Conversion of pentoses to ethanol by yeasts and fungi

Fermentation of D-xylose is of interest in enhancing the yield of ethanol obtainable from lignocellulosic hydrolysates. Such hydrolysates can contain both pentoses and hexoses, and while technology to convert hexoses to ethanol is well established, the fermentation of pentoses had been problematical. To overcome the difficulty, yeasts and fungi have been sought and identified in recent years that can convert D-xylose into ethanol. However, operation of their cultures in the presence of the pentose to obtain rapid and efficient ethanol production is somewhat more complex than in the archetype alcoholic fermentation, Saccharomyces cerevisiae on D-glucose. The complexity stems, in part, from the association of ethanol accumulation in cultures where D-xylose is the sole carbon source with conditions that limit growth, by oxygen in particular, although limitation by other nutrients might also be implicated. Aspects of screening for appropriate organisms and of the parameters that play a role in determining culture variables, especially those associated with ethanol productivity, are reviewed. Performance with D-xylose as sole carbon source, in sugar mixtures, and in lignocellulosic hydrolysates is discussed. A model that involves biochemical considerations of D-xylose metabolism is presented that rationalizes the effects of oxygen on cultures where D-xylose is the sole carbon source, notably effects of the specific rate of oxygen use on the rate and extent of ethanol accumulation. Alternate methods to direct fermentation of D-xylose have been developed that depend on its prior isomerization to D-xylose, followed by fermentation of the pentulose by certain yeasts and fungi. Factors involved in the biochemistry, use, and performance of these methods, which with some organisms involves sensitivity to oxygen, are reviewed.

[The effect of exogenous factors on prececal nutrient and amino acid absorption, ascertained from swine with ileo-rectal anastomoses. 3. The effect of crude fiber-rich coarse meal supplements to a basic ration]

In digestion trials using pigs fitted with ileo-rectal anastomoses and parallel intact pigs the influence of wheat straw meal (WSM) or grass meal (GM) supplemented on two different levels to a basal diet was examined with regard to nutrient and amino acid digestibility resp. absorption. Both roughages reduced, obviously because of their high cell wall contents, nearly at equal amounts and partly significantly the precaecal as well as the total digestibility of dry matter, organic matter, carbohydrates (= crude fibre + NFE), crude fibre and hemicelluloses (arabinose, xylose). By the roughage supplementations the crude protein digestibility at the terminal ileum was less reduced than at the end of the total digestive tract, the starch digestibility was hardly influenced and that of ether extract mostly increased. In comparison with precaecal glucose and fructose digestibilities it could be shown that the anthrone method is not suitable for determinations of the precaecal digestibility of water soluble carbohydrates. Beside these compounds other substances in the ileum digesta must evidently be dyed by anthrone too. The crude protein digestibility and the amino acid absorption were precaecally hardly or not reduced by WSM supplementations, therefore it can be concluded that it is possible to dilute the energy concentration in diets (e.g. for sows) by addition of WSM without impairing protein digestion and amino acid absorption. The GM supplementations, however, impaired protein digestibility and amino acid absorption of the whole diet, probably caused by the encrusted, possibly heat damaged protein present in the GM itself. When the roughage supplemented diets were fed, the excretion of nitrogen compounds in the faeces was enlarged due to the more intensive bacterial activity in the hind gut and the additional sorption effects to cell wall substances, so that a too low apparent and also true digestibility is made believe. The applicability of the difference method to the calculation of protein and amino acid digestibilities in roughages is very questionable because of their low contents and the missing additivity. The crude fibre and hemicellulose digestibility values calculated by the difference method demonstrate for the both roughages--in spite of high standard deviations--that the precaecal digestibility of crude fibre is about zero and that of the hard lignified hemicellulose fraction in contrast to the predominantly endospermic hemicelluloses of the basal diet is very low. The faecal crude fibre and hemicellulose digestibilities of WSM--especially on the lower supplementation levels--are markedly worse than those of GM.

Bacteriologic and clinical study of Bacteroides oris and Bacteroides buccae

We characterized clinical isolates previously identified in our laboratory as Bacteroides ruminicola, the human strains of which are now classified as Bacteroides oris and Bacteroides buccae. A total of 72 isolates (55 B. buccae isolates and 17 B. oris isolates) recovered over a 10-year period were studied. They were differentiated from each other by special-potency antibiotic disks and the RapID-ANA system. The two organisms were associated with a variety of infections, the majority being pleuropulmonary (29.2%) and infections of the head and neck region (27.8%). The infections were always polymicrobial, usually with more than five organisms per specimen. A total of 44% of the B. oris strains and 27% of the B. buccae strains were resistant to penicillin G (breakpoint, 2 U/ml), and this correlated with the presence of beta-lactamase. Although B. oris and B. buccae are found with some frequency in human infections, they are present primarily as components of a mixed flora.

Mechanism and quantitative contribution of the pentose pathway to the glucose metabolism of Morris hepatoma 5123C

An investigation of the mechanism and quantitative contribution of the pentose phosphate pathway in the glucose metabolism of Morris Hepatoma 5123C is reported. Morris Hepatoma 5123C has an active non-oxidative segment of pentose pathway as judged by its ability to convert ribose 5-P to hexose 6-P in a standard assay. Based on compliance with qualitative and quantitative criteria, the cells exhibit the L-type pentose pathway reaction sequence rather than the F-type pathway. This compliance included the formation of intermediates characteristic of the L-type pathway, namely arabinose 5-P, octulose mono- and bisphosphates and sedoheptulose 1,7-bisphosphate, during the dissimilation of ribose 5-P to hexose 6-P. The intermediary role of arabinose 5-P was suggested by the incorporation of its carbon into various intermediates and products of the pentose pathway. Intermediary roles for ido octulose mono- and bisphosphates were supported by their participation in the reaction catalyzed by the phosphotransferase enzyme of the L-type pentose pathway. Presence of L-type PP reactions was further affirmed by 14C-prediction labelling experiments using [5-14C]- and [2-14C]glucose as specifically labelled substrates. Using two methods of measurement, the F-type pentose cycle made a negligibly small contribution to glucose metabolism, while the measured value of the L-type pentose pathway accounted for 30% (approx.) of the total glucose metabolism of these cells, a value consistent with the high activity of the enzymes of the L-type pentose pathway in Morris Hepatoma 5123C cells and the very high activity of the non-oxidative segment of the pathway in vitro. The findings validate the proposal that the L-type pentose pathway reactions constitute the non-oxidative segment of the pathway in Morris Hepatoma 5123C cells. Reasons involving pyruvate recycling reactions show why there is low incorporation of 14C-isotope in C-1 of glucose 6-P, when [4,5,6-14C]glucose and [6-14C]glucose are L-type PP test substrates in intact cells.

