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  • 1
    ISSN: 1573-5176
    Keywords: agarose ; Gracilaria ; algae ; polysaccharide ; 13C-NMR ; agar
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The influence of nitrogen availability on the chemical structure of agar polymers isolated fromGracilaria verrucosa strain G-16 was ascertained by sequential solvent extraction and carbon 13 nuclear magnetic resonance (13C NMR) spectroscopy. Agar isolated fromG. verrucosa strain G-16 cultured under nitrogen limited conditions showed large non-polar components but produced spectra indicative of only minor amounts of methylation. These agars also produced spectra suggesting the presence of floridean starch. The nitrogen supplemented cultures ofG. verrucosa strain G-16 produced agars of a more polar nature (hot-water soluble) and contained little detectable starch. The data suggest that the higher gelling temperatures of agar from nitrogen limited plants is not due to markedly higher methylation. These data also suggest that nitrogen effects on agar content may not be as significant as previously thought.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: Substrate regulation of System A transport activity in rat H4 hepatoma cells is described. The uptake of several amino acids was tested in the presence of system-specific inhibitors. System A activity was increased in a RNA- and protein synthesis-dependent manner by amino acid deprivation of the cells (adaptive regulation), whereas transport by Systems ASC, N, y+, and L was unaffected. Unlike human fibroblasts, the H4 cells did not require serum to exhibit the depression of System A. At cell densities between 88 × 103 and 180 × 103 cells/cm2, the degree of adaptive regulation was inversely related to cell density. Both transport of AIB and adaptive regulation of System A were nearly abolished if either K+ or Li+ was substituted for Na+ in the medium. The presence of cycloheximide or tunicamycin blocked further increases in starvation-induced activity within 1 hr of addition, suggesting the involvement of a plasma membrane glycoprotein. In contrast, if the medium was supplemented with actinomycin after the stimulation of System A had begun, the activity continued to increase for an additional 2 hr before being slowed by the inhibitor. The contributions of trans-inhibition and repression to the amino acid-induced decay of System A activity were estimated for several representative amino acids. In general, the System A activity in normal rat hepatocytes was much less sensitive to trans-inhibition than the corresponding activity in H4 hepatoma cells. The half-life values for the amino acid-dependent decay of System A ranged from 0.5 to 2.0 hr.
    Additional Material: 7 Ill.
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  • 3
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: The transport of amino acids by normal rat hepatocytes and several hepatoma cell lines has been examined for inactivation by various protein-modifying reagents, including the sulfhydryl-preferring reagents N-ethylmaleimide (NEM) and p-chloromercuribenzene sulfonate (PCMBS). Uptake of 2-aminoisobutyric acid (AIB), a specific probe for hepatic System A-mediated transport, was equally sensitive to inhibition by the organic mercurial PCMBS in each of the cell types tested. In contrast, the sensitivity of System A to inactivation by NEM was substantially different among the five cell types. Normal hepatocytes showed the greatest sensitivity, while the hepatoma cells varied in their responsiveness from moderate to no inhibition. PCMBS inactivated greater than 85% of the System A activity in rat H4 hepatoma cells within 10 min (t1/2 = 3 min). The inhibition by PCMBS was rapidly reversed by treatment of the cells with dithiothreitol. Amino acids showing a high affinity for System A protected the transport system from inactivation, whereas non-substrates produced little or no protection. Amino acid-dependent protection was stereospecific and system-specific. L-norleucine competitively inhibited AIB uptake (Ki = 1.9 ± 0.1 mM) in H4 cells and also protected System A from PCMBS-dependent inactivation (half-maximal protection occurred at an amino acid concentration of 0.6 + 0.1 mM). N-bromosuccinimide was completely ineffective as an inhibitor of System A activity in hepatocytes, whereas treatment of H4 rat hepatoma cells with this reagent resulted in greater than 95% inhibition.
    Additional Material: 7 Ill.
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  • 4
    Publication Date: 1987-05-01
    Print ISSN: 0003-2697
    Electronic ISSN: 1096-0309
    Topics: Biology , Chemistry and Pharmacology
    Published by Elsevier
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