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  • 1
    Electronic Resource
    Electronic Resource
    Land use change and carbon exchange in the tropics: III. Structure, basic equations, and sensitivity analysis of the model (1985)
    Springer
    Environmental management 9 (1985), S. 345-353 
    Details
    ISSN: 1432-1009
    Keywords: Carbon exchange ; Computer calculation ; Land use change ; Sensitivity analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract The rationale, assumptions, structure and basic mathematical functions of the model used to produce the simulation results reported in the first two articles of this series are described in detail. Sensitivity analysis indicates that the most important parameters in the model, and, presumably, in the carbon exchange between tropical forests and the atmosphere, are: (a) the conversion rate of forests to permanent pasture and agriculture, (b) the changes that are occurring and have occurred in the shifting cultivation system, and (c) the fate of cleared vegetation. Although it is not possible to validate the model against direct measurements of carbon exchange, the model has been proven robust when subject to a series of explicit analyses and comparisons with other assessments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01867305
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  • 2
    Electronic Resource
    Electronic Resource
    Land use change and carbon exchange in the tropics: II. Estimates for the entire region (1985)
    Springer
    Environmental management 9 (1985), S. 335-344 
    Details
    ISSN: 1432-1009
    Keywords: Carbon exchange ; Land use change ; Tropical countries
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract Determining the effect of tropical land use on the carbon dioxide (CO2) content of the atmosphere requires: (a) estimates of the rates of land use change, (b) estimates of the difference between the carbon stored in forests and that stored in pastures and cultivated fields, and (c) a consideration of the fate of carbon stored in the cleared vegetation. The first article of this series analyzed land use in four tropical countries and estimated the carbon released to the atmosphere as a consequence of changes in land use. This article estimates the carbon released from the entire tropical region based on the two published studies of land use change for the tropics as a whole that distinguish between temporary and permanent land use: Seiler and Crutzen (1980) and Lanly (1982). We combine these estimates with two estimates of the difference in carbon storage between forests and fields derived from Whittaker and Likens (1975) and Brown and Lugo (1982), and the two scenarios of the fate of cleared vegetation, developed in the previous article, to produce several complete sets of data describing the necessary parameters to calculate carbon exchange. These data sets, entered into our model, produce a range of estimates of the annual release of carbon from tropical vegetation in 1980 of from 0.6 to 1.8 BMT/year, with the more likely range being 0.9–1.2 BMT/year. Our preliminary analysis suggests that the release from tropical soils due to land use change adds about an additional 0.3 BMT C/year, so that the total release is probably between 1.2 and 1.5 BMT C/year. Peng and others (1983) reported that new models of the oceanic carbon cycle can accommodate at least 1.2 BMT C/year in 1980 from forests and soils. Our results indicate that, given the uncertainties in the size of both the biotic release and oceanic uptake, the global carbon budget may be balanced if there is no significant release from nontropical ecosystems due to land use change and all mature ecosystems are in collective equilibrium with the atmosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01867304
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  • 3
    Electronic Resource
    Electronic Resource
    The metabolism of cobalamin bound to transcobalamin II and to glycoproteins that bind Cbl in HepG2 cells (human hepatoma) (1985)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 124 (1985), S. 507-515 
    Details
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: The binding, internalization, processing and release of labeled cyanocobalamin (CN[57Co]Cbl) bound to human transcobalamin II (TC II) were studied in HepG2 cells, a line of hepatocytes derived from a human hepatoma. The cells bound the TC II-Cbl by specific, high affinity receptors. Within the cell, the CN-Cbl was promptly freed from TC II and the CN-Cbl converted to more active forms including adenosyl Cbl (AdoCbl) and methyl Cbl (MeCbl). Whereas free labeled Cbl was still present at 72 hours after entry, the cells also bound Cbl to an intracellular binder (ICB) presumed to represent the holo enzymes dependent on Cbl. At levels of TC II that saturated the receptors for TC II-Cbl, much of the Cbl entering the cells remained free and was converted to AdoCbl. Under these circumstances the cells released free Cbl, mostly AdoCbl.Human R type binders of Cbl, which are glycoproteins and some having a terminal galactose, were bound by the HepG2 cells. The binding was characteristic of the receptor system responsive to a terminal galactose, or asialoglycoproteins, but was inconsistent and of low affinity. Cbl bound to R binder was internalized and converted to coenzyme forms of Cbl, but the process was much less effective than when the Cbl entered via the TC II receptor system. It was concluded that the receptors for R-Cbl were unlikely to contribute to the physiologic transport of Cbl in man, but may function in some yet unknown way.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcp.1041240322
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  • 4
    Electronic Resource
    Electronic Resource
    Cyclic activity of the receptors of cobalamin bound to transcobalamin II (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 133 (1987), S. 187-191 
    Details
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: The activity of receptors specific for human transcobalamin II-Cobalamin (TC II-Cbl) were measured in virus-transformed lymphoblasts, hepatocytes (hepatoma) and diploid fibrolasts. In all three types of human cells the receptor activity increased as cells went from a resting phase to the most actively dividing phase. Receptor activity declined as cell division slowed. The changes in activity of lymphoblasts and hepatocytes were produced by changes in receptor numbe and not by changes in affinity between receptors and TC II-Cbl. The basis of the change in fibroblasts was not clear. The Cbl-dependent methionine synthetase activity of fibroblasts, in contrast, tended to be greatest when the cultures were confluent and replication had slowed. As the fibroblasts became senescent the receptor activity for TC II-Cbl declined and the fluctuations with the phase of the cell were blunted. However, the release of apo TC II from the cells was maintained. These observations must be taken into consideration when the respective cells are used as models. Even more important are the implications of the observations of the changes in receptor activity for TC II-Cbl for the regulation of the entry of Cbl into cells.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcp.1041330125
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  • 5
    Electronic Resource
    Electronic Resource
    Cobalamin metabolism in cultured human chorionic villus cells (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 156 (1993), S. 43-47 
    Details
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: Cobalamin (Cbl, vitamin B12) metabolism was analyzed in cultures of human chorionic villus (CV) cells obtained at 9-10 weeks of gestation. CV cells were shown to synthesize transcobalamin II (TCII) and to possess a high affinity receptor for that molecule. The cells bound and internalized radioactive cyanocobalamin (CN[57Co]Cbl) complexed to TCII. This internalized CN[57Co]Cbl was found to be converted to both methylCbl and adenosylCbl, the two intracellular coenzyme forms of Cbl, and bound to the two known intracellular Cbl requiring enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase. Both enzyme systems were found to be functional in the intact cell by demonstrating the incorporation of the radioactive label from both [14C]CH3-tetrahydrofolate and [14C]propionate into acid insoluble products. MS activity was also detected in lysed cell material. CV cells were shown not to be auxotrophic for methionine since they were able to utilize homocysteine in place of methionine for cell division. Since CV cells are capable of performing many of the complex events associated with Cbl metabolism, it may be possible to use these cells to diagnose genetic defects of Cbl metabolism. © 1993 Wiley-Liss, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcp.1041560107
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