Nucleosides are efficiently absorbed by Na(+)-dependent transport across the intestinal brush border membrane in veal calves

In previous work, a comparatively high capacity for Na(+)-dependent transport of nucleosides across the intestinal brush border membrane (BBM) was observed in dairy cows, which might be related to digestion of the large amount of nucleic acids present in ruminal microorganisms in the ruminant small intestine. If this were the case, the capacity for Na(+)-dependent intestinal nucleoside transport should be much lower in veal calves, in which only small amounts of nucleic acids, nucleotides, and nucleosides reach the small intestine via the milk replacer. To test this hypothesis, we investigated Na(+)-dependent transport of 3H-labeled thymidine and guanosine across the BBM using BBM vesicles (BBMV) isolated from the small intestine of veal calves. In the presence of a transmembrane Na+ gradient both substrates were transported against a concentration gradient. Inhibitory studies showed that thymidine and guanosine are transported by two different transporters with overlapping substrate specificity, one accepting predominantly pyrimidine nucleosides (N2) and one accepting particularly purine nucleosides (N1). Nucleoside transport was inhibited by glucose along the whole small intestine. Maximal transport rates similar to those in dairy cows were obtained for the proximal, mid-, and distal small intestine. These findings suggest that the high absorptive capacity for nucleosides is a genetically fixed property in the bovine small intestine, which is already present in the preruminant state of veal calves. It may contribute to the high digestibility of nucleic acids observed by others in veal calves receiving milk replacer supplemented with RNA. Its main function may be the efficient absorption of nucleosides resulting from the digestion of nucleic acids associated with desquamated enterocytes. Due to the limited de novo synthesis of nucleotides in enterocytes intracellular uptake of nucleosides across the BBM may contribute to nucleic acid synthesis in enterocytes and thus may have a trophic effect on the intestinal epithelium.     

Allelic isoforms of the H+/nucleoside co-transporter (CaCNT) from Candida albicans reveal separate high- and low-affinity transport systems for nucleosides

Contigs 19-10196 and 19-20196 of the Stanford Candida albicans genome sequence databank encode two putative allelic isoforms of C. albicans CaCNT, a recently characterized 608 amino acid residue H+-coupled fungal member of the CNT family of concentrative nucleoside transport proteins. The single Ser/Gly difference between CaCNT/19-20196 and CaCNT occurs at position 328 in putative TM 7, and corresponds to a Ser/Gly substitution previously shown to contribute to the contrasting pyrimidine and purine nucleoside selectivities of human (h) and rat (r) Na+-dependent CNT1 and CNT2. CaCNT/19-10196 differs from CaCNT by four amino acids, but has Gly at position 328. These new proteins were recreated by site-directed mutagenesis of CaCNT and characterized functionally by heterologous expression in Xenopus laevis oocytes. In marked contrast to h/rCNT1/2, both CaCNT/19-10196 and CaCNT/19-20196 exhibited permeant selectivities for purine nucleosides (adenosine, guanosine and inosine) and uridine similar to that of CaCNT. However, although H+-coupled, CaCNT/19-20196 exhibited a approximately 10-fold higher apparent Km for uridine than either CaCNT or CaCNT/19-10196. CaCNT/19-20196 also exhibited a low apparent affinity for inosine. We conclude that the three proteins correspond to high-affinity (CaCNT, CaCNT/19-10196) and low-affinity (CaCNT/19-20196) allelic isoforms of the C. albicans CNT nucleoside transporter. This is the first example of a single amino acid residue substitution altering a CNT protein's overall apparent affinity for nucleosides.     

Functional characterization of a H+/nucleoside co-transporter (CaCNT) from Candida albicans,a fungal member of the concentrative nucleoside transporter (CNT) family of membrane proteins

Human and other mammalian concentrative (Na(+)-linked) nucleoside transport proteins belong to a membrane protein family (CNT, TC 2.A.41) that also includes Escherichia coli H(+)-dependent nucleoside transport protein NupC. Here, we report the cDNA cloning and functional characterization of a CNT family member from the pathogenic yeast Candida albicans. This 608 amino acid residue H(+)/nucleoside symporter, designated CaCNT, contains 13 predicted transmembrane domains (TMs), but lacks the exofacial, glycosylated carboxyl-terminus of its mammalian counterparts. When produced in Xenopus oocytes, CaCNT exhibited transport activity for adenosine, uridine, inosine and guanosine but not cytidine, thymidine or the nucleobase hypoxanthine. Apparent K(m) values were in the range 16-64 micro M, with V(max) : K(m) ratios of 0.58-1.31. CaCNT also accepted purine and uridine analogue nucleoside drugs as permeants, including cordycepin (3'-deoxyadenosine), a nucleoside analogue with anti-fungal activity. Electrophysiological measurements under voltage clamp conditions gave a H(+) to [(14)C]uridine coupling ratio of 1 : 1. CaCNT, obtained from logarithmically growing cells, is the first described cation-coupled nucleoside transporter in yeast, and the first member of the CNT family of proteins to be characterized from a unicellular eukaryotic organism.     

