, L-mannose and L-lyxose each elicited an alkaline pH change when added to energy-depleted suspensions of L-rhamnose-grown Salmonella typhimurium LT2, Klebsiella pneumoniae, Klebsiella aerogenes, Erwinia carotovora carotovora and Erwinia carotovora atroseptica. The relative rates of subsequent acidification varied, depending on both the organism and the sugar. L-Fucose promoted an alkaline pH change in all the L-rhamnose-induced organisms except the Erwinia species. No L-rhamnose-H+ symport occurred in any organism grown on L-fucose. 7. All these results showed that L-rhamnose transport into the micro-organisms occurred by a system different from that for L-fucose transport. Both systems are energized by the trans-membrane electrochemical gradient of protons. 8. Neither steady-state kinetic measurements nor binding-protein assays revealed the existence of a second L-rhamnose transport system in E. coli.

Effects of intestinal insulin-like peptide on glucose catabolism in mealworm larval fat body in vitro: dependence on extracellular Ca2+ for its stimulatory action

In vitro hormonally induced variations of glucose catabolism in mealworm fat body tissue were examined by a microradiorespirometric method. An insulin-like peptide (ILP) extracted from the midgut of last larval instar mealworm larvae significantly modified glucose catabolism and was dependent on energy metabolism and on the Ca2+ concentration in the culture medium. Using two different labelled substrate molecules, the stimulatory effects of ILP (compared with those of mammalian insulin) on the relative use of the pentose cycle as opposed to the glycolytic-citric acid cycle by the mealworm fat body were measured in vitro. Metabolic variations were evaluated using either [1-14C]glucose or [6-14C]glucose as substrates. Time course and dose-response curves of ILP and the hormonally induced variations in total CO2 and 14CO2 kinetics were determined. Modification in the specific radioactivity kinetics of 14CO2 derived from [1-14C] glucose and [6-14C]glucose molecules under hormonal effects were observed. As demonstrated in in vivo studies, ILP stimulated the relative utilization of the pentose cycle. However, this effect was observed much more rapidly, but for a shorter time, with fat body in vitro. Mammalian insulin produced similar, but not identical effects. Variations in transmembranous Ca2+ cellular exchanges, induced by either EGTA, nifedipine, or calcium ionophore ionomycin included in the culture medium, indicated that the stimulatory effects of ILP depends on this cation.

Evidence for catabolite inhibition in regulation of pentose utilization and transport in the ruminal bacterium Selenomonas ruminantium

Pentose sugars can be an important energy source for ruminal bacteria, but there has been relatively little study regarding the regulation of pentose utilization and transport by these organisms. Selenomonas ruminantium, a prevalent ruminal bacterium, actively metabolizes xylose and arabinose. When strain D was incubated with a combination of glucose and xylose or arabinose, the hexose was preferentially utilized over pentoses, and similar preferences were observed for sucrose and maltose. However, there was simultaneous utilization of cellobiose and pentoses. Continuous-culture studies indicated that at a low dilution rate (0.10 h-1) the organism was able to co-utilize glucose and xylose. This co-utilization was associated with growth rate-dependent decreases in glucose phosphotransferase activity, and it appeared that inhibition of pentose utilization was due to catabolite inhibition by the glucose phosphotransferase transport system. Xylose transport activity in strain D required induction, while arabinose permease synthesis did not require inducer but was subject to repression by glucose. Since an electrical potential or a chemical gradient of protons drove xylose and arabinose uptake, pentose-proton symport systems apparently contributed to transport.

Enzymes associated with metabolism of xylose and other pentoses by Prevotella (Bacteroides) ruminicola strains, Selenomonas ruminantium D, and Fibrobacter succinogenes S85