A small diffusion pore in the outer membrane of Pseudomonas aeruginosa

The permeability properties of the outer membrane of Pseudomonas aeruginosa were re-examined, since the reported conclusions are conflicting [Decad, M. G. and Nikaido, H. (1976) J. Bacteriol. 128, 325-336; Caulcott, C. A., Brown, M. R. W. and Gonda, I. (1984) FEMS Microbiol. Lett. 21, 119-123]. On the basis of the experimental evidence to be described below we conclude that the exclusion limit of the outer membrane of P. aeruginosa is smaller than the size of uncharged disaccharides but larger than the size of hexose. This conclusion is based on the following evidence. Penetration of monosaccharides into the expanded periplasm was large and that of disaccharides was small, after the cells were plasmolyzed with 600 mosM NaCl. A significant amount of protein was released after osmotic down-shock of cells treated with the hypertonic monosaccharides but not of cells treated with the hypertonic saccharides larger than disaccharides. Centrifuged pellets of cells treated with hypertonic di, tri and tetrasaccharides weighed about 15-20% less than that of cells treated with the isotonic monosaccharide, suggesting that the osmotic pressure was exerted on the outer membrane causing dehydration and shrinking of the cells. By contrast, cells treated with the hypertonic pentose and hexoses weighed about 0.1% and 6% less, respectively, than cells treated with the isotonic saccharide, suggesting that pentose diffused through the outer membrane freely.

Construction of an improved D-arabinose pathway in Escherichia coli K-12

A ribitol catabolic pathway was transduced into Escherichia coli K-12 in an effort to determine whether the ribitol pathway would confer an advantage to D-arabinose-positive mutants growing on D-arabinose as the sole carbon source. Competition studies in chemostats showed that ribitol-positive strains, with a selection coefficient of 9%/h, have a significant competitive advantage over ribitol-negative strains. Ribitol-positive strains grown in batch culture also exhibited a shorter lag period than did ribitol-negative strains when transferred from glucose to D-arabinose. Repeated transfer of a ribitol-positive strain of E. coli K-12 on D-arabinose yielded a strain with further improved growth on D-arabinose. This "evolved" strain was found to constitutively synthesize L-fucose permease, isomerase, and kinase but had lost the ability to grow on L-fucose, apparently owing to the loss of a functional aldolase. This constitutive mutation is not linked to the fucose gene cluster and may be similar to an unlinked constitutive mutation described by Chen et al. (J. Bacteriol. 159:725-729, 1984).

Biochemistry of reduction of nitro heterocycles

Misonidazole is a metabolically active drug. Its addition to cells causes an immediate alteration in cellular electron transfer pathways. Under aerobic conditions the metabolic alterations can result in futile cycling with electron transfer to oxygen and production of peroxide. Thiol levels are extremely important in protecting the cell against the peroxide formation and potentially hazardous conditions for hydroxyl radical production. Nevertheless such electron shunting out of cellular metabolism will result in alterations in pentose cycle, glycolysis and cellular capacity to reduce metabolites to essential intermediates needed in DNA metabolism (i.e. deoxyribonucleotides). Glutathione must be depleted to very low levels before toxic effects of misonidazole and other nitro compounds are manifested in cell death via peroxidative damage. Under hypoxic conditions misonidazole also diverts the pentose cycle via its own reduction; however, unlike the aerobic conditions, there are a number of reductive intermediates produced that react with non-protein thiols such as GSH as well as protein thiols. The reaction with protein thiols results in the inhibition of glycolysis and other as yet undetermined enzyme systems. The consequences of the hypoxic pretreatment of cells with nitro compounds are increased vulnerability to radiation and chemotherapeutic drugs such as L-PAM, cis-platinum and bleomycin. The role that altered enzyme activity has in the cellular response to misonidazole and chemotherapeutic agents remains to be determined. It is also clear that the GSH depleted state not only makes cells more vulnerable to oxidative stress but also to hypoxic intermediates produced by the reduction of misonidazole beyond the one electron stage. The relevancy of the present work to the proposed use of thiol depletion in vivo to enhance the radiation or chemotherapeutic response of tumor tissue lies with the following considerations. Apparently, spontaneous peroxidative damage to normal tissue such as liver can occur with GSH depletion to 10-20% of control and with other normal tissue when GSH reaches 50% of control. This situation can obviously become more critical if peroxide producing drugs are administered. The only advantage to such combined drug treatments would lie in the possibility that tumors vary in their catalase and peroxidase activity and consequently may be more vulnerable to oxidative stress (cf. review by Meister. Our tumor model, the A549 human lung carcinoma cell in vitro, appears to be an exception because it has catalase, peroxidase and a high content of GSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Anomeric specificity of glucose metabolism in the pentose cycle

The production of 3H2O from alpha- and beta-D-[5-3H]glucose and that of 14CO2 from either alpha- and beta-D-[1-14C] or alpha- and beta-D-[6-14C]glucose were measured in rat pancreatic islets and tumoral insulin-producing cells incubated at 7 degrees C. The ratio in 14CO2 output from D-[1-14C]glucose/D-[6-14C]glucose, the fraction of glucose metabolism occurring through the pentose cycle, and the flow rate through such a cycle were always higher in the presence of beta- than alpha-D-glucose. This indicates that the anomeric specificity of glucose-6-phosphate dehydrogenase is operative in intact islet cells.

