1,720,975 research outputs found
Inhibition of human pyridoxal kinase by 2-acetyl-4-((1R,2S,3R)-1,2,3,4-1 tetrahydroxybutyl)-imidazole (THI)
No full text2-Acetyl-4-((1R,2S,3R)-1,2,3,4-tetrahydroxybutyl)imidazole (THI) is observed as a minor contaminant in caramel food colourings (E 150c). Feeding experiments with rodents have revealed a significant lymphopenic effect that has been linked to the presence of THI in these food colourings. Pyridoxal kinase inhibition by THI has been suggested, but not demonstrated, as a mode of action as it leads to lowered levels of pyridoxal-5’-phosphate, which are known to cause lymphopenia. Recently, THI was also shown to inhibit sphingosine-1-phosphate lyase causing comparable immunosuppressive effects and derivatives of THI are being developed for the treatment of rheumatoid arthritis in humans. Interestingly, sphingosine-1-phosphate lyase activity depends on pyridoxal-5’-phosphate, which in turn is provided by pyridoxal kinase. This report shows that THI does inhibit pyridoxal kinase with competitive and mixed-type non-competitive behaviour towards its two substrates, pyridoxal and ATP, respectively. The corresponding inhibition constants are in the low millimolar range
Phytochemical analysis of Linaria purpurea (L.) Mill. and inhibitory activity on the production of aflatoxin B1 (AFB1) in Aspergillus flavus Link. of one of its metabolites, antirrhinoside
No full textIn this work, the first phytochemical study on the total polar fraction of Linaria purpurea (L.) Mill. was performed by means of column chromatography and NMR and MS analysis. Seven compounds were identified i.e. pheophytin a (1), methyl-pheophorbide a (2), linaride (3), antirrhide (4), antirrhinoside (5), linarioside (6) and shikimic acid (7), belonging to three different classes of natural compounds. Compound (2) represents a new compound for the family as well as compound (1) is for the genus and compounds (3, 6, 7) are for the species. The chemosystematic and ethnopharmacological relevance of these results were widely discussed. In addition, compound (5) was tested, for the first time, for its potential antifungal activity, showing to drastically reduce the production of aflatoxin B1 in Aspergillus flavus Link. This further property of compound (5) makes it a potential natural and green anti-aflatoxin B1 agent to be used in the food industry
Structural stability of cold-adapted serine hydroxymethyltransferase, a tool for ß-hydroxy-α-amino acid biosynthesis
No full textSerine hydroxymethyltransferase (SHMT) is a pyridoxal-5'-phosphate-dependent enzyme that catalyses the reversible conversion of l-serine and tetrahydropteroylglutamate to glycine and 5,10-methylenetetrahydropteroylglutamate. This enzyme represents a good model for analysing the intricate relationship between activity and stability, since it is ubiquitous in nature and well characterized from different organisms. Besides its physiological role, SHMT catalyses the reversible cleavage of several beta-hydroxy amino acids varying in substituent and stereochemistry at C-beta and, for this reason, it represents a good tool for biotechnological applications. SHMT from the psychrophilic bacterium Psychromonas ingrahamii (piSHMT) displays the interesting feature of having high specific activity in the cleavage of beta-hydroxy amino acids at all temperatures. In the present study, we compare the temperature dependence of psychrophilic piSHMT and mesophilic Escherichia coli SHMT (ecSHMT) in catalysing the physiological hydroxymethyltransferase reaction. We also investigate the structural stability of both enzymes by performing equilibrium unfolding experiments. Unexpectedly, our results show that piSHMT is a less efficient catalyst than ecSHMT in the hydroxymethyltransferase activity at all temperatures. Moreover, the two enzymes have comparable structural stability, with piSHMT showing even higher resistance to chemical denaturation by urea and to inactivation by formaldehyde. This unusual structural stability of piSHMT and its high efficiency at low temperatures as catalyst of beta-hydroxy amino acids cleavage make this enzyme an attractive tool for industrial applications
On the mechanism of Escherichia coli pyridoxal kinase inhibition by pyridoxal and pyridoxal 5'-phosphate
