PubMed Health⌕ Search

Biomedical subjects

F A Loewus

Publications and source records attributed to F A Loewus.

At least 19 recordsLinked to original sources
PubMed2001

Biosynthesis of L-ascorbic acid and conversion of carbons 1 and 2 of L-ascorbic acid to oxalic acid occurs within individual calcium oxalate crystal idioblasts.

L-Ascorbic acid (AsA) and its metabolic precursors give rise to oxalic acid (OxA) found in calcium oxalate crystals in specialized crystal idioblast cells in plants; however, it is not known if AsA and OxA are synthesized within the crystal idioblast cell or transported in from surrounding mesophyll cells. Isolated developing crystal idioblasts from Pistia stratiotes were used to study the pathway of OxA biosynthesis and to determine if idioblasts contain the entire path and are essentially independent in OxA synthesis. Idioblasts were supplied with various (14)C-labeled compounds and examined by micro-autoradiography for incorporation of (14)C into calcium oxalate crystals. [(14)C]OxA gave heavy labeling of crystals, indicating the isolated idioblasts are functional in crystal formation. Incubation with [1-(14)C]AsA also gave heavy labeling of crystals, whereas [6-(14)C]AsA gave no labeling. Labeled precursors of AsA (L-[1-(14)C]galactose; D-[1-(14)C]mannose) also resulted in crystal labeling, as did the ascorbic acid analog, D-[1-(14)C]erythorbic acid. Intensity of labeling of isolated idioblasts followed the pattern OxA > AsA (erythorbic acid) > L-galactose > D-mannose. Our results demonstrate that P. stratiotes crystal idioblasts synthesize the OxA used for crystal formation, the OxA is derived from the number 1 and 2 carbons of AsA, and the proposed pathway of ascorbic acid synthesis via D-mannose and L-galactose is operational in individual P. stratiotes crystal idioblasts. These results are discussed with respect to fine control of calcium oxalate precipitation and the concept of crystal idioblasts as independent physiological compartments.

Araceae↗
PubMed2000

L-Ascorbic acid and L-galactose are sources for oxalic acid and calcium oxalate in Pistia stratiotes.

Axenic Pistia stratiotes L. plants were pulse-chase labeled with [14C]oxalic acid, L[1-14C]ascorbic acid, L-6-14C]ascorbic acid, D-[1-14C]erythorbic acid, L-[1-14C]galactose, or [1-14C]glycolate. Specific radioactivities of L-ascorbic acid (AsA), free oxalic acid (OxA) and calcium oxalate (CaOx) in labeled plants were compared. Samples of leaf tissue were fixed for microautoradiography and examined by confocal microscopy. Results demonstrate a biosynthetic role for AsA as precursor of OxA and its crystalline deposition product, CaOx, in idioblast cells of P. stratiotes and support the recent discovery of Wheeler, Jones and Smirnoff (Wheeler, G.L., Jones M.A., & Smirnoff, N. (1998). The biosynthetic pathway of vitamin C in higher plants. Nature, 393, 365-369) that L-galactose is a key intermediate in the conversion of D-glucose to AsA in plants. D-[1-14C]erythorbic acid (a diastereomeric analog of AsA) is utilized also by P. stratiotes as a precursor of OxA and its calcium salt deposition product in idioblasts. Labeled OxA is rapidly incorporated into CaOx in idioblasts, but microautoradiography shows there is also significant incorporation of carbon from OxA into other components of growing cells, contrary to the dogma that OxA is a relatively stable end product of metabolism. Glycolate is a poor substrate for synthesis of OxA and CaOx formation, further establishing AsA as th immediate precursor in the synthesis of OxA used for calcium precipitation in crystal idioblasts.

Ascorbic Acid↗
PubMed1998

5-O-(alpha-D-galactopyranosyl)-D-glycero-pent-2-enono-1,4-lactone: characterization in the oxalate-producing fungus, Sclerotinia sclerotiorum.