Prevotella (Bacteroides) ruminicola strains B(1)4 and S23 and Selenomonas ruminantium strain D used xylose as the sole source of carbohydrate for growth, whereas Fibrobacter succinogenes was unable to metabolize xylose. Prevotella ruminicola strain B(1)4 exhibited transport activity for xylose. In contrast, F. succinogenes lacked typical xylose uptake activity but did exhibit low binding potential for the sugar. Prevotella ruminicola strains B(1)4 and S23 as well as S. ruminantium D showed low xylose isomerase activities but higher xylulokinase activities, using assays that gave high activities for these enzymes in Escherichia coli. Xylose isomerase appeared to be produced constitutively in these ruminal bacteria, but xylulokinase was induced to varying degrees with xylose as the source of carbohydrate. Fibrobacter succinogenes lacked xylose isomerase and xylulokinase. All three species of ruminal bacteria possessed transketolase, xylulose-5-phosphate epimerase, and ribose-5-phosphate isomerase activities. Neither P. ruminicola B(1)4 nor F. succinogenes S85 showed significant phosphoketolase activity. The data indicate that F. succinogenes is unable to either actively uptake or metabolize xylose as a result of the absence of functional xylose permease, xylose isomerase, and xylulokinase activities, although it and both P. ruminicola and S. ruminantium possess the essential enzymes of the nonoxidative branch of the pentose phosphate cycle.

[Effect of manganese on transketolase activity and total pentose content in rabbits]

Different doses of manganese are shown to exert a positive effect on the transketolase activity in the blood serum and in certain tissues as well as on the amount of total pentoses in the blood serum of rabbits. This confirms intensification of the pentose-phosphate exchange of carbohydrates under the effect of manganese.

Lyase activity of nucleoside 2-deoxyribosyltransferase: transient generation of ribal and its use in the synthesis of 2'-deoxynucleosides

In the absence of acceptors nucleoside 2-deoxyribosyltransferase catalyzes the slow hydrolysis of 2'-deoxynucleosides. During this hydrolytic reaction, D-ribal (1,4-anhydro-2-deoxy-D-erythro-pent-1-enitol), a glycal of ribose hitherto encountered only as a reagent in organic synthesis, is generated spontaneously, disappearing later as 2'-deoxynucleoside hydrolysis approaches completion. Nucleoside 2-deoxyribosyltransferase is found to catalyze the hydration of D-ribal in the absence of nucleic acid bases and the synthesis of deoxyribonucleosides from ribal in their presence, affording a new method for the preparation of 2'-deoxyribonucleosides. The stereochemistry of nucleoside formation from ribal supports the intervention of deoxyribosyl-enzyme intermediate. The equilibrium constant for the covalent hydration of ribal is found to be approximately 65.

L-lyxose metabolism employs the L-rhamnose pathway in mutant cells of Escherichia coli adapted to grow on L-lyxose

Escherichia coli cannot grow on L-lyxose, a pentose analog of the 6-deoxyhexose L-rhamnose, which supports the growth of this and other enteric bacteria. L-Rhamnose is metabolized in E. coli by a system that consists of a rhamnose permease, rhamnose isomerase, rhamnulose kinase, and rhamnulose-1-phosphate aldolase, which yields the degradation products dihydroxyacetone phosphate and L-lactaldehyde. This aldehyde is oxidized to L-lactate by lactaldehyde dehydrogenase. All enzymes of the rhamnose system were found to be inducible not only by L-rhamnose but also by L-lyxose. L-Lyxose competed with L-rhamnose for the rhamnose transport system, and purified rhamnose isomerase catalyzed the conversion of L-lyxose into L-xylulose. However, rhamnulose kinase did not phosphorylate L-xylulose sufficiently to support the growth of wild-type E. coli on L-lyxose. Mutants able to grow on L-lyxose were analyzed and found to have a mutated rhamnulose kinase which phosphorylated L-xylulose as efficiently as the wild-type enzyme phosphorylated L-rhamnulose. Thus, the mutated kinase, mapped in the rha locus, enabled the growth of the mutant cells on L-lyxose. The glycolaldehyde generated in the cleavage of L-xylulose 1-phosphate by the rhamnulose-1-phosphate aldolase was oxidized by lactaldehyde dehydrogenase to glycolate, a compound normally utilized by E. coli.