Rates of pentose cycle flux in perfused rat liver. Evaluation of the role of reducing equivalents from the pentose cycle for mixed-function oxidation

Rates of NADPH production via the pentose phosphate cycle were determined in perfused livers from phenobarbital-treated rats by measuring 14CO2 production from [1-14C]glucose infused in the presence and absence of p-nitroanisole (0.2 mM), a substrate for mixed-function oxidation. In the fed state, basal rates of NADPH generation were 34-44 mumol/g/hr. p-Nitroanisole, which was metabolized at rates of 8.9 mumol/g/hr, stimulated pentose cycle-dependent NADPH production by 21-24 mumol/g/hr. Fasting for 24 hr prior to perfusion diminished pentose cycle flux by 80% and largely abolished the stimulation of the pentose cycle by p-nitroanisole. In contrast, rates of p-nitroanisole O-demethylation were only diminished slightly, to 5.7 mumol/g/hr. Fasting decreased hepatic glucose, glucose 6-phosphate, and 6-phosphogluconate contents drastically as expected. Pretreatment of rats with 6-aminonicotinamide, which is metabolized to a potent inhibitor of 6-phosphogluconate dehydrogenase, decreased rates of NADPH generation via the pentose cycle to 6.9 mumol/g/hr but did not alter rates of p-nitroanisole metabolism (8.8 mumol/g/hr). Basal rates of NADPH generation decreased from 38 to 26 mumol/g/hr during infusion of potassium cyanide (2 mM), an inhibitor of mitochondrial energy metabolism. Cyanide also decreased rates of p-nitroanisole O-demethylation by over 60%; however, stimulation of NADPH generation via the pentose cycle by p-nitroanisole was as great in the presence (17-21 mumol/g/hr) as in the absence of cyanide. Since rates of mixed-function oxidation were unaffected after virtually complete inhibition of the pentose cycle with 6-amino-nicotinamide, it is concluded that reducing equivalents for the mixed-function oxidation of p-nitroanisole are not provided by the pentose cycle under these conditions.

An investigation of the equilibria between aqueous ribose, ribulose, and arabinose

The thermodynamics of the equilibria between aqueous ribose, ribulose, and arabinose were investigated using high-pressure liquid chromatography and microcalorimetry. The reactions were carried out in aqueous phosphate buffer over the pH range 6.8-7.4 and over the temperature range 313.15-343.75 K using solubilized glucose isomerase with either Mg(NO3)2 or MgSO4 as cofactors. The equilibrium constants (K) and the standard state Gibbs energy (delta G degrees) and enthalpy (delta H degrees) changes at 298.15 K for the three equilibria investigated were found to be: ribose(aq) = ribulose(aq) K = 0.317, delta G degrees = 2.85 +/- 0.14 kJ mol-1, delta H degrees = 11.0 +/- 1.5 kJ mol-1; ribose(aq) = arabinose(aq) K = 4.00, delta G degrees = -3.44 +/- 0.30 kJ mol-1, delta H degrees = -9.8 +/- 3.0 kJ mol-1; ribulose(aq) = arabinose(aq) K = 12.6, delta G degrees = -6.29 +/- 0.34 kJ mol-1, delta H degrees = -20.75 +/- 3.4 kJ mol-1. Information on rates of the above reactions was also obtained. The temperature dependencies of the equilibrium constants are conveniently expressed as R in K = -delta G degrees 298.15/298.15 + delta H degrees 298.15[(1/298.15)-(1/T)] where R is the gas constant (8.31441 J mol-1 K-1) and T the thermodynamic temperature.

Blood group B-specific lectin of Plecoglossus altivelis (Ayu fish) eggs

A lectin that agglutinates human blood group B erythrocytes but not blood group A and O erythrocytes was isolated from eggs of Ayu sweet fish (Plecoglossus altivelis). The lectin also agglutinates Ehrlich ascites carcinoma cells but not rat ascites hepatoma AH109 or rat sarcoma 150 cells tested. The lectin agglutination was most effectively inhibited by monosaccharides with the first type of configuration, i.e., L-rhamnose, L-mannose and L-lyxose at a concentration of 0.03 mM. The lectin agglutination was moderately inhibited by monosaccharides with the second type of configuration, i.e., D-galactose, D-fucose and D-galacturonic acid at a concentration of 0.4 mM. However, the agglutination was not inhibited by various other monosaccharides and oligosaccharides that have other types of configuration. The basis for an apparent B-specific hemagglutination may be due to the steric similarity of the C2 and C4 of the galactosyl series, the B-specific determinant, and the L-rhamnosyl-Sepharose column and was characterized as a homogeneous low molecular weight protein (Mr 14000) with an abundance of hydrophobic amino acids and dicarboxylic amino acid.

Inhibition of 6-phosphogluconate dehydrogenase by glucose 1,6-diphosphate in human normal and malignant colon extracts

Increased activity of 6-phosphogluconate dehydrogenase was found in human colon tumors as compared to the adjacent unaffected mucosa. Glucose 1,6-diphosphate (Glc-1,6-P2), an endogenous potent regulator of glucose metabolism, markedly inhibited the activity of 6-phosphogluconate dehydrogenase (6-PGD) in extracts of the normal and malignant human colon. Glc-1,6-P2 also inhibited the activity of hexokinase in these extracts. The endogenous levels of Glc-1,6-P2 in the colon and tumors were measured. Since the pentose cycle can be inhibited by Glc-1,6-P2, means to increase endogenous levels of Glc-1,6-P2 or to introduce it into cells, might result in antitumor effects.