Extracts of sclerotia from Sclerotinia sclerotiorum, a fungal phytopathogen, contain two electrochemically-active constituents, D-glycero-pent-2-enono-1,4-lactone (trivial name: D-erythroascorbic acid), and a previously unidentified compound, here characterized as 5-O-(alpha-D-galactopyranosyl)-D-glycero-pent-2-enono-1,4-lactone on the basis of its physical and chemical properties and its two hydrolytic products, D-galactose and D-erythroascorbic acid. Treatment of this galactoside with alkaline hydrogen peroxide produces oxalic acid as observed earlier with erythroascorbic acid.

4-Butyrolactone↗
PubMed1995

Conversion of D-arabinose to D-erythroascorbic acid and oxalic acid in Sclerotinia sclerotiorum.

D-glycero-Pent-2-enono-1,4-lactone (trivial name: D-erythroascorbic acid) occurs in the phytopathogen, Sclerotinia sclerotiorum (Lib.) de Bary, where it has a potential role as precursor of oxalic acid. On Glc/yeast/malt medium, S. sclerotiorum produces only nominal amounts of D-erythroascorbic acid but even partial replacement of Glc by D-Ara increases production of erythroascorbic acid and oxalic acid. Use of D-[1-14C]-, -[3-14C]-, or -[6-14C]Glc and D-[5-3H]-, -[2-14C,5-3H]-, or -[UL-14C]Ara provide additional information on erythroascorbic acid biosynthesis and cleavage. The latter process resembles that obtained by peroxygenation of erythroascorbic acid in alkaline solution. An unknown erythroascorbic acid-like compound also occurs in both Glc- and Ara-based cultures.

Arabinose↗
PubMed1990

Conversion of l-Sorbosone to l-Ascorbic Acid by a NADP-Dependent Dehydrogenase in Bean and Spinach Leaf.

An NADP-dependent dehydrogenase catalyzing the conversion of l-sorbosone to l-ascorbic acid has been isolated from Phaseolus vulgaris L. and Spinacia oleracea L. and partially purified. It is stable at -20 degrees C for up to 8 months. Molecular masses, as determined by gel filtration, were 21 and 29 kilodaltons for bean and spinach enzymes, respectively. K(m) for sorbosone were 12 +/- 2 and 18 +/- 2 millimolar and for NADP(+), 0.14 +/- 0.05 and 1.2 +/- 0.5 millimolar, for bean and spinach, respectively. Lycorine, a purported inhibitor of l-ascorbic acid biosynthesis, had no effect on the reaction.

Journal Article↗
PubMed1990

d-Glucosone and l-Sorbosone, Putative Intermediates of l-Ascorbic Acid Biosynthesis in Detached Bean and Spinach Leaves.

d-[6-(14)C]Glucosone that had been prepared enzymically from d-[6-(14)C]glucose was used to compare relative efficiencies of these two sugars for l-ascorbic acid (AA) biosynthesis in detached bean (Phaseolus vulgaris L., cv California small white) apices and 4-week-old spinach (Spinacia oleracea L., cv Giant Noble) leaves. At tracer concentration, (14)C from glucosone was utilized by spinach leaves for AA biosynthesis much more effectively than glucose. Carbon-14 from [6-(14)C]glucose underwent considerable redistribution during AA formation, whereas (14)C from [6-(14)C]glucosone remained almost totally in carbon 6 of AA. In other experiments with spinach leaves, l-[U-(14)C]sorbosone was found to be equivalent to [6-(14)C]glucose as a source of (14)C for AA. In the presence of 0.1% d-glucosone, conversion of [6-(14)C] glucose into labeled AA was greatly repressed. In a comparable experiment with l-sorbosone replacing d-glucosone, the effect was much less. The experiments described here give substance to the proposal that d-glucosone and l-sorbosone are putative intermediates in the conversion of d-glucose to AA in higher plants.

