Kinetics of soil microbial uptake of free amino acids

 Amino acids and proteins typically form the biggest input of organic-N into most soils and provide a readily available source of C and N for soil microorganisms. Amino acids can also be taken up directly by plant roots, providing an alternative source of available soil N. However, the degree to which plants can compete against the soil microbial population for amino acids in soil solution remains poorly understood. The aim of this study was to measure the rate of microbial uptake of three contrastingly charged ¹⁴C-labelled amino acids (glutamate^1–, glycine⁰, lysine^0.9+) over a wide concentration range (0.1–5 mM) and in two contrastingly managed soils varying in their degree of erosion, organic-C content and microbial biomass. Amino acid uptake was concentration dependent and conformed to a single Michaelis-Menten equation. The mean maximum amino acid uptake rate (V _max) for the non-eroded (control) soil (high organic-C, high biomass) was 0.13±0.02 mmol kg^–1 h^–1, while half maximal uptake occurred at a concentration (K _m) of 2.63±0.07 mM. Typically, V _max was fourfold lower and K _m twofold lower in the eroded soil (low available organic-C, low biomass) compared to the non-eroded (control) soil. Amino acid substrate concentration had little effect on the proportion of amino acid utilized in catabolic versus anabolic metabolism and was similar for both. While the results obtained here represent the summation of kinetics for a mixed soil population, they indicate that amino acid uptake is saturated at concentrations within the millimolar range. Because the affinity constants also were similar to those described for plant roots, we hypothesized that competition for amino acids between plants and microbes will be strong in soil but highly dependent upon the spatial distribution of roots and microbes in soil. Received: 2 March 2000     

土壤中积累态磷活化动力学的研究Ⅰ.有机质的影响

研究了潮土和红壤中积累态磷的活化动力学及其有机质的影响。四种动力学方程均能很好地描述积累态磷的活化。用H2 O2 去除有机质加快了潮土中积累态磷的活化 ,对红壤影响不明显。施加 2 %葡萄糖培育促进了红壤中积累态磷的活化 ,对潮土影响不大     

脉动流化床的空气动力学特性与干燥特性

对小麦、大豆、豌豆种籽进行了一系列干燥试验,通过试验结果对脉动流化床干燥的空气动力学特性和干燥特性进行了分析。提出了干燥速率与干燥介质温度、气流表观速度、床层静止高度、气流脉动频率以及气流分布板开孔率之间的关系,同时也得出了床层压降与床层静止高度的关系。     

l-Asparaginase activity of soils

Summary A simple, precise, and sensitive method to assay l-asparaginase (l-asparagine amidohydrolase, EC 3.5.1.1) activity in soils is described. This method use steam distillation to determine the NH _inf4 ^sup+ produced by l-asparaginase activity when soil is incubated with buffered (0.1 M THAM, pH 10) l-asparagine solution and toluene at 30°C for 2 h. The procedure developed gives quantitative recovery of NH _inf4 ^sup+ -N added to soils and does not cause chemical hydrolysis of l-asparagine. The optimum buffer pH for NH _inf4 ^sup+ -N released by l-asparaginase activity in soils was 10. This enzyme was saturated with 50 mM l-asparagine, and the reaction rate essentially followed zero-order kinetics. The d-isomer of asparagine was also hydrolyzed in soils, but at only 16% of the activity of the l-isomer at a saturating concentration of the substrate. The optimal temperature for the soil l-asparaginase reaction occurred at 60°C and denaturation began at 65°C. The Arrhenius equation plot for l-asparaginase activity in three selected soils was linear between 10 and 50°C. The activation energy values of this enzyme ranged from 20.2 to 34.1 (average 26.6) kJ mol^-1. Application of three linear transformations of the Michaelis-Menten equation showed that the K _m values of l-asparaginase in nine soils ranged from 2.6 to 10.0 (average 6.1) mM and the V _max values ranged from 9 to 131 g NH _inf4 ^sup+ -N released g^-1 soil 2 h^-1. The temperature coefficients (Q ₁₀) for soil l-asparaginase activity ranged from 1.12 to 1.70 (average 1.39). Steam sterilization (121°C for 1 h), formaldehyde, and NaF decreased the activity but the presence of toluene increased the amount of NH _inf4 ^sup+ released. Treatment of soils with dimethylsulfoxide completely destroyed l-asparaginase activity. The use of sulfhydryl reagents indicated that a free sulfhydryl moiety was required to maintain the active enzyme. l-Asparaginase activity in soils was increased by 13 to 18% in the presence of THAM buffer prepared to contain 5 mM Ca²⁺ and Mg²⁺, respectively.     

