Our current understanding of silicon (Si) availability in agricultural soils is reviewed and knowledge gaps are highlighted. Silicon is a beneficial rather than essential plant nutrient and yield ...responses to its application have been frequently demonstrated in Si‐accumulator crops such as rice and sugarcane. These crops are typically grown on highly weathered (desilicated) soils where soil solution Si concentrations are low. Increased yields are the result of simultaneous increases in plant tolerance to a wide range of biotic (plant pathogens, insect pests) and abiotic (water shortage, excess salts, metal toxicities) stresses. Traditionally, soil solution Si is viewed as being supplied by dissolution of primary and secondary minerals and buffered by adsorption/desorption of silicate onto Al and Fe hydrous oxide surfaces. In recent years it has become recognized that phytogenic cycling of Si uptake of Si by plants, formation of phytogenic silica (SiO2 · nH2O) mainly in leaves and subsequent return of this silica to soils in plant litter is the main determinant of soil solution Si concentrations in natural forests and grasslands. Considerable diminution of the phytogenic Si pool in agricultural soils is likely due to regular removal of Si in harvested products. A range of extractants (unbuffered salts, acetate‐based solutions, and acids) can provide valuable information on the Si status of soils and the likelihood of a yield response in rice and sugarcane. The most common Si fertilizers used are industrial byproducts (e.g., blast furnace slag, steel slag, ferromanganous slag, Ca slag). Since agriculture promotes soil desilication and Si is presently being promoted as a broad spectrum plant prophylactic, the future use of Si in agriculture is likely to increase.
Aspects that require future research include the role of specific adsorption of silicate onto hydrous oxides, the significance of phytogenic Si in agricultural soils, the extent of loss of phytogenic Si due to crop harvest, the role of hydroxyaluminosilicate formation in fertilized soils, and the effect of soil pH on Si availability.
The artemisinin derivatives, dihydroartemisinin (DHA), artesunate, atemether and arteether, are currently used for treatment of malaria in artemisinin combination therapies (ACT) with longer ...half-life drugs. The demand is enormous--in 2005, the estimated global demand for one such ACT alone, artemether-lumifantrine, which constitutes about 70% of all current clinically-used ACTs, is for 120 million adult treatment courses. At 0.5 gm of artemether per total dose regimen, the amount of artemisinin required is approximately 114 tons. This has placed substantial stress on total artemisinin supplies world-wide, and considerable attention is being focussed on enhancing availability of artemisinin by improvement in horticultural practice and extraction of artemisinin from Artemisia annua. Artemisinic acid, which also occurs in A. annua, can be converted into artemisinin and is the ultimate target of a biotechnological approach, which if successful, will augment artemisinin supply in the future. The conversion of artemisinin into the known artemisinin derivatives, and problems with the methods are critically reviewed. Some attention is paid to mechanistic aspects which clarify stereochemistry. The current artemisinins are by no means ideal drugs. Artesunate in particular is incompatible with basic quinolines by virtue of proton transfer, and has intrinsic chemical instability. At pH 1.2, conversion to DHA is rapid, with t(1/2) 26 min, and at pH 7.4, t(1/2) is about 10 hours. With a pK(a) of 4.6, over 99% of artesunate will be ionized at pH 7.4, and thus uptake by passive diffusion from the intestinal tract will be minimal. Although a considerable effort has been vested in the search for new artemisinins, largely through functionalization of artemisinin at C-10, O-11 or at C-15 via artemisitene, or of DHA at C-10, deliberate enhancement of the 'druggability' of artemisinins by reducing lipophilicity, which at the same time will attenuate the neurotoxicity characteristic of the current derivatives, and enhance absorption, by and large has not been considered. A review of the various types of newer derivatives is given together with a consideration of medicinal chemistry aspects.