Journal Article↗
PubMed1987

Localization of Phosphorus and Cation Reserves in Lilium longiflorum Pollen.

Transmission electron microscopy of pollen from Lilium longiflorum Thunb. reveals electron-dense inclusions in storage body organelles ubiquitous in the cytosol. In ungerminated pollen, these inclusions are rounded in appearance and appressed to the inner surface of the smooth membrane of the storage body. During pollen germination, these inclusions become less rounded, smaller, and enclosed in storage bodies that have developed crenated membranes. Energy dispersive x-ray analysis reveals high levels of P, Mg, K, and Ca in the inclusions relative to other regions of the cytosol in which elemental signals can be obtained. The elemental composition and the degradation of inclusions during germination are offered as evidence for storage of phytin in these structures which are thus analogous to phytin storage globoids of seed tissues.

Journal Article↗
PubMed1986

A Calcium-Activated Phytase from Pollen of Lilium longiflorum.

A phytase was isolated and partially purified from the pollen of Lilium longiflorum Thumb. Optimum activity was at pH 8.0. The phytase was activated by Ca(2+) and Sr(2+) but not by the other divalent cations tested. Activity was inhibited by ethylenediaminetetraacetate. The phytase had a temperature optimum of 55 to 60 degrees C and an activation energy of about 12,700 calories/mole. Extraction of L. longiflorum pollen with 0.1% Triton X-100 increased recovery of the phytase by nearly 4-fold. The phytase had a molecular weight of about 88,000 as determined by gel filtration chromatography and a K(m) value of 7.2 micromolar for phytic acid in the presence of Ca(2+).

Journal Article↗
PubMed1985

Pollen sporoplasts: dissolution of pollen walls.

4-Methylmorpholine N-oxide monohydrate (MMNO.H(2)O), a potent solvent for polysaccharides, is an effective vehicle for release of membrane-enclosed male gametophytes (sporoplasts) from spore walls. This release occurs in minutes when pollen (Lilium longiflorum Thunb.) is suspended in a melt of MMNO.H(2)O at 75 degrees C. Continued heating at 75 degrees C leads to distintegration of the exine ;shell' which coalesces into immiscible globules in the MMNO melt. These observations provide a general procedure for preparation of pollen sporoplasts and sporoplast outer membranes, and offer a new method for dissolving the sporopollenin component of the spore wall.

Journal Article↗
PubMed1984

1L-myo-inositol 1-phosphate synthase from pollen of Lilium longiflorum. An ordered sequential mechanism.

1L-Inositol 1-phosphate synthase (EC 5.5.1.4) devoid of bound NAD+ was isolated from mature pollen of Lilium longiflorum ( Easter lily ). The enzyme has a molecular weight of 157,000 +/- 15,000 and a subunit weight of 61,000 +/- 5,000. Kinetic studies of the uninhibited reaction and of inhibition by 2-deoxy-D-glucose 6-phosphate and NADH show the reaction to be ordered sequential with NAD+ adding first. The Michaelis constants for NAD+ and D-glucose 6-phosphate are 2.4 and 65 microM, respectively. The Ki for 2-deoxy-D-glucose 6-phosphate was 8.7 and 2.0 microM, respectively, when D-glucose 6-phosphate or NAD+ was varied. The Ki for NADH and variable NAD+ was 4.7 microM and, for NADH and variable D-glucose 6-phosphate, 3.9 microM.

Carbohydrate Epimerases↗
PubMed1984

myo-Inositol-1-phosphate synthase from pine pollen: sulfhydryl involvement at the active site.

myo-Inositol-1-phosphate synthase [EC 5.5.1.4; 1L-myo-inositol-1-phosphate lyase, (isomerizing)] from Pinus ponderosa pollen has been partially purified and characterized. It has a pH optimum between 7.25 and 7.75. The km for D-glucose 6-phosphate (NAD+ constant at 1 mM) is 0.33 mM. Inhibition by p-chloromercuribenzoate and N-ethylmaleimide, and partial protection against this inhibition by D-glucose 6-phosphate in the presence of NAD+, suggests that there is sulfhydryl group involvement at the substrate binding site.

