Quelques aspects de la chimie de l'acide ascorbique

Résumé D'après les résultats obtenus in vivo et in vitro concernant l'acide ascorbique, nous essayons d'éclaircir son rôle depuis sa génèse à sa répartition dans les végétaux. L'acide ascorbique est simple d'après sa structure et son action biologique est considérable. In vivo: l'acide ascorbique atteint son taux maximum dans les organes végétaux dans les phases déterminées de chaque période de végétation, mais les valeurs numériques de ce maximum dépendent des conditions écologiques de la période de végétation. La composition chimique des matières non azotées de ces organes est formée, dans la phase initiale de leur croissance et de leur développement, presqu'entièrement des combinaisons labiles comme ène-diols et matières fortement réductrices, et dans la phase finale des saccharides, de cellulose, des pentosanes etc. La formation la plus intense des combinaisons stables se fait dans les phases quand les fruits atteignent leur poids maximum au moment où la matière organique remplace l'eau.In vitro: le pH des solutions aqueuses et tamponnées de l'acide ascorbique et de son produit d'oxydation (l'acide déhydroascorbique) est un des facteurs qui déterminent sa propriété chimique et physique. Ainsi la solution équimoléculaire de ces deux acides, aqueuse et tamponnée, de valeurs différentes du pH se comporte de manière qu'on pourrait supposer, qu'il s'agit de l'interaction entre eux. Ensuite les valeurs du pouvoir rotatoire, calculées et trouvées pour les solutions aqueuses et tamponnées du mélange de ces acides, sont différentes, en dépendance du pH. Le pH du suc des végétaux pourrait être, donc, un des facteurs conditionnant la transformation, la dégradation et l'isomérisation de l'acide ascorbique et de l'acide déhydroascorbique in vivo par les voies différentes.La question se pose, si ce comportement de l'acide ascorbique mène à la synthèse d'autres composés nécessaires pour le développement de la plante.

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Summary On the basis of the results obtained in vivo and in vitro concerning ascorbic acid, we are attempting to clarify its role from its genesis to its distribution in the plants. Ascorbic acid is simple in its structure, and its biological action is considerable.In vivo: the maximum content of ascorbic acid occurs in plant organs in determined phases of each vegetation period, but the numerical values of these maxima are dependent on the ecological conditions of the vegetation period. The chemical composition of the non-nitrogen components of these organs is formed, in the initial phases of their growth and development, almost entirely of labile compounds, such as ene-diols and strongly reducing substances, and in the final phases of saccharides, of cellulose, of pentosans, etc. The most intensive formation of these stable combinations is effected during the phases when the fruits attain their maximum weight at the moment when water is replaced by organic substances. In vitro: the pH of aqueous and buffered solutions of ascorbic acid and of the product of its oxydation (dehydroascorbic acid) is one of the factors that determine its chemical and physical properties. Thus the equimolecular solution, aqueous and buffered, of different pH values of these two acids, behaves as one would suppose when it comes to an interaction between them. Furthermore the rotatory power values, calculated and found for aqueous and buffered solutions of these acids, differ, depending on the pH. The pH of the vegetable sap could then be one of the factors conditioning the transformation, the degradation, and the isomerization in vivo of ascorbic acid and of dehydroascorbic acid by different pathways.The question remains, whether or not this behaviour of ascorbic acid leads to the synthesis of other compounds that are necessary to the development of the plant.

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Zusammenfassung Auf Grund der Ergebnisse von Versuchen in vivo und in vitro, betreffend Askorbin-Säure, probieren wir ihre Rolle zu erklären, vom Anfang ihrer Entstehung bis zu ihrer Verteilung in den Pflanzen. Die Askorbin-Säure ist einfach, ihrer Struktur nach, und ihre biologische Wirkung ist bedeutend.In vivo: Die Maxima der Ascorbinsäure-Gehalte treten in pflanzlichen Organen in bestimmten Phasen jeder Vegetationsperiode auf. Die absolute Höhe ist jedoch von den ökologischen Bedingungen der Wachstumsperiode abhängig. Die stickstofffreien Substanzen dieser Organe setzen sich in der Anfangsphase des Wachstums und während der Entwicklung fast ausschließich aus labilen Verbindungen, wie Endiolen und stark reduzierenden Verbindungen, zusammen und in der Endphase aus Sacchariden, Cellulose, Pentosanen usw. Die stärkste Bildung dieser stabilen Verbindungen erfolgt in der Phase, in der die Früchte bei Ersatz des Wassers durch organische Substanzen ihr höchstes Gewicht erreichen.In vitro: Der pH-Wert der wäßrigen und gepufferten Lösungen von Ascorbinsäure und ihres Oxydationsproduktes — Dehydroascorbinsäure — ist einer der Faktoren, der die chemische Reaktionsfähigkeit bestimmt. So verhält sich die äquimolare wäßrige und gepufferte Lösung dieser beiden Säuren bei verschiedenen pH-Werten so, daß man eine gegenseitige Beeinflussung annehmen kann. Auch die optische Drehung der wäßrigen und gepufferten Lösungen von Mischungen dieser Säuren variiert in Abhängigkeit von pH-Wert. Der pH-Wert des Pflanzensaftes könnte somit einer der die Umwandlung, den Abbau und die Isomerisierung der Ascorbinsäure und der Dehydroascorbinsäure in vivo bedingenden Faktoren sein.