Evidence that aldolase and D-arabinose 5-phosphate are components of pentose pathway reactions in liver in vitro

An immunochemical procedure involving the reaction of liver aldolase antibody and rat liver enzyme preparation shows that conversion of ribose 5-P to hexose 6-P by reactions of the non-oxidative pentose pathway fails to occur in the absence of aldolase activity. Radioautography of pentose pathway products formed by liver enzyme catalysis of [U-14C] arabinose 5-P and unlabelled ribose 5-P illustrates the incorporation of 14C into ketopentose, sedoheptulose, fructose and glucose phosphates. There is approximate congruity of the mole specific radioactivity of the pentose and hexose phosphates. These findings are consistent with the proposal that L-pentose pathway reactions constitute the non-oxidative segment of the pathway in liver.

Biosynthesis of pentosyl lipids by pea membranes

Pea membranes were incubated with UDP-[14C]xylose or UDP-[14C]arabinose and sequentially extracted with chloroform/methanol/water (10:10:3, by vol.) and sodium dodecyl sulphate (2%, w/v). An active epimerase in the membranes rapidly interconverted the two pentosyl nucleotides. Chromatographic analysis of the lipid extract revealed that both substrates gave rise to xylose- and arabinose-containing neutral lipids, xylolipid with properties similar to a polyisoprenol monophosphoryl derivative, and highly charged lipid-linked arabinosyl oligosaccharide. When UDP-[14C]pentose or the extracted lipid-linked [14C]arabinosyl oligosaccharide were used as substrates, their 14C was also incorporating into sodium dodecyl sulphate-soluble and -insoluble fractions as major end products. Polyacrylamide-gel electrophoresis of sodium dodecyl sulphate-soluble products indicated the formation of mobile components with Mr values between 40 000 and 200 000 (Sepharose CL-6B). The lipid-linked [14C]arabinosyl oligosaccharide possessed properties comparable with those of unsaturated polyisoprenyl pyrophosphoryl derivatives. It was hydrolysed by dilute acid to a charged product (apparent Mr 2300) that could be fractionated in alkali. It was degraded to shorter labelled oligosaccharides by slightly more concentrated acid and eventually to [14C]arabinose as the only labelled component. Susceptibility to acid hydrolysis, and methylation analysis, indicated that the oligosaccharide contained approximately seven sequential alpha-1,5-linked arabinofuranosyl units at the non-reducing end. Several acidic residues appear to be interposed between the terminal arabinosyl units and the charged lipid.

Involvement of oxygen and mitochondrial function in the metabolism of D-xylulose by Saccharomyces cerevisiae

Mitochondrial function associated with oxygen was required for growth of Saccharomyces cerevisiae on D-xylulose. The requirement was shown by (i) the inhibition of growth of a wild-type strain under anaerobic conditions, (ii) the inhibition of aerobic growth after treatment with inhibitors of mitochondrial function, and (iii) the lack of aerobic and anaerobic growth of nuclear and cytoplasmic petites. The mitochondrial function was associated with the channeling of catabolites of D-xylulose to growth processes, since ethanol was formed even when growth was inhibited. Mitochondrial function was implicated as well in determining the extent of growth and the concentration of ethanol in aerobic cultures of the wild-type. In such cultures, the concentration of ethanol decreased and growth increased concomitantly as aeration rate increased. A factor in this relation was considered to be the relatively poor ability of D-xylulose to inhibit the oxidative utilization of ethanol.

Pentose cycle flux and fatty acid synthesis in bovine adipose tissue slices incubated with 6-aminonicotinamide

The effects of the purported inhibitor of 6-phosphogluconate dehydrogenase, 6-aminonicotinamide, on lipogenesis from acetate and the metabolism of glucose were investigated in bovine adipose tissue. The incorporation of [U-14C]acetate and tritium from [3-3H]glucose into fatty acids was stimulated by 6-aminonicotinamide proportionately, indicating that the pentose cycle provided the same percentage of NADPH required for fat synthesis in the absence and presence of 6-aminonicotinamide. Tissue samples incubated with 6-aminonicotinamide displayed higher maximal activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase than control samples. The cellular content of 6-phosphogluconate was increased by 6-aminonicotinamide by 40% in samples incubated with 2 mM glucose (plus 33 mU/ml insulin) and 10 mM acetate; 6-aminonicotinamide stimulated the production of L-lactate in either the absence or presence of acetate. Studies with 1-, 6-, and U-14C-labeled glucose indicated that 6-aminonicotinamide increased the proportion of utilized glucose metabolized by the pentose cycle in the absence, but not in the presence of acetate. Unlike results observed in rat adipose tissue, the primary effect of 6-aminonicotinamide was to increase the proportion of NADPH produced by the pentose cycle that was utilized for fat synthesis secondarily to the stimulation of lipogenesis by an unknown mechanism.

Carbohydrate metabolism in lactic acid bacteria

The term "lactic acid bacteria" is discussed. An overview of the following topics is given: main pathways of homo- and heterofermentation of hexoses, i.e. glycolysis, bifidus pathway, 6-phosphogluconate pathway; uptake and dissimilation of lactose (tagatose pathway); fermentation of pentoses and pentitols; alternative fates of pyruvate, i.e. splitting to formate and acetate, CO2 and acetate or formation of acetoin and diacetyl; lactate oxidation; biochemical basis for the formation of different stereoisomers of lactate.

