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Kinetics of cyanide and carbon monoxide dissociation from ferrous human haptoglobin:hemoglobin(II) complexes
No full textHaptoglobin (Hp) counterbalances the adverse effects of extra-erythrocytic hemoglobin (Hb) trapping the alpha beta dimers of Hb. In turn, the Hp:Hb complexes display heme-based reactivity. Here, the kinetics of cyanide and carbon monoxide dissociation from ferrous-ligated Hp:Hb complexes are reported at pH 7.0 and 20.0 degrees C. Cyanide dissociation from Hp1-1:Hb(II)-CN- and Hp2-2:Hb-CN- has been followed upon the dithionite-mediated conversion of ferric to ferrous-ligated Hp:Hb complexes. Values of k(on) for the dithionite-mediated reduction of Hp1-1:Hb(III)-CN- and Hp2-2:Hb(III)-CN- are (7.3 +/- 1.1) x 10(6) M-1 s(-1) and (6.2 +/- 1.0) x 10(6) M-1 s(-1), respectively. Values of the first-order rate constant (i.e., h) for cyanide dissociation from Hp1-1:Hb(II)-CN- and Hp2-2:Hb(II)-CN- are (1.2 +/- 0.2) x 10(-1) s(-1) and (1.3 +/- 0.2) x 10(-1) s(-1), respectively. CO dissociation from Hp:Hb(II)-CO complexes has been followed by replacing CO with NO. Values of the first-order rate constant (i.e., l) for CO dissociation from Hp1-1:Hb(II)-CO are (1.4 +/- 0.2) x 10(-2) s(-1) and (6.2 +/- 0.8) x 10(-3) s(-1), and those from Hp2-2:Hb(II)-CO are (1.3 +/- 0.2) x 10(-2) s(-1) and (7.3 +/- 0.9) x 10(-3) s(-1). Values of k(on), h, and l correspond to those reported for the R-state of tetrameric Hb and isolated alpha and beta chains. This highlights the view that the conformation of the Hb alpha beta-dimers bound to Hp1-1 and Hp2-2 matches that of the R-state of the Hb tetramer. Furthermore, unlike ferric Hb(III), ligated ferrous Hb(II) does not show an assembly-linked structural change. Graphic abstrac
Ferric nitrosylated myoglobin catalyzes peroxynitrite scavenging
No full textMyoglobin (Mb), generally taken as the molecular model of monomeric globular heme-proteins, is devoted: (i) to act as an intracellular oxygen reservoir, (ii) to transport oxygen from the sarcolemma to the mitochondria of vertebrate heart and red muscle cells, and (iii) to act as a scavenger of nitrogen and oxygen reactive species protecting mitochondrial respiration. Here, the first evidence of (NO)-N-center dot inhibition of ferric Mb- (Mb(III)) mediated detoxification of peroxynitrite is reported, at pH 7.2 and 20.0 degrees C. (NO)-N-center dot binds to Mb(III) with a simple equilibrium; the value of the second-order rate constant for Mb(III) nitrosylation (i.e., (center dot NO)k(on)) is (6.8 +/- 0.7) x 10(4) M-1 s(-1) and the value of the first-order rate constant for Mb(III)-NO denitrosylation (i.e., (center dot NO)k(off)) is 3.1 +/- 0.3 s(-1). The calculated value of the dissociation equilibrium constant for Mb(III)-NO complex formation (i.e., (center dot NO)k(off)/(center dot NO)k(on) = (4.6 +/- 0.7) x 10(-5) M) is virtually the same as that directly measured (i.e., K-center dot NO = (3.8 +/- 0.5) x 10(-5) M). In the absence of (NO)-N-center dot, Mb(III) catalyzes the conversion of peroxynitrite to NO3-, the value of the second-order rate constant (i.e., (P)k(on)) being (1.9 +/- 0.2) x 10(4) M-1 s(-1). However, in the presence of (NO)-N-center dot, Mb(III)-mediated detoxification of peroxynitrite is only partially inhibited, underlying the possibility that also Mb(III)-NO is able to catalyze the peroxynitrite isomerization, though with a reduced rate ((P)k(on)* = (2.8 +/- 0.3) x 10(3) M-1 s(-1)). These data expand the multiple roles of (NO)-N-center dot in modulating heme-protein actions, envisaging a delicate balancing between peroxynitrite and (NO)-N-center dot, which is modulated through the relative amount of Mb(III) and Mb(III)-NO. Graphic abstrac
Hydroxylamine-induced oxidation of ferrous nitrobindins
No full textHemoglobin and myoglobin are generally taken as molecular models of all-α-helical heme-proteins. On the other hand, nitrophorins and nitrobindins (Nb), which are arranged in 8 and 10 β-strands, respectively, represent the molecular models of all-β-barrel heme-proteins. Here, kinetics of the hydroxylamine- (HA-) mediated oxidation of ferrous Mycobacterium tuberculosis, Arabidopsis thaliana, and Homo sapiens nitrobindins (Mt-Nb(II), At-Nb(II), and Hs-Nb(II), respectively), at pH 7.0 and 20.0 °C, are reported. Of note, HA displays antibacterial properties and is a good candidate for the treatment and/or prevention of reactive nitrogen species- (RNS-) linked aging-related pathologies, such as macular degeneration. Under anaerobic conditions, mixing the Mt-Nb(II), At-Nb(II), and Hs-Nb(II) solutions with the HA solutions brings about absorbance spectral changes reflecting the formation of the ferric derivative (i.e., Mt-Nb(III), At-Nb(III), and Hs-Nb(III), respectively). Values of the second order rate constant for the HA-mediated oxidation of Mt-Nb(II), At-Nb(II), and Hs-Nb(II) are 1.1 × 104 M-1 s-1, 6.5 × 104 M-1 s-1, and 2.2 × 104 M-1 s-1, respectively. Moreover, the HA:Nb(II) stoichiometry is 1:2 as reported for ferrous deoxygenated and carbonylated all-α-helical heme-proteins. A comparative look of the HA reduction kinetics by several ferrous heme-proteins suggests that an important role might be played by residues (such as His or Tyr) in the proximity of the heme-Fe atom either coordinating it or not. In this respect, Nbs seem to exploit somewhat different structural aspects, indicating that redox mechanisms for the heme-Fe(II)-to-heme-Fe(III) conversion might differ between all-α-helical and all-β-barrel heme-proteins
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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