A SIMPLE METHOD FOR THE ISOLATION OF PURINE NUCLEOSIDES FROM WORT AND BEER USING COLUMN CHROMATOGRAPHY

Column chromatography using a variety of dextran based gel matrices was used to fractionate nucleosides in wort and beer. The gel matrices Sephadex G10 and Sephasorb HP Ultrafine were found to elute guanosine, deoxyguanosine, adenosine and deoxyadenosine in a single fraction whereas Sephadex LH20 and Sephadex G25 both yielded two fractions containing nucleosides. The first nucleoside containing fraction from Sephadex LH20 contained guanosine and the second fraction contained deoxyguanosine, adenosine and deoxyadenosine. In contract the first nucleoside containing fraction from Sephadex G25 contained both adenosine and deoxyadenosine and the second fraction contained guanosine and deoxyguanosine. Fractionation of low molecular weight components from wort and beer by column chromatography provides a simple technique for the isolation of purine nucleosides. This may be used to monitor directly the presence of purine nucleosides throughout the brewing process and to obtain quantitative estimates of the content of purine nucleosides in beer.     

细脚拟青霉深层发酵菌丝体核苷类成分分析

以已知核苷为标样,并以冬虫夏草生药作对比,利用反相高效液相色谱法对细脚拟青霉RCEF0441 深层发酵菌丝体中核苷类化学成分进行分析。结果表明,细脚拟青霉RCEF0441 菌丝体中主要含有胞苷、尿苷、鸟苷、肌苷、胸苷和腺苷6 种核苷,其中胞苷的含量为冬虫夏草生药的3 倍左右,肌苷和鸟苷的含量则分别是冬虫夏草生药的7 和9 倍左右,尿苷和腺苷的含量也显著高于对照品冬虫夏草生药(p<0.05)。     

基于逐步回归分析何首乌中核苷类成分

分析何首乌中10 种核苷类成分,探讨10 种核苷类成分含量对总核苷含量的影响程度。采用超高效液相色谱-串联四极杆-线性离子阱质谱技术测定何首乌中10 种核苷类成分的含量,应用逐步回归分析方法得到10 种核苷类成分含量对总核苷含量影响程度的最优方程。结果显示何首乌核苷类成分中以尿苷、腺嘌呤、鸟苷、胞苷含量较高;逐步回归分析显示尿苷、胞苷和胸苷含量对总核苷含量的影响最为显著。尿苷、胞苷和胸苷含量可标示总核苷作为何首乌药材质量评价的一项指标,为何首乌药材质量的多指标评价体系构建提供科学依据。     

蜂头虫草无性型——蜂头层束梗孢活性成分分析

测定了蜂头虫草无性型——蜂头层束梗孢(Hymenostilbe sphecophila Kob)HS01胞外多糖含量、5种核替代谢量和氨基酸含量。结果表明,胞外多糖含量最高可达1.75 g/L,胞内多糖含量最高可达2.423%;发酵液与菌丝体中均含有尿苷、鸟苷、肌苷、胸腺嘧啶核苷和腺苷5种核苷,其含量均高于对照品冬虫夏草生药;HS01发酵菌丝体与天然虫草生药在氨基酸种类上一致,但氨基酸总量和人体所必须氨基酸总量明显高于冬虫夏草生药,氨基酸总量为8.330 mg/g。     

Determination of individual purine and pyrimidine bases in carbohydrate-rich food

The following method was developed for the qualitative and quantitative determination of purine and pyrimidine bases in carbohydrate rich food: The bases were liberated from nucleic acids, nucleotides or nucleosides by acid hydrolysis in a pressure digestion vessel. A complete liberation without losses of purine bases occurs upon hydrolysis for 15 min at 240 degrees C with trifluoroacetic and formic acids (1+1; V + V), pyrimidine bases need 45 min at 240 degrees C. The products arising from side reactions (such as hydroxymethylfurfural from hexoses and furfural from pentoses) could be removed from the hydrolysate by extraction with dichlormethane. The liberated bases could be separated upon stepwise elution by cation exchange chromatography. They were detected and determined by UV-measurements, continuously monitoring at lambda = 260 nm, and integrating electronically. The evaluation was carried out by a method with internal standard.     