The involvement of liver fructokinase in the metabolism of D-xylulose and xylitol in isolated rat hepatocytes

Hepatocytes isolated from fed, male, Sprague-Dawley rats accumulate xylulose-1-phosphate and glycolaldehyde as well as xylulose-5-phosphate when incubated with 2-20 mM D-xylulose. Fructokinase inhibitors (fructose and 1-deoxyfructose) decreased xylulose-1-phosphate and glycolaldehyde (but not xylulose-5-phosphate) levels in xylulose-treated hepatocytes, demonstrating the role of fructokinase in xylulose-1-phosphate and glycolaldehyde formation. As the fructokinase inhibitors had no overall effects on the conversion of D-xylulose to glucose, the overall flux through the pathway involving fructokinase was less than 27% of the total D-xylulose utilized. In hepatocytes from fed or fasted rats there was no detectable accumulation of either xylulose-1-phosphate or glycolaldehyde after treatment with 20 mM xylitol. Other differences between xylitol and D-xylulose metabolism in rat hepatocytes included a slower rate of xylitol metabolism in all preparations and a difference in the relative conversion of xylitol to glucose in hepatocytes from fasted rats. Rats adapted to 20% xylitol (diarrhea-free) had a lower water consumption than those fed a control cornstarch diet; there were no differences in weight gain, food consumption or in rates or metabolite patterns of xylitol metabolism in hepatocytes isolated from these rats. Despite the minor role of fructokinase in the overall metabolism of xylitol and of D-xylulose as shown by these results, it is not possible to exclude the possibility of some flux through the pathway involving xylulose-1-phosphate and glycolaldehyde formation as a possible route for oxalate formation.

Down-regulation of the hexose transport system: metabolic basis studied with a fibroblast mutant lacking phosphoglucose isomerase

Down-regulation ("curb") of hexose transport in Chinese hamster lung fibroblasts has been studied in a metabolic mutant highly defective in phosphoglucose isomerase (PGI; glucosephosphate isomerase; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9). In the parental strain (PGI+) glucose as well as glucosamine and mannose were able to elicit a curb of the hexose transport system. In the PGI mutant, only glucose was able to mediate a transport curb. The inability of glucosamine and mannose to promote a transport curb in the PGI strain must be ascribed to the fact that the 6-esters of these aldohexoses are converted by their own specific deaminase and isomerase to fructose 6-phosphate, which initiates the pyruvate-tricarboxylate energy-yielding pathway but cannot be converted to glucose 6-phosphate in the mutant. The latter ester can be metabolized, but its metabolism in the mutant is confined to the pentose shunt. It is shown that inhibitors such as 2,4-dinitrophenol and malonate exert only slight inhibition of the pentose shunt yet release the glucose-mediated curb elicited by glucose and glucosamine in the parental PGI+ strain and also the glucose transport curb persisting in the PGI mutant.

Rates of flux through the pentose cycle in perfused rat liver. A procedure for the calculation of rates of substrate flux from 14CO2 production from [1-14C]glucose

A method for the determination of substrate flux through the pentose cycle was developed employing [1-14C]glucose in experiments with perfused rat livers. The method consists first of a kinetic analysis which differentiates between the production of 14CO2 from [1-14C]glucose via the pentose cycle and via the citrate cycle and, second of a calculation of the specific radioactivity of the hexose monophosphate pool from measured rates of glycolysis and the specific radioactivity of lactate released into the perfusate. The method was validated by experiments comparing the results of tracer infusions with [1-14C]glucose, [6-14C]glucose and [3-14C]pyruvate. In livers from fed rats perfused with 10 mM glucose, the rate of substrate flux through the pentose cycle was around 0.2 mumol X min-1 X g-1; it was about 20% of the substrate flux via glycolysis. The kinetic data were inconsistent with the existence of an L-type pentose cycle in liver.

The participation of the Shemin and C5 pathways in 5-aminolaevulinate and chlorophyll formation in higher plants and facultative photosynthetic bacteria

Chlorophyll and bacteriochlorophyll formation from 14C-labelled precursors was studied during the illumination of etiolated maize leaves excised from dark-grown seedlings and in cell suspensions of respiring, dark-, aerobically-grown Rhodopseudomonas spheroides adapting to the photosynthetic state in the light under anaerobic conditions. It was found that 1-14C-labelled glutamate and 2-oxoglutarate were incorporated into the tetrapyrrole moieties of chlorophyll and bacteriochlorophyll. This suggests that the C5 pathway of tetrapyrrole biosynthesis operates in both Zea mays and R. spheroides since in the alternative Shemin pathway the label would have been lost as 14CO2 during the formation of succinyl-CoA prior to the condensation with glycine to form 5-aminolaevulinate. It was also found that 5(-14)C-labelled glutamate and 2-oxoglutarate were incorporated into these chlorophylls which is consistent with the operation of both the C5 and Shemin pathways. That the Shemin pathway is also involved was confirmed by the incorporation of [2(-14)C]glycine into both chlorophylls. None of these substrates were incorporated into the phytol moieties of either plant or bacterial chlorophyll or into the carotenoids. However, when [1(-14)C]acetate was added to greening maize leaves not only the tetrapyrrole and phytol moieties were labelled but also the carotenoids: the labelling of these lipids is consistent with their formation from acetate via the isopentenyl pyrophosphate pathway. By comparing the incorporation of [1(-14)C]2-oxoglutarate with that of [5(-14)C]2-oxoglutarate the approximate relative contribution of each pathway to chlorophyll biosynthesis was determined. In maize leaves both pathways contributed almost equally but in R. spheroides the contribution by the Shemin and C5 pathways was 90 and 10%, respectively.