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Gamma-lactone de l'acide 2-céto L(+)-gulonique. Vitamine C.     

THE EFFECT OF NITROGEN APPLICATION AND AUTUMN MANAGEMENT ON AUTUMN GROWTH, WINTER 'BURN' AND SPRING GROWTH OF LOLIUM PERENNE L. AT ABERYSTWYTH, EDINBURGH AND CAMBRIDGE.

A trial was carried out to find management practices which would permit the best discrimination for winter performance of erennial ryegrass cultivars. Due to the mild winter experienced only a few of the plants were killed outright. The cultivars were therefore assessed for percentage of green herbage and for spring growth. There were four sites: an upland and lowland site at Aberystwyth and one site each at Edinburgh and Cambridge. At all sites two N rates (totals of 125 or 550 kg/ha in the first year after sowing) and 5 autumn cutting treatments were used to give differences in the amounts of herbage remaining at the onset of winter. The last dates of defoliation in the autumn were: C₁ end of August; C₂ end of September; C₃ end of October; C₄ mid-November; C₅ was cut on all these dates. Four cultivars (Grasslands Ruanui, S321, Premo and Argo) which differed in their autumn growth potential, frost susceptibility and degree of winter dormancy were grown. The autumn yield of S321 was, in general, higher than that of the other cultivars but there were interactions with N, management and location. The data on percentage of herbage remaining green in February illustrated differences due to the siting of the trial. In the conditions prevailing the two Aberystwyth sites facilitated greater discrimination between cultivars than those at Edinburgh and Cambridge. In three of the four sites the management which produced most winter ‘burn’(including both that due to natural senescence and that due to winter damage) involved accumulation of herbage in the autumn but there were considerable sites × managements interactions. For example at Cambridge management produced no significant effect whereas at the Aberystwyth lowland site frequent defoliation in the autumn had more effect than allowing herbage to remain uncut after the end of August. High N significantly decreased the percentage of green herbage only at the two Aberystwyth sites. Over five-fold differences in spring yield were obtained, the heaviest yields being recorded at Edinburgh and Cambridge. However, there were large interactions between environments and cultivars; for example the higher N rate reduced spring yield at Aberystwyth but increased spring yield at Edinburgh and Cambridge. Premo and S321 had similar yields in spring at the Aberystwyth lowland and Cambridge sites but Premo was higher yielding than S321 at the Aberystwyth upland site and at Edinburgh. The large environmental effects and their interactions with cultivars illustrate the difficulties of cultivar assessment and the dangers inherent in national recommendations for grass cultivars.     

Photosynthetic capacity of red spruce during winter

We measured the photosynthetic capacity (P(max)) of plantation-grown red spruce (Picea rubens Sarg.) during two winter seasons (1993-94 and 1994-95) and monitored field photosynthesis of these trees during one winter (1993-94). We also measured P(max) for mature montane trees from January through May 1995. Changes in P(max) and field photosynthesis closely paralleled seasonal changes in outdoor air temperature. However, during thaw periods, field photosynthesis was closely correlated with multiple-day temperature regimes, whereas P(max) was closely correlated with single-day fluctuations in temperature. There was a strong association between short-term changes in ambient temperature and P(max) during the extended thaw of January 1995. Significant increases in P(max) occurred within two days of the start of this thaw. Repeated measurements of cut shoots kept indoors indicated that temperature-induced increases in P(max) can occur within 3 h. Although significant correlations between P(max) and stomatal conductance (g(s)) or intracellular CO(2) concentration (C(i)) raised the possibility that increases in P(max) resulted from increases in stomatal aperture, fluctuations in g(s) or C(i) explained little of the overall variation in P(max). Following both natural and simulated thaws, P(max) increased considerably but plateaued at only 37% of the mean photosynthetic rate reported for red spruce during the growing season. Thus, even though shoots were provided with near-optimal environmental conditions, and despite thaw-induced changes in physiology, significant limitations to winter photosynthesis remained.     