End-stage renal disease and diabetes catalyze the formation of a pentose-derived crosslink from aging human collagen

Structure elucidation of a specific fluorophore from the aging extracellular matrix revealed the presence of a protein crosslink formed through nonenzymatic glycosylation of lysine and arginine residues. The unexpected finding that a pentose instead of a hexose is involved in the crosslinking process suggested that the crosslink, named pentosidine, might provide insight into abnormalities of pentose metabolism in aging and disease. This hypothesis was investigated by quantitating pentosidine in hydrolysates of 103 human skin specimens obtained randomly at autopsy. Pentosidine level was found to increase exponentially from 5 to 75 pmol/mg collagen over lifespan (r = 0.86, P less than 0.001). A three- to tenfold increase was noted in insulin-dependent diabetic and nondiabetic subjects with severe end-stage renal disease requiring hemodialysis (P less than 0.001). Moderately elevated levels were also noted in some very old subjects, some subjects with non-insulin dependent diabetes, and two subjects with cystic fibrosis and diabetes. The cause of the abnormal pentose metabolism in these conditions is unknown but may relate to hemolysis, impaired pentose excretion, cellular stress, and accelerated breakdown of ribonucleotides. Thus, pentosidine emerges as a useful tool for assessment of previously unrecognized disorders of pentose metabolism in aging and disease. Its presence in red blood cells and plasma proteins suggests that it might be used as a measure of integrated pentosemia in analogy to glycohemoglobin for the assessment of cumulative glycemia.

Fructose-6-phosphate cycling and the pentose cycle in hyperthyroidism

Hepatic fructose-6-phosphate (fructose-6-P) cycling and pentose cycle activity were quantified in hyperthyroid patients. A measure of the fructose-6-P cycle was the incorporation of 14C, on administering [3-3H,6-14C]galactose, into carbon 1 of blood glucose and the 3H/14C ratio in blood glucose. The measure of the pentose cycle was the randomization of 14C to carbon 1 of blood glucose on administering [2-14C]galactose. [2-3H]Galactose was also administered, so the 3H/14C ratio in blood glucose measured the extent of equilibration of glucose-6-P with fructose-6-P. Patients given [3-3H,6-14C]galactose were restudied when euthyroid. Of the 14C from [3-3H,6-14C]galactose, 7.7-9.5% was in carbon 1 of glucose in both states. 3H/14C ratios were also the same in both states. Fructose-6-P cycling was estimated to be 13 +/- 1% the rate of glucose turnover in the euthyroid and 15 +/- 1% that in the hyperthyroid state. The pentose cycle contributed about 2% to glucose utilization, similar to previous estimates in healthy humans. As in healthy individuals, about 25% of 3H was retained in the conversion of [2-3H]glucose-6-P to glucose. Thus, the fractions of glucose turnover participating in hepatic fructose-6-P and pentose cycling are similar in hyperthyroid and healthy subjects. As a result, augmented fructose-6-P cycling does not substantially contribute to increased hepatic oxygen consumption in hyperthyroidism.

No effect of long-term DHEA treatment on either hepatocyte and adipocyte pentose pathway activity or adipocyte glycerol release

The effects of long-term dehydroepiandrosterone treatment on the pentose shunt in adipocytes and hepatocytes from female lean Zucker rats were determined. No significant effects could be attributed to this treatment. There was also no effect on basal or epinephrine-stimulated glycerol release from adipocytes.

Clostridium methylpentosum sp. nov.: a ring-shaped intestinal bacterium that ferments only methylpentoses and pentoses

An intestinal bacterium isolated from a human subject utilized only two methylpentoses (L-rhamnose and L-fucose) and two pentoses (L-lyxose and D-arabinose) as fermentable substrates, among many compounds tested. The isolate was obligately anaerobic and had a distinctive morphology, its cells being rods bent in the shape of rings with the ends slightly overlapping. Single ring-shaped cells and left-handed helical chains of cells were present in cultures. The cells were surrounded by large capsules which appeared as thick, fibrous masses when examined by electron microscopy. Capsules were formed by cells growing in media containing any one of the four fermentable substrates. Terminally located, heat-resistant endospores were formed on plates of an enriched agar medium supplemented with L-rhamnose. End products of L-rhamnose or L-fucose fermentation included acetate, propionate, n-propanol, CO2, and H2. The isolate represented a new species of Clostridium for which the name Clostridium methylpentosum (type strain R2, ATCC 43829) is proposed. This organism may participate in intestinal digestive processes by metabolizing rhamnose released via the enzymatic depolymerization of dietary pectin.