1,720,958 research outputs found
Lipase-mediated production of defensive toxins in the marine mollusc Oxynoe olivacea
No full textMetabolites related to caulerpenyne (1), a toxic sesquiterpene featured by two enol-acetate residues, play a major
role in the chemical defence of both algae of the genus Caulerpa and a few molluscs of the order Sacoglossa. Here
we report the direct evidence that cell-free preparations of Oxynoe olivacea, a Mediterranean sacoglossan, transform
efficiently the algal metabolite 1 to oxytoxin-2 (3), the main defensive metabolite of the mollusc. The process
implies two distinct hydrolytic activities, here named LIP-1 and LIP-2, able to operate either hydrolysis of the acetyl
residue at C-1 or concerted elimination of the acetyl groups at C-4 and C-13. Incubation experiments with tissue
homogenates of O. olivacea or with commercially available lipases suggest a two-step mechanism that involves,
in vitro, an unstable metabolite characterized as preoxytoxin-2 (4). The course of the entire process can be easily
monitored by reverse phase HPLC/ESI-MS, as well as by NMR measurements, which provides direct evidence of
the enzymatic mechanism leading to the formation of this last compound (4). In agreement with the literature, both
fresh and frozen tissues of Mediterranean Caulerpa prolifera also have the capability to transform 1 into aldehydic
derivatives, namely oxytoxin-1 (2) and oxytoxin-2 (3), through hydrolysis of the acetyl groups. However, differently
from experiments with mollusc homogenates, the conversion is not complete and caulerpenyne (1) can be detected in
the algal suspension for a few hours. HPLC/ESI-MS monitoring of this transformation suggests that the hydrolytic
route involves different activities in the mollusc and seaweed
Chemistry of oxylipin pathways in marine diatoms
No full textOxylipins are important signal transduction molecules widely distributed in animals
and plants where they regulate a variety of events associated with physiological and
pathological processes. The family embraces several different metabolites that share a common
origin from the oxygenase-catalyzed oxidation of polyunsaturated fatty acids. The biological
role of these compounds has been especially studied in mammalians and higher
plants, although a varied and very high concentration of these products has also been reported
from marine macroalgae. This article gives a summary of our results concerning the oxylipin
chemistry of marine diatoms, a major class of planktonic microalgae that discourage predation
from their natural grazers, zooplanktonic copepods, using chemical warfare. These apparently
harmless microscopic cells produce a plethora of oxylipins, including short-chain
unsaturated aldehydes, hydroxyl-, keto-, and epoxyhydroxy fatty acid derivatives, that induce
reproductive failure in copepods through abortions, congenital malformations, and reduced
larval growth. The biochemical process involved in the production of these compounds
shows a simple regulation based on decompartmentation and mixing of preexisting enzymes
and requires hydrolysis of chloroplast-derived glycolipids to feed the downstream activities
of C16 and C20 lipoxygenases
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
New C16 fatty-acid-based oxylipin pathway in the marine diatom Thalassiosira rotula
No full textAn unprecedented series of C16 oxylipins (1–8) has been characterized from the marine diatom Thalassiosira rotula.
Absolute stereochemistry of the major alcohols 2 and 3 was determined to be 9S by spectroscopic and chemical
methods. All the described products are formally derived by unprecedented enzymatic oxidation of C16 fatty acids.
Conversion of hexadeca-6,9,12-trienoic acid (C16:3 x-4) into 3 unequivocally established the occurrence of (at least)
a specific 9S-oxygenase activity. To the best of our knowledge, the present data reveal for the first time the existence
of an organic network of oxygenase-mediated transformations that require C16 fatty acids as substrates in living cells
New birth-control aldehydes from the marine diatom Skeletonema costatum: characterization and biogenesis
No full textThe paper describes the identification of short-chain aldehydes (4–7) from the marine diatom Skeletonema costatum,
and their effects on copepod egg viability and sea urchin cell division. Compounds 4–7 were isolated as carboethoxyethylidene
(CET) derivatives and their characterization was performed by NMR and GCMS analyses. Evidence is presented to support the
defensive role and biosynthetic origin of these compounds in S. costatum
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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