Growth of Lactobacillus bulgaricus in milk. 2. Characteristics of purine nucleotides, pyrimidine nucleotides, and nucleic acid synthesis

Lactobacillus bulgaricus incorporated exogenous guanine only into guanine nucleotides and not into adenine nucleotides. Lactobacillus bulgaricus lacks the ability to interconvert adenine nucleotides and guanine nucleotides. Biosynthesis of pyrimidine nucleotides from orotic acid was depressed by addition of guanosine monophosphate or adenosine monophosphate. Some step in pyrimidine biosynthesis from orotic acid may be negatively regulated by the intracellular amount of purine nucleotides. Lactobacillus bulgaricus could not grow in milk from which orotate was removed. This indicates that the activity of de novo pyrimidine synthesis is very low in this organism. When the intracellular amount of purine or pyrimidine base was limited, ribonucleic acid synthesis was markedly depressed, resulting in cell elongation, whereas deoxyribonucleic acid synthesis was not so much affected.     

Human NKCC2 cation–Cl– co‐transporter complements lack of Vhc1 transporter in yeast vacuolar membranes

Cation–chloride co‐transporters serve to transport Cl^– and alkali metal cations. Whereas a large family of these exists in higher eukaryotes, yeasts only possess one cation–chloride co‐transporter, Vhc1, localized to the vacuolar membrane. In this study, the human cation–chloride co‐transporter NKCC2 complemented the phenotype of VHC1 deletion in Saccharomyces cerevisiae and its activity controlled the growth of salt‐sensitive yeast cells in the presence of high KCl, NaCl and LiCl. A S. cerevisiae mutant lacking plasma‐membrane alkali–metal cation exporters Nha1 and Ena1‐5 and the vacuolar cation–chloride co‐transporter Vhc1 is highly sensitive to increased concentrations of alkali–metal cations, and it proved to be a suitable model for characterizing the substrate specificity and transport activity of human wild‐type and mutated cation–chloride co‐transporters. Copyright © 2013 John Wiley & Sons, Ltd.     

Cloning of the Candida albicans nucleoside transporter by complementation of nucleoside transport-deficient Saccharomyces

The nucleoside permease gene (i.e. NUP) from Candida albicans was cloned by complementation of Saccharomyces cerevisiae deficient in nucleoside transport capability. The permease transported adenosine and guanosine and was sensitive to the mammalian nucleoside transport inhibitors: dipyridamole and NBMPR. It did not transport uridine, cytidine, adenine, guanine or uracil. The inability to transport uridine indicated that the NUP gene product was different from the Candida uridine permease, which also transported cytosine and adenosine. The NUP gene coded for a protein of 407 amino acids in size which was approximately the size of the human, Giardia and E. coli nucleoside permeases. It did not, however, exhibit any significant degree of homology with these transporters. The GenBank accession number for the Candida NUP gene is AF016246. © 1998 John Wiley & Sons, Ltd.     

Human placental amino acid transporter genes: expression and function

Recent findings on amino acid transporter genes are reviewed with particular focus on matching previously described transport systems to individual genes. Functional studies using cloned and expressed transporters are considered as the critical tool allowing identification of the functional properties of individual genes. Specifically, these experiments allow identification of the transported substrate amino acids and of the transport mechanism. We focus on the very recent discovery and properties of the heterodimeric family of amino acid transport proteins where two subunits encoded in different genes are required. For these transporters, co-expression of both subunits is mandatory for functional studies. The field of placental amino acid transport is further complicated by complexities arising from both gestational age-specific and species-specific gene expression. The function of the transporter also depends on its cellular localization in the trophoblast. In addition, for transporters that are coupled to ion gradients, both membrane potential and ion pumping will contribute to the rate of amino acid delivery to the fetus. Regulation of function is important not only for fetal nutrition but also for specific additional aspects of placental biology.     