活化钾矿的钾释放动力学研究

【目的】建立活化钾矿的钾释放动力学模型.【方法】采用活化剂对非水溶性钾矿(钾长石、富钾页岩)进行活化,通过阳离子盐溶液(NH+4、Na+)连续振荡浸提,分析阳离子盐溶液对活化钾矿动态释钾过程的影响,研究活化钾矿的钾释放动力学模型.【结果和结论】各浸提剂浸提钾矿的释放曲线均表现出前期快速,之后缓慢释放的变化趋势.浸提剂浸提钾长石、活化钾长石钾的初始释放量顺序为:NH4ClNa ClH2O,累积释放量顺序为:Na ClNH4Cl≈H2O;富钾页岩、活化页岩的整个动态释钾过程均表现为:H2ONa ClNH4Cl.分别用双常数模型、一级动力学模型、扩散模型和Elovich模型对钾矿及其活化钾矿累积释钾量进行拟合,钾长石、活化钾长石的最优动力学模型是Elovich模型或双常数模型;富钾页岩、活化页岩的最优动力学模型是Elovich模型.钾长石、活化钾长石的一级动力学模型在2种浸提剂NH4Cl和Na Cl间的拟合性无明显差异;NH4Cl浸提的动力学模型拟合性均优于Na Cl.活化钾矿的钾初始释放速率、平均释放速率以及最大平衡释放量均有显著提高.初始释放速率的增大是活化钾矿的钾最大平衡释放量增多的主要原因.     

Flow-cytometric Studies of the Phagocytic Capacities of Equine Neutrophils

Methodological aspects of flow-cytometric evaluation of the phagocytic properties of equine neutrophils were elucidated. The kinetics of attachment and ingestion were studied, and the phagocytic process was more rapidly completed when serum-opsonized yeast cells were used than with use of IgG-opsonized yeast cells. Trypan blue was successfully used to quench fluorescence of non-ingested yeast cells. There were only minor differences in the kinetics of phagocytosis between quenched and un-quenched samples, indicating that attachment is rapidly followed by ingestion. Trypan blue quenching caused loss of cells with light scattering properties of granulocytes, although this did not affect the determined frequencies of truly phagocytic neutrophils. Aggregation of yeast cells proved to be a disturbance but not an obstacle to the determination of frequencies of actively phagocytic cells. Flow cytometry is well suited for studies of phagocytosis of yeast cells by equine neutrophils, and the trypan blue quenching provides a means of eliminating false-positive events due to aggregation of yeast cells. The main advantage of the flow-cytometric method is the possibility of rapid processing of a large number of samples, making the method useful for studies of herds.     

Calcium and magnesium interactions in browning of ‘Golden Delicious’ apples with bitter pit

The role of Ca and Mg in the enzymatic browning of ‘Golden Delicious’ apples was explored. Enzymatic browning due to polyphenol oxidase (PPO) was stimulated with the addition of 0.8–10 mM MgCl₂ to the reaction mixture, whereas 0.8–10 mM CaCl₂ had little effect or decreased the rate of enzymatic browning. Within a week after vacuum infiltration with MgCl₂, treated fruits exhibited browning symptoms that were similar to bitter pit. Six months after vacuum infiltration, when analyzed for enzyme activity, MgCl₂-treated fruits exhibited higher rates of PPO activity, whereas CaCl₂-treated fruits had lower activity than the MgCl₂-treated fruits and the controls. A hypothesis is advanced that the initial visible symptoms of bitter pit, i.e. the localized browning, is caused by PPO, its activity being stimulated by the localized Mg/Ca imbalance present in the tissue. The results indicate that Ca is predominant in the prevention of this disorder, whereas a deficiency of Ca is involved in its induction.     