No full textPyridoxal 5'-phosphate (PLP), the catalytically active form of vitamin B6, plays a crucial role in several cellular processes. In most organisms, PLP is recycled from nutrients and degraded B6-enzymes in a salvage pathway that involves pyridoxal kinase (PLK), pyridoxine phosphate oxidase and phosphatase activities. Regulation of the salvage pathway is poorly understood. Escherichia coli possesses two distinct pyridoxal kinases, PLK1, which is the focus of the present work, and PLK2. From previous studies dating back to thirty years ago, pyridoxal (PL) was shown to inhibit E. coli PLK1 forming a covalent link with the enzyme. This inhibition was proposed to play a regulative role in vitamin B6 metabolism, although its details had never been clarified. Recently, we have shown that also PLP produced during PLK1 catalytic cycle acts as an inhibitor, forming a Schiff base with Lys229, without being released in the solvent. The question arises as to which is the actual inhibition mechanism by PL and PLP. In the present work, we demonstrated that also PL binds to Lys229 as a Schiff base. However, the isolated covalent PLK1-PL complex is not inactive but, in the presence of ATP, is able to catalyse the single turnover production of PLP, which binds tightly to the enzyme and is ultimately responsible for its inactivation. The inactivation mechanism mediated by Lys229 may play a physiological role in controlling cellular levels of PLP. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications
Thiamine-dependent regulation of mammalian brain pyridoxal kinase in vitro and in vivo
No full textVitamins B1 (thiamine) and B6 (pyridox(al/ine/amine)) are crucial for CNS function and neurogenesis due to the coenzyme action of their phosphorylated derivatives in the brain metabolism of glucose and neurotransmitters. Here, the non-coenzyme action of thiamine on the major mammalian producers of pyridoxal-5'-phosphate (PLP), such as pyridoxal kinase (PdxK) and pyridoxine 5'-phosphate oxidase (PNPO), is characterized. Among the natural thiamine compounds, thiamine triphosphate (ThTP) is the best effector of recombinant human PdxK (hPdxK) in vitro, inhibiting hPdxK in the presence of Mg2+ , but activating the Zn2+ -dependent reaction. Inhibition of hPdxK by thiamine antagonists decreases from amprolium to pyrithiamine to oxythiamine, highlighting possible dysregulation of both the B1 - and B6 -dependent metabolism in the chemical models of thiamine deficiency. Compared to the canonical hPdxK, the D87H and V128I variants show a 2-fold increase in Kapp of thiamine inhibition, and the V128I and H246Q variants show a 4-fold and a 2-fold decreased Kapp of ThDP, respectively. Thiamine administration changes diurnal regulation of PdxK activity and phosphorylation at Ser213 and Ser285, expression of the PdxK-related circadian kinases / phosphatases in the rat brain, and ECG. In contrast to PdxK, PNPO is not affected by thiamine or its derivatives, either in vitro, or in vivo. Dephosphorylation of the PdxK Ser285, potentially affecting mobility of the ATP-binding loop, inversely correlates with the enzyme activity. Dephosphorylation of the PdxK Ser213, which is far away from the active site, does not correlate with the activity. The correlations analysis suggests the PdxK Ser213 to be a target of kinase MAP2K1 and phosphatase Ppp1ca. Diurnal effects of thiamine administration on the metabolically linked ThDP- and PLP-dependent enzymes may support the brain homeostatic mechanisms and physiological fitness
The potential of plant-based bioactive compounds on inhibition of aflatoxin B1 biosynthesis and down-regulation of aflR, aflM and aflP genes
Full text linkThe use of plant extracts in pre- and post-harvest disease management of agricultural crops to cope with aflatoxin B1 contamination has shown great promise due to their capability in managing toxins and safe-keeping the quality. We investigated the anti-aflatoxigenic effect of multiple doses of eight plant extracts (Heracleum persicum, Peganum harmala, Crocus sativus, Trachyspermum ammi, Rosmarinus officinalis, Anethum graveolens, Berberis vulgaris, Berberis thunbergii) on Aspergillus flavus via LC-MS and the down-regulatory effect of them on aflR, aflM and aflP genes involved in the aflatoxin B1 biosynthesis pathway using RT-qPCR analyses. Our results showed that H. persicum (4 mg/mL), P. harmala (6 mg/mL) and T. ammi (2 mg/mL) completely stopped the production of aflatoxin B1, without inducing significant changes in A. flavus growth. Furthermore, our findings showed a highly significant correlation between the gene expression and the aflatoxin B1 biosynthesis, such that certain doses of the extracts reduced or blocked the expression of the aflR, aflM and aflP and consequently reduced the synthesis of aflatoxin B1. Interestingly, compared to the regulatory gene (aflR), the down-regulation of expression in the structural genes (aflM and aflP) was more consistent and correlated with the inhibition of aflatoxin B1 production. Overall, this study reveals the anti-aflatoxigenic mechanisms of the selected plant extracts at the gene expression level and provides evidence for their use in plant and crop protection
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
- …