Controversy surrounds the effects of pH on Si availability in agricultural soils with some workers reporting a negative and others a positive relationship. The factors contributing to this ...contradiction are examined and discussed. Because soil development typically involves both desilication and decreasing pH, when a group of soils from a region with different ages and weathering rates are compared, there is typically a positive relationship between pH and Si extractability. Raising pH by liming can effect Si availability through a number of mechanisms including weathering. Over the normal range of liming (i.e. pH 4.5 to 6.5) increased pH decreases the solubility of crystalline and amorphous aluminosilicates but above about pH 7 their solubility increases. Specific adsorption of monomeric monosilicic acid (the major form found in soil solution) is at a maximum between pH 9 and 10 (pK1 for silicic acid is 9.8). Thus, increasing pH increases the proportion of Si adsorbed and decreases that present in soil solution leading to a negative relationship between pH and soil solution Si. With time, polymerization of monomeric Si can occur at the surface of metal oxide surfaces and this may hinder desorption reactions. Nonetheless, Si is generally highly mobile in soils and easily leached so that in the longer term, it is conceivable that increased adsorption may be a retention mechanism which effectively increases Si availability. Soils contain a biogenic pool of amorphous silica (mainly of plant phytolith origin) and its solubility has been shown to increase greatly with increasing pH from 2 up to pH 12. Thus, liming would increase Si availability in the short-term by increasing dissolution of biogenic Si but in the longer term it would tend to decrease Si availability through diminution of the biogenic pool of Si. The most common Si fertilizers are slag materials which are both sources of Si and liming materials. Thus when soils with different histories of slag application are compared, there is often a positive relationship between pH and Si extractability. The relationship between pH and Si solubility/availability can therefore be positive or negative depending on the situation and which pool(s) of potentially available Si (lithogenic/pedogenic, biogenic, adsorbed or residual fertilizer Si) is the most important source of soluble and available Si.
•Both positive and negative relationships between soil pH and Si extractability are reported.•Increased pH favours Si adsorption onto soil colloids and a reduction in soluble Si.•Positive relationships are seen where a group of soils is compared and in slag-fertilized soils.•Increased pH favours dissolution of biogenic Si and this may increase Si availability.
•Biogenic Si is the major pool of Si through which Si is cycled in soils under natural ecosystems.•For arable crops, residue returns recycle large quantities of phytolith Si back to the biogenic pool ...of soil Si.•Under grazed pastures the bulk of phytolith Si is recycled back to the biogenic soil Si pool.•Use of a soil test to extract the biogenic Si pool (alkali extractant) would improve estimation of potentially available Si in many agricultural soils.
Although the biogenic pool of Si in soils is known to be of central importance to plant uptake and Si cycling in natural forest and grassland ecosystems, its role in agricultural systems is controversial and unclear. The biogenic pool is mainly composed of phytogenic (plant-derived) amorphous silica (deposited in plant shoots as phytoliths) but there are also minor components of zoogenic, microbial and protistic silica. In natural ecosystems the pool of biogenic Si in the soil is typically several orders of magnitude greater than annual plant uptake so slow dissolution of this Si pool supplies the plant with Si (as silicic acid) while litter fall replenishes the pool with newly-formed phytolith Si. However except for grazed pastures, such cycling of Si is much decreased under agriculture because phytolith Si is removed from the field in harvested products and crop residues. For graminaceous crops, which commonly accumulate Si and are Si responsive (e.g. rice and sugarcane) only about 20% of accumulated Si is present in harvested products (e.g. harvested grain or cane) and the remaining 80% is present in harvest residues (straw or cane trash). The extent of phytolith Si removal, and thus rate of diminution of the biogenic pool of soil Si, is therefore greatly dependant on the magnitude of Si uptake by the crop and whether crop residues are retained or removed. Where crop residues are regularly returned to the soil, and for pastoral soils, biogenic Si will remain a significant source of potentially available Si. Thus, in addition to routine soil tests for Si using neutral salt or acidic reagents, an additional broad estimate of biogenic Si (e.g. alkali-soluble Si) is likely to improve evaluation of potentially plant available Si in many agricultural soils.