Binding Sites↗
PubMed1984

Further Studies on myo-Inositol-1-phosphatase from the Pollen of Lilium longiflorum Thunb.

myo-Inositol-1-phosphatase has been purified to homogeneity from Lilium longiflorum pollen using an alternative procedure which includes pH change and phenyl Sepharose column chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis shows that the enzyme is a dimer (subunit molecular weight, 29,000 daltons). The enzyme is stable at low pH values and is inactivated only below pH 3.0. In addition to 1l-and 1d-myo-inositol-1-phosphate, it shows high specificity for 1l-chiro-inositol-3-phosphate. As observed earlier with other primary phosphate esters, d-glucitol-6-phosphate and d-mannitol-6-phosphate are hydrolyzed very slowly. No activity is observed with inorganic pyrophosphate or myo-inositol pentaphosphate as substrate. The enzyme is inhibited by fluoride, sulfate, molybdate, and thiol-directed reagents. Partial protection against N-ethylmaleimide inhibition by substrate and Mg(2+) together suggests sulfhydryl involvement at the active site.

Journal Article↗
PubMed1982

Redistribution of Tritium during Germination of Grain Harvested from myo-[2-H]Inositol- and scyllo-[R-H]Inositol-Labeled Wheat.

Wheat kernels from myo-[2-(3)H]inositol- or scyllo-[R-(3)H]inositol-labeled plants (Sasaki and Loewus 1980 Plant Physiol 66: 740-745) were used to study redistribution of (3)H into growing regions during germination. Most of the labeled 1-alpha-galactinol (or the analogous scyllo-inositol galactoside) was hydrolyzed within 1 day. Water-soluble phytate was dephosphorylated within 3 days. A large reserve of bound phytate continued to release myo-inositol over several days. Translocation of free myo-inositol to growing regions provided substrate for the myo-inositol oxidation pathway and incorporation of (3)H into new cell wall polysaccharides.Cell wall polysaccharides in the kernel were degraded during germination. The labeled residues were translocated to growing regions and reutilized for new cell wall formation. Pentosyl residues accounted for most of this label.Free scyllo-inositol followed a path of translocation from kernel to seedling similar to that of myo-inositol. Unlike myo-inositol, it did not furnish substrate for the myo-inositol oxidation pathway but accumulated as free scyllo-inositol in the seedling.The fate of phytate-derived myo-inositol during germination of wheat is discussed in relation to a recent scheme of phytate metabolism proposed by De and Biswas (1979 J Biol Chem 254: 8717-8719) for germinating mung bean seedlings.

Journal Article↗
PubMed1982

l-Ascorbic Acid Biosynthesis in Ochromonas danica.

Ochromonas danica Pringsheim, a freshwater chrysomonad, converts d-glucose into l-ascorbic acid over a metabolic pathway that ;inverts' the carbon chain of the sugar. In this respect, l-ascorbic acid formation resembles that found in ascorbic acid-synthesizing animals. It differs from this process in that d-galacturonate and l-galactono-1,4-lactone, rather than d-glucuronate and l-gulono-1,4-lactone, enhance production of ascorbic acid and repress the incorporation of (14)C from d-[1-(14)C]glucose into ascorbic acid.