The impact of phages on interspecific competition in experimental populations of bacteria

Background  

Phages are thought to play a crucial role in the maintenance of diversity in natural bacterial communities. Theory suggests that phages impose density dependent regulation on bacterial populations, preventing competitive dominants from excluding less competitive species. To test this, we constructed experimental communities containing two bacterial species (Pseudomonas fluorescens and Pseudomonas aeruginosa) and their phage parasites. Communities were propagated at two environmental temperatures that reversed the outcome of competition in the absence of phage.     

Treatment with rhBMP-2 of extreme radial bone atrophy secondary to fracture management in an Italian Greyhound

rhBMP-2 solution on a collagen sponge was placed along the diaphysis of an atrophicradius, which had a history of recurring fractures. Two months after rhBMP-2 treatment, new mineralized bone was present, which significantly increased the diameter of the radius and allowed the removal of the external skeletal fixator (ESF). Due to carpo-metacarpal joint compromise, a pancarpal arthrodesis was performed seven months later. At follow-up evaluation two years later the dog was only very mildly lame.     

Adenosine A2A receptor agonists inhibit lipopolysaccharide-induced production of tumor necrosis factor-alpha by equine monocytes

Adenosine is an endogenous nucleoside that regulates many physiological processes by activating one or more adenosine receptor subtypes, namely A1, A2A, A2B and A3. The results of previous studies indicate that adenosine analogues inhibit lipopolysaccharide (LPS)-induced production of reactive oxygen species (ROS) by equine neutrophils primarily through activation of A2A receptors. Because peripheral blood monocytes produce cytokines that are responsible for many of the deleterious effects of LPS, the current study was performed to evaluate the effects of an array of novel adenosine receptor agonists on LPS-induced production of tumor necrosis factor-alpha (TNF-alpha), and to assess the selectively of these agonists for equine adenosine A2A over the A1 receptor. Radioligand binding studies performed with equine tissues expressing adenosine A1 and A2A receptor subtypes yielded a rank order of affinity for the equine A2A receptor of ATL307>ATL309 approximately ATL310 approximately ATL313>ATL202 approximately ATL361 approximately ATL376>ATL372>CGS21680>NECA. Co-incubation of equine peripheral blood monocytes with LPS and these agonists resulted in inhibition of TNF-alpha production with a rank order of potency that strongly correlated with their binding affinities for equine adenosine A2A receptors. Results of experiments performed with one of the adenosine receptor agonists (ATL313) and selective adenosine receptor antagonists confirmed that inhibition of LPS-induced production of TNF-alpha occurred via stimulation of A2A receptors. Although incubation of monocytes with IB-MECA, a compound purported to act as an adenosine A3 receptor agonist, reduced LPS-induced TNF-alpha production, this effect of IB-MECA was inhibited by the A2A selective antagonist ZM241385 but not by the A3 receptor antagonist MRS1220. These results indicate that the adenosine receptor subtype responsible for regulation of LPS-induced cytokine production by equine monocytes is the A2A receptor. To address the signal transduction mechanism responsible for the anti-inflammatory effects of ATL313 in equine monocytes, production of cAMP was compared in the presence and absence of either the adenosine A2A receptor antagonist ZM241385 or the adenosine A2B receptor antagonist MRS1706. In the absence of the antagonists, ATL313 increased production of cAMP; ZM241385 inhibited this effect of ATL313, whereas MRS1706 did not. Furthermore, incubation of monocytes with either the stable analogue of cAMP, dibutyryl cAMP, or forskolin, an activator of adenylyl cyclase, also inhibited LPS-induced production of TNF-alpha production by equine monocytes. Collectively, the results of the current study indicate that adenosine analogues inhibit LPS-induced production of TNF-alpha by equine monocytes primarily via activation of adenosine A2A receptors and do so in a cAMP-dependent manner. The results of this study indicate that stable adenosine analogues that are selective for adenosine A2A receptors may be suitable for development as anti-inflammatory drugs in horses.