The effects of increasing pH on the adsorption and extractability of Si in two Si-deficient Australian sugarcane soils was investigated and the effects of increasing rates of fertilizer Si (as blast ...furnace slag) on pH and extractable Si were also examined. Equilibrium studies showed that maximum adsorption of Si by the two soils occurred in the pH range 9–10. When soil pH was increased from 5.0 to 6.5, subsequent adsorption of Si by the two soils, as measured by adsorption isotherms, increased. After incubation with progressive lime additions there was a decline in CaCl2− extractable Si due to its increased adsorption and an increase in acid (H2SO4− and acetic acid)-extractable (mainly adsorbed) Si. The increase in acid extractable Si was greater than the decrease in CaCl2− extractable Si suggesting a supply from an additional source. Alkali (Na2CO3 and Tiron)-extractable Si decreased greatly with increasing pH suggesting dissolution of the amorphous (mainly biogenic) pool of silica was occurring with increasing pH. When increasing rates of slag were incubated with the soils, pH, CaCl2− and acid- extractable Si were all increased because upon dissolution slags release both silicic acid and OH− ions. There was, therefore, a positive relationship between extractable Si and soil pH. However, Na2CO3−and Tiron-extractable Si decreased with increasing slag rates (and increasing soil pH) suggesting dissolution of the biogenic pool of soil Si. It was concluded that future research needs to examine the desorption potential of adsorbed Si and the effects of liming on dissolution of the biogenic pool of soil silica under field conditions.
•Maximum adsorption of Si by soils occurred at pH 9-10.•Lime and slag addition both led to an increase in acid-extractable Si.•Concentrations of alkali (Na2CO3− and Tiron)-extractable Si were increased.•Increased pH increases adsorption Si but decreases that in the biogenic soil pool.•Slag increases pH, CaCl2-and acid- extractable Si.
Lipoprotein (a) Lp(a) and its measurement, structure and function, the impact of ethnicity and environmental factors, epidemiological and genetic associations with vascular disease, and new prospects ...in drug development have been extensively examined throughout this Thematic Review Series on Lp(a). Studies suggest that the kidney has a role in Lp(a) catabolism, and that Lp(a) levels are increased in association with kidney disease only for people with large apo(a) isoforms. By contrast, in those patients with large protein losses, as in the nephrotic syndrome and continuous ambulatory peritoneal dialysis, Lp(a) is increased irrespective of apo(a) isoform size. Such acquired abnormalities can be reversed by kidney transplantation or remission of nephrosis. In this Thematic Review, we focus on the relationship between Lp(a), chronic kidney disease, and risk of cardiovascular events.
Artemisinins: their growing importance in medicine Krishna, Sanjeev; Bustamante, Leyla; Haynes, Richard K ...
Trends in pharmacological sciences (Regular ed.),
10/2008, Letnik:
29, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Artemisinins are derived from extracts of sweet wormwood ( Artemisia annua ) and are well established for the treatment of malaria, including highly drug-resistant strains. Their efficacy also ...extends to phylogenetically unrelated parasitic infections such as schistosomiasis. More recently, they have also shown potent and broad anticancer properties in cell lines and animal models. In this review, we discuss recent advances in defining the role of artemisinins in medicine, with particular focus on their controversial mechanisms of action. This safe and cheap drug class that saves lives at risk from malaria can also have important potential in oncology.
A 32-week leaching column study was carried out in the greenhouse to investigate the effects of incorporation of gypsum, cattle manure, biosolids, gypsum plus cattle manure and gypsum plus biosolids ...into the topsoil layer (0–10 cm) on growth of Rhodes grass, and on root distribution and chemical and microbial properties in the topsoil and subsoil (10–30 cm) layers of seawater neutralized bauxite residues. The columns were leached for a period of 8 weeks prior to sowing Rhodes grass and during that time the bulk of the salts accumulated during seawater neutralization were leached out. The main cation leached was Na+ and the main balancing anions were Cl− and SO42−. During this period the pH of leachates rose from 7 to 8 up to 9–10. At the end of the study, exchangeable Na and pH were lowered in the surface horizon by all treatments with a combination of gypsum plus organic amendments having the greatest effect. The latter treatments also caused a significant decrease in pH, extractable Al and exchangeable Na in the subsoil. Rhodes grass dry matter production followed the order Control < gypsum < cattle manure = gypsum plus cattle manure < biosolids = gypsum plus biosolids. Growth of roots into the subsoil layer was inhibited in the Control and gypsum treatments but when organic amendments were applied, 50% or more of root dry matter was recovered in the subsoil layer. It was concluded that incorporating a combination of gypsum and organic matter into the surface soil is an effective strategy for revegetation of bauxite residue.