Pseudomonas cepacia mutants blocked in the Entner-Doudoroff pathway

Pseudomonas cepacia mutants deficient in either 6-phosphogluconate (6PGA) dehydratase (Edd-) or 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase (Eda-) failed to utilize glucose or gluconate despite the prominence of of 6-phosphogluconate dehydrogenase (6PGAD) ii this bacterium and the potential for utilizing the pentose shunt suggested by its growth on ribitol and xylose. The Eda- strains grew normally on glucuronic acid, indicating that in P. cepacia its degradation does not depend upon KDPG aldolase as it does in Escherichia coli. Both 6PGA dehydratase and KDPG aldolase were inducible enzymes, with 6PGA rather than gluconate the apparent inducer. Edd- as well as Eda- strains were sensitive to growth inhibition by glucose, gluconate, fructose, and related carbohydrates when these substrates were present in combination with alternate carbon sources such as citrate or phthalate, presumably as a consequence of accumulation and toxicity of 6PGA, KDPG, or both. Edd- mutants were somewhat less sensitive to such inhibition than were Eda- strains. Certain derivatives of the Edd- strains we examined were able to utilize gluconate despite their deficiency of 6PGA dehydratase. Such mutants formed higher levels of 6PGAD than did the wild type. It is likely that the elevated levels of 6PGAD in these strains prevents accumulation of toxic levels of 6PGA that would otherwise result from a block in he Entner-Doudoroff pathway. The results suggest that P. cepacia can mutate to grow slowly on gluconate utilizing only the pentose shunt.

A pathway for the interconversion of hexose and pentose in the parasitic amoeba Entamoeba histolytica

Isotope studies indicate that hexose-to-pentose interconversion by axenic Entamoeba histolytica conserves the C-1 and C-6 hexose carbon atoms. Transketolase was readily identified in amoebal extracts, and transaldolase could not be demonstrated. However, sedoheptulose 7-phosphate is a substrate for the PPi-dependent amoebal phosphofructokinase, and sedoheptulose 1,7-bisphosphate is cleaved by amoebal aldolase to dihydroxyacetone phosphate and erythrose phosphate. Since these three enzymes catalyse physiologically reversible reactions, a non-oxidative pathway for hexose-pentose interconversion exists in amoebae in the absence of transaldolase. By using known amoebal enzyme, the conversion of ribose into fructose was confirmed in vitro. Some kinetic parameters of amoebal phosphofructokinase, transketolase and aldolase were determined.

Fermentation of D-xylose, xylitol, and D-xylulose by yeasts

Fifteen yeasts which can assimilate D-xylose were examined for the ability to convert this pentose to ethanol. In six of the seven genera investigated the conversion was enhanced when air had access to the medium. Therefore, the ability to convert D-xylose to ethanol under these conditions is probably common among yeasts. Growth under the same conditions on xylitol, a putative catabolite of D-xylose, led to only traces of ethanol. The effects of growth on another putative catabolite, D-xylose, were complex, but some of the strains which were among the better producers of ethanol from D-xylose produced less from D-xylulose.

Stimulation of the hexosemonophosphate-pentose pathway by pyrroline-5-carboxylate in cultured cells

delta 1-Pyrroline-5-carboxylic acid, an intermediate in the interconversions of proline, ornithine, and glutamate, is a potent stimulator of glucose oxidation through the hexosemonophosphate-pentose pathway. The effect is observed in cultured human fibroblasts, Chinese hamster ovary cells (CHO-K1), and rabbit kidney cells (LLC-RK1). In human fibroblasts, the magnitude of the stimulation of the hexosemonophosphate-pentose pathway is dependent on the concentration of added pyrroline-5-carboxylate and the effect is observed over a wide range of glucose concentrations. The mechanism of the effect is related to the generation of oxidizing potential in the form of NADP+ by pyrroline-5-carboxylate reductase concomitant with the conversion of pyrroline-5-carboxylate to proline. In LLC-RK1 cells, a cell line unique in having proline oxidase activity, proline also stimulated hexosemonophosphate-pentose pathway activity. Although pyrroline-5-carboxylate markedly stimulated the hexosemonophosphate-pentose pathway, it has no effect on glucose metabolism in the Embden-Meyerhof pathway or the tricarboxylic acid cycle. Since the hexosemonophosphate-pentose pathway is a source of ribose-5-phosphate, the precursor of phosphoribosyl pyrophosphate, the effect of pyrroline-5-carboxylate on the hexosemonophosphate-pentose pathway may link amino acid and nucleic acid metabolism.

[Specific and metabolic effect of oxythiamine]

Increasing doses of oxythiamine were studied as exerting the effect on transketolase inactivation in rat tissues. A conclusion is made that in the process of synthesis de novo there is a transient form of the enzyme accessible for interaction with oxythiamine pyrophosphate. Injection of oxythiamine in the increasing doses are accompanied by a decrease in the glycogen amount, increase in the intracellular level most of the studied intermediates of glycolysis and pentose cycle as well as cAMP. The probable biochemical mechanism of the oxythiamine action is connected with the activation of processes dependent on cAMP.

Evidence for biochemical and physiological differences between enzyme genotypes in Drosophila melanogaster

The in vivo flux of carbon through the pentose shunt is investigated as a function of different 6-phosphogluconate dehydrogenase (6Pgd) and glucose-6-phosphate dehydrogenase (G6pd) genotypes by using differential radioactive labeling of the C-1 and C-6 positions of glucose. Alternative 6Pgd-G6pd genotypes are shown to differ in relative in vivo carbon flux through the pentose shunt. The relative in vitro specific activity differences between the 6PgdSS and 6PgdFF genotypes appear to be primarily responsible for these differences. In addition, the pentose-shunt activity is correlated with the rate of lipid synthesis. This correlation is consistent with the major metabolic function of the pathway, which is to produce NADPH for lipid synthesis. Taken together, the results of these experiments show that different genotypes of 6Pgd are associated with measurable biochemical and physiological differences. Higher order phenotypic differences of this kind must be demonstrated to support the hypothesis that natural selection can discriminate among allozymes of a given genetic locus.

