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31.
Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion 总被引:2,自引:2,他引:0
Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO4-Pmg–1C in zooplankton (C
zp
) h–1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg–1C
zp
h–1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO4-P mg–1 C h–1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO4-P mg–1 C hp–1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the real rates. 相似文献
32.
Mittal Chandra K. Harrell William B. Mehta Chander S. 《Molecular and cellular biochemistry》1995,149(1):263-265
This study was designed to evaluate thein vitro effects of transition heavy metal cations on activity of constitutive isoform of nitric oxide synthase (cNOS) in rat brain. NOS activity was determined in the cytosolic fractions of rat cerebral hemispheres by conversion of3H-L-arginine to3H-L-citrulline. Different concentrations of mercury (Hg2+), nickel (Ni2+), manganese (Mn2+), zinc (Zn2+), cadmium (Cd2+), lead (Pb2+) and calcium (Ca2+) were tested on NOS activity. While all the cations caused inhibition, there were differences in the apparent inhibition constants (Ki) among the cations. With the exception of calcium ion no other cation required preincubation with the enzyme preparation. These results indicate that while calcium ion modulate cNOS activity at regulatory site(s), inhibitory influence of toxic heavy metal cations may be exerted on the catalytic site(s) either by direct binding to it or by interfering with the electron transfer during catalysis. 相似文献
33.
34.
Excised shoot tips of Cuscuta reflexa Roxb. (dodder), a rootless and leafless angiospermic plant parasite, were cultured in vitro for the study of the control of lateral bud development by the apex. In a chemically defined medium lacking hormones, the basal bud alone developed into a shoot. The addition of coconut milk to the growth medium induced the activation of multiple lateral buds, but only a single bud developed further into a shoot. The decapitation of this shoot induced the development of another shoot and the process could be repeated. This showed the controlling effect of the apex in correlative control of bud development. Application of indole-3-acetic acid to the shoot tip explant delayed the development of the lateral bud. Gibberellic acid A3 induced a marked elongation growth of the explant and reinforced apical dominance. The direct application of cytokinin to an inhibited bud relieved it from apical dominance. A basipetally decreasing concentration gradient of auxin may prevail at the nodes. Bud outgrowth is probably stimulated by cytokinin produced locally in the bud. 相似文献
35.
Thirty isolates of mungbean Rhizobium were tested for the presence of H2-recycling system. All the isolates were preliminary screened for detecting H2-recycling system in free culture using triphenyltetrazolium chloride reduction as screening procedure. The isolates which reduced the dye rapidly at early stages of growth were found to recycle hydrogen both in vivo as well as in vitro. Nitrogen fixing efficiency of hydrogenase positive, hydrogenase negative isolates and Hup– mutants was compared by green house experiments. There was 13–56% increase in dry matter and 21–46% increase in total nitrogen of the plants inoculated with H2-recycling isolates over the plants inoculated with non-recycling isolates. There was reduction in dry matter and total nitrogen content of the plants inoculated with Hup– mutants as compared to plants inoculated with wild type strain. The per cent decrease due to inoculation with Hup– mutants over wild type strain was 19–22 and 20–26 of dry weight and total nitrogen in plants, respectively.Abbreviations TTC
triphenyltetrazolium chloride 相似文献
36.
Degradation of (+/-)-synephrine by Arthrobacter synephrinum. Oxidation of 3,4-dihydroxyphenylacetate to 2-hydroxy-5-carboxymethyl-muconate semialdehyde. 总被引:2,自引:0,他引:2 下载免费PDF全文
1. Cell-free extracts of Arthrobacter synephrinum catalyse the oxidation of 3,4-dihydroxy-phenylacetate. 2. The product of oxidation was characterized as 2-hydroxy-5-carboxymethylmuconate semialdehyde from its chemical behaviour as well as from nuclear-magnetic-resonance spectra. 3. A 3,4-dihydroxyphenylacetate 2,3-dioxygenase (EC 1.13.11.15) was partially purified from A. synephrinum. 4. The enzyme had a Km of 25 micrometer towards its substrate and exhibited typical Michaelis-Menten kinetics. 5. The enzyme also catalysed the oxidation of 3,4-dihydroxymandelate and 3,4-dihydroxyphenylpropionate, at reaction rates of 0.5 and 0.04 respectively of that for 3,4-dihydroxyphenylacetate. 6. The enzyme was sensitive to treatment with thiol-specific reagents. 7. The molecular weight of the enzyme as determined by Sephadex G-200 chromatography was approx. 282000. 相似文献
37.