Journal Article↗
PubMed1982

Metabolism of l-Threonic Acid in Rumex x acutus L. and Pelargonium crispum (L.) L'Hér.

l-Threonic acid is a natural constituent in leaves of Pelargonium crispum (L.) L'Hér (lemon geranium) and Rumex x acutus L. (sorrel). In both species, l-[(14)C]threonate is formed after feeding l-[U-(14)C]ascorbic acid to detached leaves. R. acutus leaves labeled with l-[4-(3)H]- or l-[6-(3)H]ascorbic acid produce l-[(3)H]threonate, in the first case internally labeled and in the second case confined to the hydroxymethyl group. These results are consistent with the formation of l-threonate from carbons three through six of l-ascorbic acid. Detached leaves of P. crispum oxidize l-[U-(14)C] threonate to l-[(14)C]tartrate whereas leaves of R. acutus produce negligible tartrate and the bulk of the (14)C appears in (14)CO(2), [(14)C]sucrose, and other products of carbohydrate metabolism. R. acutus leaves that are labeled with l-[U-(14)C]threonate release (14)CO(2) at linear rate until a limiting value of 25% of the total [U-(14)C]threonate is metabolized. A small quantity of [(14)C]glycerate is also produced which suggests a process involving decarboxylation of l-[U-(14)C]threonate.

Journal Article↗
PubMed1982

myo-Inositol-1-Phosphatase from the Pollen of Lilium longiflorum Thunb.

A Mg(2+)-dependent, alkaline phosphatase has been isolated from mature pollen of Lilium longiflorum Thunb., cv. Ace and partially purified. It hydrolyzes 1l- and 1d-myo-inositol 1-phosphate, myo-inositol 2-phosphate, and beta-glycerophosphate at rates decreasing in the order named. The affinity of the enzyme for 1l- and 1d-myo-inositol 1-phosphate is approximately 10-fold greater than its affinity for myo-inositol 2-phosphate. Little or no activity is found with phytate, d-glucose 6-phosphate, d-glucose 1-phosphate, d-fructose 1-phosphate, d-fructose 6-phosphate, d-mannose 6-phosphate, or p-nitrophenyl phosphate. 3-Phosphosphoglycerate is a weak competitive inhibitor. myo-Inositol does not inhibit the reaction. Optimal activity is obtained at pH 8.5 and requires the presence of Mg(2+). At 4 millimolar, Co(2+), Fe(2+) or Mn(2+) are less effective. Substantial inhibition is obtained with 0.25 molar Li(+). With beta-glycerophosphate as substrate the K(m) is 0.06 millimolar and the reaction remains linear at least 2 hours. In 0.1 molar Tris, beta-glycerophosphate yields equivalent amounts of glycerol and inorganic phosphate, evidence that transphosphorylation does not occur.In higher plants this myo-inositol-1-phosphatase links myo-inositol biosynthesis to the myo-inositol oxidation pathway to produce an alternative path from d-glucose 6-phosphate to UDP-d-glucuronate that bypasses UDP-d-glucose dehydrogenase. myo-Inositol-1-phosphatase also furnishes free myo-inositol for reactions that lead to other cyclitols and cyclitol-containing compounds of biosynthetic and/or regulatory significance in plant growth and development.

Journal Article↗
PubMed1982

Loss of Hydrogen from Carbon 5 of d-Glucose during Conversion of d-[5-H,6-C]Glucose to l-Ascorbic Acid in Pelargonium crispum (L.) L'Hér.

Conversion of d-[5-(3)H,6-(14)C]glucose to l-ascorbic acid in detached apices of Pelargonium crispum (L.) L'Hér cv Prince Rupert (lemon geranium) was accompanied by complete loss of tritium in the product. Chemical degradation of d-glucose which was recovered from the labeled apices yielded d-glyceric acid (corresponding to carbons 4, 5, and 6 of glucose) with a (3)H:(14)C ratio of 4 to be compared with 9, the ratio in d-[5-(3)H,6-(14)C]glucose initially. Conversion of d-[6-(3)H,6-(14)C]glucose in the same tissue was accompanied by retention of tritium in l-ascorbic acid with a (3)H:(14)C ratio comparable to that of compounds from the hexose pool. Results indicate that during l-ascorbic acid biosynthesis from glucose in Pelargonium crispum hydrogen at carbon 5 undergoes exchange with the medium, suggesting an epimerization at this carbon atom.

Journal Article↗