A comparison of 13C nuclear magnetic resonance and 14C tracer studies of hepatic metabolism

The gluconeogenic pathway from 13C-labeled substrates, each of which contained the 14C-labeled counterpart at a tracer level, has been followed in isolated rat liver cells and in isolated perfused mouse liver. The gluconeogenic flux from glycerol, the synthesis of glycogen, the stimulation of glycogenolysis by glucagon, the recycling of triacylglycerol, and an increase in pentose cycle activity under the influence of phenazine methosulfate were all observed directly in the 13C NMR spectra of perfused liver or isolated hepatocytes. The relative concentrations of 13C label at specific carbons measured by the NMR spectra under these conditions agreed closely with 14C isotopic distributions measured in extracts of the same doubly labeled samples for specific activities of greater than or equal to 3%. The label distributions measured by both methods were the same to within the experimental errors, which ranged from +/- 2% to +/- 7% in these experiments.

The role of dietary fibre in the human colon

Several effects of dietary fibre on colonic function have been documented by experiment or deduced from epidemiologic observation. The magnitude of these changes depends on the source and the physical and chemical composition of the fibre used, and on the individual response of the subjects. Three theories of the mode of action of fibre are discussed; they relate to the water-holding capacity of fibre, the production of short-chain fatty acids from fibre in the colon and the alteration by fibre of the colonic microflora.

Growth of yeasts on D-xylulose 1

Nine of eleven yeasts of different species or genera grew in the presence of air on the intermediate of D-xylose catabolism, D-xylulose (D-threo-pentulose). Growth on this substrate was efficient as judged by the optical density in stationary phase being generally similar to that after growth on glucose. Yeasts which grew on D-xylose also did so on D-xylulose, but among those which grew are included several which utilise neither D-xylose nor xylitol: Saccharomyces cerevisiae, Saccharomyces carlsbergensis, and Schizosaccharomyces pombe. Since catabolism of a sugar generally requires an initial phosphorylation step, growth of these strains suggests that they contain an enzyme which can function as a D-xylulose kinase. The D-xylulose-5-phosphate formed thereby is considered to enter the pentose-phosphate pathway. Glucose-grown inocula of S. carlsbergensis and Schizosaccharomyces pombe, and of several other yeasts, began to grow logarithmically when placed on D-xylulose with no apparent delay, or one which was minimal, suggesting that the D-xylulose kinase was already present in such cells, or was rapidly induced. Petites of S. cerevisiae did not grow on D-xylulose indicating that, in this species, mitochondria are involved in its utilisation.

Metabolic disposition of glucose carbon by sensory ganglia of 15-day-old chicken embryos, with new dynamic models of carbohydrate metabolism

Dorsal root ganglia from the lumbar region of chicken embryos 14--16 days old were incubated at 37 degrees in modified McCoy's culture medium containing [1-(14)C]glucose or [6-(14)C]glucose and sometimes 32Pi. A volume of 10 microliters of medium was used for four ganglia (dry weight approx. 90 micrograms). The output of 14CO2 was measured continuously. Incorporation of 14C into tissue constituents and into products released to the medium was measured after incubation for 3--17 h. Among nine radioactive components resolved in paper chromatograms of ganglion constituents, the most 14C was found in lipids and on materials remaining at the origin. Only the lipids and the origin materials were detectably labeled by 32Pi. Only relatively small amounts of 14C from [6-(14)]glucose were found in chromatographic regions that should contain intermediates of the pentose cycle. At least five labeled products were released to the bathing medium. Among these, the largest amount of 14C was found in lactic acid. A second component released, possibly alanine, also received considerable 14C. No 32P was detected in products in the medium. The rate of glucose uptake remained relatively constant as the concentration of glucose in the medium declined nearly 10-fold during prolonged experiments. Two new dynamic models of glucose metabolism successfully explained the time courses and magnitudes of previously reported 14CO2 outputs from [1-(14)C]glucose, [2-(14)C]-glucose, and [6-(14)C]glucose. These models are based on the assumption that glucose carbon was delayed on its way to CO2 in a pool of intermediates early in the metabolic chain and in a second pool either in or before the citric acid cycle. Both models assigned the pentose cycle to one cellular compartment, and incorporation into slowly-turning-over substances to another cellular compartment. According to both models, not more than one-half of the glyceraldehyde-P produced by the pentose cycle was converted to fructose-6-P, while at least half of this and other fructose-6-P from the pentose cycle was recycled into it. These conclusions differ from those from a previous model, which assumed that glucose carbon was delayed in a pool related to the pentose cycle; that model had suggested full recycling of both the glyceraldehyde-P and the fructose-6-P produced by the pentose cycle. In the citric acid cycle the efficiency of recycling was over 80%, according to all models. All models demonstrated the large differences that can occur in the metabolic handling of carbons 1, 2, and 6 of glucose. These differences need consideration in any description of the partitioning of glucose metabolism between alternative pathways.

Clearance rates of metabolizable and nonmetabolizable sugars in insulin-infused dogs and in exercising dogs

Dogs with indwelling arterial and venous catheters were trained to run on a treadmill (slope, 15% speed, 100 m/min). The clearance rates (CR) of 3H-inulin, 3-3Hglucose (G), 14C-L-arabinose (L-A), 14C-D-xylose (D-X), 14C-3-methyl-O-glucose (3-m-G) and 2-deoxy-glucose (2-d-G) were measured using the primed constant infusion tracer technics. With the exception of glucose, all substances were excreted in the urine; therefore the CR is the sum of the urinary clearance rate (UCR) plus the tissue clearance rate (tCR). By measuring CRinuMn and the concentration of 3H-inulin in the urine, the flow rate of urine and the UCR and tCR of the simultaneously infused 14C-sugars were calculated. Infusion of insulin (1.5 mU/kg min) and glucose (6 mg/kg min) into the resting dog raised plasma insulin to 60–40μU/ml at normoglycemia. The tCR of all sugars was increased: G>2-d-G>3-m-G>D-X>L-A. There was a great discrepancy between the phosphorylable (G, 2- d-G) and the nonmetabolizable sugars (3-m-G, D-X, LA). Insulin increased tCR of glucose almost threefold, and it was estimated that 27% of the increase could be ascribed to the unassisted direct effect of membrane- transport (3-m-G) and 55% if the glucose could be phosphorylated but not metabolized further (2-d-G). Exercise decreased plasma insulin by almost 50%, hepatic glucose production rose more than twofold, and the tCR-glucose more than 21/2-fold. The tCR of L-A remained unchanged and that of the other sugar derivatives was increased by the run in the following order G>2-d-G>3-m-G>D-X. It was estimated that only 16% of the increase of tCR-glucose could be ascribed to the effect of exercise on the membrane-transport if it was not assisted by the hexokinase, and 53% with the help of the enzyme but without the accelerated glycolysis. It is concluded that, under physiologic conditions, a relatively small fraction (16–20%) of the twoto threefold increase of glucose utilization can be ascribed to a direct membrane effect if the activity of flux-generating steps towards either glycogen formation (insulin) or glycolysis (exercise) is accelerated.