Microbial Oxidation of Methane and Methanol: Crystallization of Methanol Dehydrogenase and Properties of Holo- and Apo-Methanol Dehydrogenase from Methylomonas methanica 下载免费PDF全文
Procedures are described for the purification and crystallization of methanol dehydrogenase from the soluble fraction of the type I obligate methylotroph Methylomonas methanica strain S1. The crystallized enzyme is homogeneous as judged by acrylamide gel electrophoresis and ultracentrifugation. The enzyme had a high pH optimum (9.5) and required ammonium salt as an activator. In the presence of phenazine methosulfate as an electron acceptor, the enzyme catalyzed the oxidation of primary alcohols and formaldehyde. Secondary, tertiary, and aromatic alcohols were not oxidized. The molecular weight as well as subunit size of methanol dehydrogenase was 60,000, indicating that it is monomeric. The sedimentation constant (s(20,w)) was 3.1S. The amino acid composition of the crystallized enzyme is also presented. Antisera prepared against the crystalline enzyme were nonspecific; they cross-reacted with and inhibited the isofunctional enzyme from other obligate methylotrophic bacteria. The crystalline methanol dehydrogenase had an absorption peak at 350 nm in the visible region and weak fluorescence peaks at 440 and 470 nm due to the presence of a pteridine derivative as the prosthetic group. A procedure was developed for the preparation of apo-methanol dehydrogenase. The molecular weights, sedimentation constants, electrophoretic mobilities, and immunological properties of apo- and holo-methanol dehydrogenases are identical. Apo-methanol dehydrogenase lacked the absorption peak at 350 nm and the fluorescence peaks at 440 and 470 nm and was catalytically inactive. All attempts to reconstitute an active enzyme from apo-methanol dehydrogenase, using various pteridine derivatives, were unsuccessful. 相似文献
38.
39.
Summary Root exudate from seedlings ofCicer arietinum L. was collected in a chamber under aseptic conditions. The exudate was fractionated into anion, cation and neutral fractions.
The anionic fraction was made up of galacturonic acid, gluconic acid, mannuronic acid and two unidentified compounds withR
f
values 0.56 and 0.62. The cationic fraction contained alanine, arginine, aspartic acid, cystine, glycine, histidine, isoleucine,
leucine, lysine and serine. The neutral fraction was made up of arabinose, galactose, glucose, ribose and xylose. The amino
acids contributed to the bulk of the root exudate. The ratio of anionic, cationic and neutral fraction was 1∶7∶2. The crude
root exudate was tested for its chemotactic ability using the capillary tube method. It was highly chemotactic for theRhizobium sp. The individual fractions and their various combinations were tested for chemotaxis. The chemotactic response of the Cicer
strain of Rhizobium was least with anionic fraction most with cationic fraction and intermediate with neutral fraction. Maximum
chemotactic response among the fractional combinations was obtained with all the three fractions and least with cationic plus
neutral factions. Individual compounds constituting the various fractions were also tried for their ability to elicit chemotactic
response. The organism exhibited maximum positive chemotactic response to histidine and negative response to alanine among
the amino acids and to glucose and gluconic acid among the sugars and sugar acids. 相似文献
40.
The Escherichia coli trp repressor binds to the trp operator in the presence of tryptophan, thereby inhibiting tryptophan biosynthesis. Tryptophan analogues lacking the alpha-amino group act as inducers of trp operon expression. We have used one- and two-dimensional 1H-NMR spectroscopy to compare the binding to the repressor of the corepressors L-tryptophan, D-tryptophan and 5-methyl-DL-tryptophan with that of the inducer indole-3-propionic acid. We have determined the chemical shifts of the indole ring protons of the ligands when bound to the protein, principally by magnetization-transfer experiments. The chemical shifts of the indole NH and C4 protons differ between corepressors and inducer. At the same time, the pattern of intermolecular NOE between protons of the protein and those of the ligand also differ between the two classes of ligand. These two lines of evidence indicate that corepressors and inducers bind differently in the binding site, and the evidence suggests that the orientation of the indole ring in the binding site differs by approximately 180 degrees between the two kinds of ligand. This is in contrast to a previous solution study [Lane, A.N. (1986) Eur. J. Biochem. 157, 405-413], but consistent with recent X-ray crystallographic work [Lawson, C.L. & Sigler, P.B. (1988) Nature 333, 869-871]. D-Tryptophan and 5-methyltryptophan, which are more effective corepressors than L-tryptophan, bind similarly to L-tryptophan. The indole ring of D-tryptophan appears to bind in essentially the same orientation as that of the L isomer. There are, however, some differences in chemical shifts and NOE for 5-methyltryptophan, which indicate that there are significant differences between the two corepressors L-tryptophan and 5-methyltryptophan in the orientation of the indole ring within the binding site. 相似文献