Altitudinal analysis of carbon stocks in the Antisana páramo,Ecuadorian Andes

Aims The importance of quantifying carbon stocks in terrestrial ecosystems is crucial for determining climate change dynamics. However, the present regional assessments of carbon stocks in tropical grasslands are extrapolated to unsampled areas with a high degree of uncertainty and without considering the carbon and nitrogen composition of vegetation and soil along altitudinal ranges. This study aims to assess carbon and nitrogen concentrations in soil and vegetation, aboveground carbon stocks distribution and soil organic carbon stocks along an altitudinal range in the páramo region in the Ecuadorian Andes.Methods The vegetation inventory was conducted using 15×15 m sampling plots distributed in three altitudinal ranges. Based on the patterns exhibited by the dominant vegetation growth forms, biomass and soil were sampled to quantify the corresponding carbon and nitrogen concentrations. Subsequently, the aboveground live biomass along the páramo altitudinal range was estimated using allometric equations. Finally, soil and vegetation carbon stocks were estimated for the entire basin.Important findings Altitudinal analysis supported a potential distribution of carbon and nitrogen concentrations in soil, litter and live tissues, where higher concentrations were found in the low altitudinal range mainly for tussocks and acaulescent rosettes. Cellulose in litter showed higher concentrations at low altitudinal ranges for acaulescent rosettes and cushions only. For the same growth forms, lignin patterns in litter were higher in high altitudinal ranges. Soil texture provided complementary information: high percentage of silt was highly correlated to high soil nitrogen and carbon concentration. Tussocks were found to be responsive to altitude with their, highest aboveground carbon stocks occurring at the low altitudinal range, but cushions and acaulescent rosettes responded differently. The established relationships among soil, vegetation and altitude shown in this study must be taken into account to estimate both aboveground and soil organic carbon stocks in páramo regions—such estimates will be considerably inaccurate if these relationships are ignored.     

Oxidative denitrification of 2-nitropropane and propane-2-nitronate by mouse liver microsomes: lack of correlation with hepatocytotoxic potential

2-Nitropropane (2-NP) is an industrial chemical with hepatotoxic and genotoxic properties. It exists in chemical equilibrium with propane-2-nitronate, which is much more genotoxic than 2-NP. In this work the link between toxicity and metabolism of 2-NP and its nitronate was investigated. To that end 2-NP or propane-2-nitronate were incubated with murine hepatic microsomes at concentrations of up to 10 mM, and generation of nitrite was measured as product of metabolic oxidation of the two species. Under the acidic reaction conditions of the colorimetric nitrite assay propane-2-nitronate decomposed chemically to nitrite. Therefore an ion-pair HPLC assay at neutral pH was developed which enabled determination of nitrite formed from the nitronate. The rate of metabolic nitrite generation from propane-2-nitronate was 5-10-fold that obtained with 2-NP. Metabolism of either species to nitrite was dependent on the presence in the incubate of viable microsomes and of NADPH, and it was inhibited in the presence of carbon monoxide or the cytochrome P-450 inhibitor SKF525A. Acetone could also be measured as a metabolite of 2-NP. Optical difference spectra were recorded in mixtures of propane-2-nitronate with liver microsomes from phenobarbital-pretreated rats. The spectral dissociation constant was found to be 30 mM, which compares with 10 mM reported for 2-NP. 2-NP and propane 2-nitronate were incubated with mouse hepatocytes in suspension and cytotoxicity was determined by measurement of leakage of cellular lactate dehydrogenase into the medium. Both species were hardly toxic, as concentrations of 20 mM were required to elicit significant damage to the cells. The results demonstrate that propane-2-nitronate, like 2-NP, undergoes microsomal oxidative denitrification, probably catalysed by cytochrome P-450. Metabolism of both species occurs at markedly different rates, but the difference in metabolism is not reflected by a difference in hepatocytotoxic potential.     

SNAT4 isoform of system A amino acid transporter is expressed in human placenta

The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.     

Metabolism of phaseollin by Septoria nodorum and other non-pathogens of Phaseolus vulgaris

Phaseollin is metabolised by cultures of Septoria nodorum, a non-pathogen of bean, into cis and trans isomers of 12,13-dihydrodihydroxyphaseollin. These products are much less fungitoxic than phaseollin which suggests that the capacity to detoxify phytoalexins is not confined to pathogenic fungi.     

Increased Susceptibility to Streptozotocin-Induced β-Cell Apoptosis and Delayed Autoimmune Diabetes in Alkylpurine- DNA-N-Glycosylase-Deficient Mice

Type 1 diabetes is thought to occur as a result of the loss of insulin-producing pancreatic beta cells by an environmentally triggered autoimmune reaction. In rodent models of diabetes, streptozotocin (STZ), a genotoxic methylating agent that is targeted to the beta cells, is used to trigger the initial cell death. High single doses of STZ cause extensive beta-cell necrosis, while multiple low doses induce limited apoptosis, which elicits an autoimmune reaction that eliminates the remaining cells. We now show that in mice lacking the DNA repair enzyme alkylpurine-DNA-N-glycosylase (APNG), beta-cell necrosis was markedly attenuated after a single dose of STZ. This is most probably due to the reduction in the frequency of base excision repair-induced strand breaks and the consequent activation of poly(ADP-ribose) polymerase (PARP), which results in catastrophic ATP depletion and cell necrosis. Indeed, PARP activity was not induced in APNG(-/-) islet cells following treatment with STZ in vitro. However, 48 h after STZ treatment, there was a peak of apoptosis in the beta cells of APNG(-/-) mice. Apoptosis was not observed in PARP-inhibited APNG(+/+) mice, suggesting that apoptotic pathways are activated in the absence of significant numbers of DNA strand breaks. Interestingly, STZ-treated APNG(-/-) mice succumbed to diabetes 8 months after treatment, in contrast to previous work with PARP inhibitors, where a high incidence of beta-cell tumors was observed. In the multiple-low-dose model, STZ induced diabetes in both APNG(-/-) and APNG(+/+) mice; however, the initial peak of apoptosis was 2.5-fold greater in the APNG(-/-) mice. We conclude that APNG substrates are diabetogenic but by different mechanisms according to the status of APNG activity.     

软计算在生态模型中的应用

由于生态系统的高度复杂性和非线性以及空间数据采集技术的快速发展,近年来越来越多的软计算方法开始应用到生态模拟中来。软计算是个非常广泛的领域,在模式上主要包括元胞自动机、基于个体和盒式模式等;在方法上代表性的有人工神经网络、模糊数学、遗传算法、混沌理论等。重点介绍元胞自动机和规律方法在生态模型中的应用,具体实例包括种群动态模拟、水华预警和生境栖息地模拟。     

FUGE: an analytical ultracentrifuge simulation for teaching purposes

An IBM-compatible microcomputer program for teaching purposesis described which simulates the operation of a sedimentationvelocity determination of a protein in an analytical ultracentrifugeusing schlieren optics. The program operates in speeded-up timeand simulates the major procedures which would need to be carriedout to operate such an instrument. The position of the sedimentingboundary can be observed at any time during the run, and upto six ‘photographs’ can be recorded for subsequentanalysis. Calculation of sedimentation coefficient, diffusioncoefficient and mol. wt can be made from a dot-matrix printout.Ten representative proteins are stored within the program, butprovision exists for user-supplied data. Received on June 25, 1987; accepted on September 9, 1987