[Enzymatic activity of pentose cycle and the contractile function of rabbit womb as dependent on the carbon dioxide level]

The in vitro experiments were carried out to study the electrical and mechanical activities of the myometrium of nonpregnant and pregnant female rabbits as well as the enzymic activity of the pentose cycle in this tissue as affected by changes in the concentration and a fixation degree of carbon dioxide. It is shown that an increase in the carbon dioxide concentration in the incubation medium decreases the enzymatic activity of nonoxidative unit of the pentose cycle, but has no effect on the carbon dioxide fixation, electrical and mechanical activity of the myometrium. The carbon dioxide fixation in the myometrium tissue during pregnancy is established to be intensified. A higher content of carbon dioxide in the medium weakens the womb with the tonic properties of prostaglandins being significantly deteriorated.

Pentose pathway in pulmonary fibrosis due to chronic paraquat poisoning

In 14 beagle dogs, paraquat was infused in fractional doses to produce pulmonary fibrosis while avoiding fatal liver and kidney lesions. Activity of the three enzymes of the pentose pathway: glucose-6-phosphate dehydrogenase (G-6-PDH), glutathione reductase (GR) and glutathione peroxidase (GSH Px), which supply reduced equivalents against oxidant agents, were measured in the mediastinal lobe of the lung. After a single low dose (2-3 mg/kg body weight), GR and GSH Px activities were reduced. After repeated paraquat doses, pentose pathway enzyme activities were higher than after a single low dose; however, they did not significantly exceed the normal values as determined in control dogs. The activities of G-6-PDH, GR and GSH Px correlated with the total paraquat dose and with the extent of pulmonary fibrosis measured with an electronic image analyzer. The activity of pulmonary lactate dehydrogenase, which was also reduced after a single low dose of paraquat, did not show the same correlations.

[Enzymes catalyzing the reduction of aldopentoses and the oxidation of pentitols in Mycobacteria]

M. phlei, grown on synthetic Sauton medium (with 6% glycerol as carbon source), had NADPH- and NADH-aldopentose reductase, as well as NAD-pentitol dehydrogenase activities; some of their properties are studied. These activities are not present in BCG grown on the same medium. All experiments of aldopentose-reductase induction in BCG on a D(+)xylose medium were negative.

Effects of 2,4-dihydroxybutyrate on lipogenesis in rat hepatocytes

Hepatocytes were prepared from rats fasted 2 days and refed a high carbohydrate diet for 1 day. These cells contained very high levels of glycogen (about half the defatted dry weight) and carried out high rates of lipogenesis (up to 800 micron at tritium incorporation from 3HOH/g (defatted dry weight)/h), even in the absence of added substrates. Pentose cycle flux was estimated by a method involving the use of [1-14C]galactose (Rognstad, R. (1976) Int. J. Biochem. 7, 221-228). In hepatocytes from normal fasted refed rats, the amount of NADPH produced by the pentose cycle was sufficient for about one-half to three-fourths of that required for fatty acid synthesis. 2,4-Dihydroxybutyrate, a malic enzyme inhibitor (Schimerlik, M.I. & Cleland, W.W. (1977) Biochemistry 16, 565-570) markedly depressed the randomization of 14C in lactate from [6-14C]hexoses, indicating an inhibition of the pyruvate cycle. 2,4-Dihydroxybutyrate (10 mM) had only a slight inhibitory effect on overall lipogenesis, but increased the rate of the pentose cycle by 40 to 90%.

Removal of an adenine-like molecule during activation of dinitrogenase reductase from Rhodospirillum rubrum

During the activation of the inactive dinitrogenase reductase from Rhodospirillum rubrum, an adenine-like molecules is lost and phosphate is found on both active and inactive forms of the protein. ATP and divalent metals are required for activation of the reduced protein, but ATP is not required for activation of phenazine methosulfate-oxidized dinitrogenase reductase. Snake venom diesterase and spleen diesterase have no effect on the inactive protein; alkaline phosphatase removes phosphate from the activated protein but not from the inactive protein. ATP binds to both active and inactive forms of the protein.

Studies on dehydrogenases of the glucuronate-xylulose cycle in the livers of diabetic mice and rats

In view of reports that accessory pathways of glucose oxidation are enhanced in the diabetic state, we have determined the levels of key enzymes of the glucuronate-xylulose cycle in the livers of diabetic mice and rats. Genetically diabetic mice (db/db) were found to have increased levels of two NADP-linked enzymes of this cycle [NADP-xylitol dehydrogenase and NADP-L-hexonate dehydrogenase (aldehyde reductase II)], whereas other NAD- and NADP-linked dehydrogenase activities of the pathway were not changed. On the other hand, the livers of streptozotocin-diabetic mice and rats showed normal levels of all these enzymes. In the course of this study, evidence was obtained for the presence in db/db mouse liver of low molecular weight material inhibitory for glucose 6-phosphate dehydrogenase. The use of these animal models in diabetes research is briefly discussed.