Investigation of the immediate analgesic effects of extracorporeal shock wave therapy for treatment of navicular disease in horses

OBJECTIVES: To measure the acute analgesic properties of extracorporeal shock wave therapy (ESWT) in horses with navicular disease using objective ground reaction forces (GRF). STUDY DESIGN: Prospective study. ANIMALS: Nine client-owned horses with unilateral forelimb lameness caused by navicular disease. METHODS: Horses were trotted at 3 m/s over a force plate to record pre-treatment GRF of the lame and contra-lateral forelimbs. After ESWT of the lame limb, applied with a radial unit to each of the frog and heel bulb areas with 1500 pulses at 4 bar and 10 Hz, GRF measurements were repeated at 15 minutes and 24-hour intervals for 7 days. From the 5 trials recorded at each time, peak vertical force (PVF) was calculated and averaged. Data were compared between limbs and over time using a 2-way repeated-measures ANOVA. RESULTS: Mean (+/-SD) PVF was significantly different between lame (day 1; 4339+/-626 N) and contra-lateral (day 1; 5236+/-515 N) limbs pre-treatment (P<.05). After ESWT treatment, this difference remained unchanged at 15 minutes and from days 1 to 7. To confirm navicular disease, palmar digital analgesia significantly reduced the lameness (lame 5144+/-430 N; contralateral 5082+/-586 N; P>.05). CONCLUSIONS: Single ESWT treatment applied in this manner does not influence lameness in horses with navicular disease. CLINICAL RELEVANCE: Although many equine regulatory commissions currently ban the use of ESWT before competition, our results suggest that such regulations may be unfounded. Further research is required to determine the long-term analgesic properties of this frequently used treatment modality in equine athletes.     

Isolation and biological properties of virulent subpopulations from lentogenic Newcastle disease virus strains

From each of two lentogenic Newcastle disease virus (NDV) strains of the type LaSota and Hitchner B1 a virulent subpopulation could be obtained. The two subpopulations were—in comparison to the two parent viruses—more resistant to the lipid solvent chloroform and more stable against thermal degradation. Also, the glycoproteins haemagglutinin and F (fusion) were more stable against thermal inactivation. Electron microscopic observations revealed in terms of size and morphology all of the characteristics of NDV. Both subpopulations possessed, however, the same elution kinetics as their respective parent strains. The intracerebral and intravenous pathogenicity indices as well as the mean death times of the two subpopulations allow to classify these viruses as virulent Newcastle disease viruses.     

Fipronil modulation of GABAA receptor single-channel currents

Fipronil is the first phenylpyrazole insecticide introduced for pest control. Although fipronil is known to inhibit GABA receptors, the detailed mechanism of action remains to be seen. In order to elucidate the mechanism of fipronil interaction with the mammalian GABAA system, single-channel patch clamp experiments were performed using rat dorsal root ganglion neurons. The amplitude of main conductance state (27pS) current was not significantly altered by co-application of 10 microM fipronil and 10 microM GABA. The histograms of open time distribution were fitted to a sum of three exponential functions. After application of 10 microM fipronil, the proportion of the fastest component increased slightly and that of the slowest component decreased slightly. Thus, the mean open time was decreased from 11.4 ms to 7.8 ms by fipronil. The histograms of closed time distribution were fitted to a sum of four exponential functions. Fipronil 10 microM prolonged the slowest time constant resulting in a prolongation of the mean closed time from 29.7 ms to 52.8 ms. Thus, the frequency of channel openings was reduced. Thus, the fipronil suppression of GABA-induced whole-cell currents is caused in part by decreases in the channel open time and the frequency of channel openings.