•The bulk of accumulated salts were leached over the first 5 leaching events.•The main cation leached was Na+ and the main balancing anions were Cl− and SO42-.•Gypsum plus organic manures decreased pH, extractable Na and Al in the topsoil and subsoil layers.•Root growth into the subsoil layer was greatly promoted by addition of organic manures.•Shoot yields were greater for additions of cattle manure and biosolids than gypsum alone.
Seawater neutralization is a technique that can be used to treat bauxite residue prior to its storage but, as yet, no attempts have been made to revegetate it. Seawater neutralized bauxite residue ...was found to have a pH1:5 of 9.3 and to be highly saline (EC1.5 16.5 dS m−1). After leaching pH1:5 rose to 9.7 and the residue was still highly sodic (ESP = 64–69%). Addition of 1% gypsum, prior to leaching, arrested this increase in pH while with 5% gypsum addition the pH1:5 was lowered to 8.9. Addition of 5% gypsum also reduced ESP to 38% and increased watercress germination in the residue from 58% in control treatments to 88%. The major ions in leachates were Na+ and Cl− and gypsum application increased the quantities of Na+, Ca2+ and SO42− leached. Addition of 6% biosolids or 6% poultry manure added exchangeable cations to the mud and lowered ESP by 5–11%.The EC was 2.8–3.7 (mean 3.1) times higher and pH 0.2–0.7 (mean 0.43) units lower in saturation paste compared with 1:5 soil:water extracts. Addition of amendments had only small effects on physical properties. While organic C content was increased more by biosolids than poultry manure addition the reverse was the case for soluble organic C, microbial biomass C and basal respiration. It was concluded that although seawater neutralization initially lowers the pH of bauxite residues it is unlikely to increase the ease with which they can be revegetated.
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•We studied the properties of seawater neutralized bauxite residue.•Despite partial neutralization it had a high EC, pH, exchangeable Na and ESP.•The pH of the material increased from 9.3 up to 9.7 during leaching.•A rate of 5% gypsum reduced the pH to 8.9 and the ESP to 38% after leaching.•Germination of watercress in residue was increased from 58% to 88% by of 5% gypsum.
Abstract Artemisone is a 10-amino-artemisinin derivative that is markedly superior in vitro and in vivo to current artemisinins against malaria and also possesses antitumor activity. In seeking to ...capitalise on the last property, we have examined the encapsulation of artemisone in nano-vesicular niosomes and solid lipid nanoparticles, and have evaluated efficacies of the free and encapsulated artemisone against human melanoma A-375 cells and effects on human keratinocytes (HaCaT). Artemisone is successfully encapsulated into the nano-vesicles with encapsulation efficiencies of 67 ± 6% and 79 ± 5%, and with average particle sizes being 211 ± 10 nm and 295 ± 18 nm respectively. The formulations displayed highly selective cytotoxicity towards the melanoma cells with negligible toxicity towards the normal skin cells. The artemisone-loaded nano-vesicles almost completely inhibited the melanoma cells compared to the free drug. The results overall suggest a potentially more useful therapeutic strategy that needs to be evaluated for the treatment of melanoma and other cancers. From the Clinical Editor Apart from being an effective anti-malarial drug, a surprising action of artemisone also has antitumor activity. Nonetheless, its low water solubility and bioavailability has limited its clinical use. In this article, the authors enacapsulated artemisone in nano- vesicles and solid lipid nano-particles (SLNs). In-vitro studies confirmed the selective cytotoxicity towards melanoma cells. Further in-vivo and pre-clinical studies are awaited.
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