Characterization of bovine glucose transporter 1 kinetics and substrate specificities in Xenopus oocytes

Glucose is an essential substrate for lactose synthesis and an important energy source in milk production. Glucose uptake in the mammary gland, therefore, plays a critical role in milk synthesis. Facilitative glucose transporters (GLUT) mediate glucose uptake in the mammary gland. Glucose transporter 1 (GLUT1) is the major facilitative glucose transporter expressed in the bovine mammary gland and has been shown to localize to the basolateral membrane of mammary epithelial cells. Glucose transporter 1 is, therefore, thought to play a major role in glucose uptake during lactation. The objective of this study was to determine the transport kinetic properties and substrate specificity of bovine GLUT1 using the Xenopus oocyte model. Bovine GLUT1 (bGLUT1) was expressed in Xenopus oocytes by microinjection of in vitro transcribed cRNA and was found to be localized to the plasma membrane, which resulted in increased glucose uptake. This bGLUT1-mediated glucose uptake was dramatically inhibited by specific facilitative glucose transport inhibitors, cytochalasin B, and phloretin. Kinetic analysis of bovine and human GLUT1 was conducted under zero-trans conditions using radio-labeled 2-deoxy-D-glucose and the principles of Michaelis-Menten kinetics. Bovine GLUT1 exhibited a Michaelis constant (K(m)) of 9.8 ± 3.0mM for 2-deoxy-d-glucose, similar to 11.7 ± 3.7 mM for human GLUT1. Transport by bGLUT1 was inhibited by mannose and galactose, but not fructose, indicating that bGLUT1 may also be able to transport mannose and galactose. Our data provides functional insight into the transport properties of bGLUT1 in taking up glucose across mammary epithelial cells for milk synthesis.     

鸡桑、华桑和桑的根皮中7种核苷类成分的含量测定

目的:采用高效液相色谱法（HPLC）对不同产地鸡桑、华桑和桑的根皮中腺嘌呤、腺苷、胞苷、次黄嘌呤、鸟苷、尿嘧啶和2'-脱氧腺苷等7种核苷进行测定与分析,为制订质量标准提供参考。方法:采用Venusil MP C₁₈（5 μm,250 mm×4.6 mm）色谱柱,以甲醇和水为流动相,梯度洗脱,检测波长为260 nm,流速为1.0 mL/min,柱温30 ℃。结果:7种核苷成分在各自质量浓度范围内线性关系良好（r≥0.9998）,平均回收率为97.49%~101.45%,RSD≤3%。不同产地鸡桑、华桑和桑的根皮中7种核苷的含量差异较大,其中华桑与鸡桑的根皮中尿嘧啶和次黄嘌呤含量整体较高,而不同样品次黄嘌呤含量差异性较大,胞苷含量最低。桑的根皮中鸟苷和腺嘌呤含量较高,2'-脱氧腺苷含量较低。鸡桑与华桑中核苷成分略低于桑。独立样本t检验结果表明,鸡桑、华桑和桑的根皮中7种核苷类成分无显著性差异（P>0.05）,其核苷类成分具有一定的等同性。主成分分析和聚类分析结果表明,鸡桑、华桑和桑不能依据核苷类成分含量的差异加以区分,表明以华桑和鸡桑来源的崖桑皮可作为桑白皮的代替使用。结论:鸡桑、华桑和桑的根皮中核苷类成分含量无明显差异,本研究为鸡桑、华桑和桑质量等同性的研究提供了科学依据。     

New vectors for simplified construction of BrdU-Incorporating strains of Saccharomyces cerevisiae

The thymidine analogue BrdU is a powerful tool for analysing nucleotide incorporation in studies of DNA replication or repair. S. cerevisiae lacks the thymidine salvage pathway that enables efficient cellular uptake and incorporation of thymidine analogues into DNA. Recent in vivo reconstitution of this pathway in yeast by high-level expression of Herpes simplex virus thymidine kinase (HSV-TK) or combined expression of HSV-TK and human equilibrative nucleoside transporter (hENT1) has enabled analysis of BrdU incorporation in yeast. While the BrdU incorporation systems are highly valuable, the construction and use of strains utilizing these systems can be complicated by specific requirements of the available systems. We have created a set of vectors that simplify the construction and use of BrdU-Incorporating (BrdU-Inc) strains of budding yeast. Each vector in the set contains HSV-TK and hENT1 under the control of promoters that express constitutively, and one of four different selectable markers (HIS3, TRP1, LEU2 or URA3) for genomic integration. With these BrdU-Inc vectors, one-step integration of a single copy produces yeast that efficiently incorporate BrdU upon its addition to the medium. These vectors ease strain construction and maintenance, thereby facilitating routine use of BrdU for analysis in yeast.     

Novel transporters from Kluyveromyces marxianus and Pichia guilliermondii expressed in Saccharomyces cerevisiae enable growth on l‐arabinose and d‐xylose

Genes encoding l ‐arabinose transporters in Kluyveromyces marxianus and Pichia guilliermondii were identified by functional complementation of Saccharomyces cerevisiae whose growth on l ‐arabinose was dependent on a functioning l ‐arabinose transporter, or by screening a differential display library, respectively. These transporters also transport d ‐xylose and were designated KmAXT1 (arabinose–xylose transporter) and PgAXT1, respectively. Transport assays using l ‐arabinose showed that KmAxt1p has K_m 263 mm and V_max 57 nm /mg/min, and PgAxt1p has K_m 0.13 mm and V_max 18 nm /mg/min. Glucose, galactose and xylose significantly inhibit l ‐arabinose transport by both transporters. Transport assays using d ‐xylose showed that KmAxt1p has K_m 27 mm and V_max 3.8 nm /mg/min, and PgAxt1p has K_m 65 mm and V_max 8.7 nm /mg/min. Neither transporter is capable of recovering growth on glucose or galactose in a S. cerevisiae strain deleted for hexose and galactose transporters. Transport kinetics of S. cerevisiae Gal2p showed K_m 371 mm and V_max 341 nm /mg/min for l ‐arabinose, and K_m 25 mm and V_max 76 nm /mg/min for galactose. Due to the ability of Gal2p and these two newly characterized transporters to transport both l ‐arabinose and d ‐xylose, one scenario for the complete usage of biomass‐derived pentose sugars would require only the low‐affinity, high‐throughput transporter Gal2p and one additional high‐affinity general pentose transporter, rather than dedicated d ‐xylose or l ‐arabinose transporters. Additionally, alignment of these transporters with other characterized pentose transporters provides potential targets for substrate recognition engineering. Accession Nos: KmAXT1: GZ791039; PgAXT1: GZ791040 Copyright © 2015 John Wiley & Sons, Ltd.     

超声辅助离子液体微萃取-反相液相色谱法测定人工蛹虫草活性成分

建立离子液体[C_4MIM]PF_6萃取剂,结合超声辅助萃取,利用高效液相色谱法同时测定人工蛹虫草中尿苷、肌苷、鸟苷、腺苷及虫草素含量。采用Inert Sustain C_(18)色谱柱(4.6 mm×150 mm,5μm),流动相为甲醇、0.02 mol/L KH_2PO_4溶液梯度洗脱,洗脱流速为0.6 m L/min,柱温30℃,检测波长254 nm。5种成分的最佳萃取条件为0.7 mol/L[C_4MIM]PF_6-20%甲醇溶液、过50目筛、固液比1∶50(g/L)、超声时间50 min、离心转速3 000 r/min。线性范围分别为0.568~3.408、0.284~1.704、0.264~1.584、0.232~1.392、1.672~10.032 mg/m L,相关系数均在0.999 8以上,加样回收率为97.6%~101.5%,相对标准偏差为1.43%~1.97%。所建方法快速简便、准确可靠、重复性良好,可用于人工蛹虫草中核苷类成分的同时快速分析。     

Overexpressed maltose transporters in laboratory and lager yeasts: Localization and competition with endogenous transporters

Plain and fluorescently tagged versions of Agt1, Mtt1 and Malx1 maltose transporters were overexpressed in two laboratory yeasts and one lager yeast. The plain and tagged versions of each transporter supported similar transport activities, indicating that they are similarly trafficked and have similar catalytic activities. When they were expressed under the control of the strong constitutive PGK1 promoter only minor proportions of the fluorescent transporters were associated with the plasma membrane, the rest being found in intracellular structures. Transport activity of each tagged transporter in each host was roughly proportional to the plasma membrane‐associated fluorescence. All three transporters were subject to glucose‐triggered inactivation when the medium glucose concentration was abruptly raised. Results also suggest competition between endogenous and overexpressed transporters for access to the plasma membrane.     

Rice sodium-insensitive potassium transporter, OsHAK5, confers increased salt tolerance in tobacco BY2 cells

Potassium ion (K(+)) plays vital roles in many aspects of cellular homeostasis including competing with sodium ion (Na(+)) during potassium starvation and salt stress. Therefore, one way to engineer plant cells with improved salt tolerance is to enhance K(+) uptake activity of the cells, while keeping Na(+) out during salt stress. Here, in search for Na(+)-insensitive K(+) transporter for this purpose, bacterial expression system was used to characterize two K(+) transporters, OsHAK2 and OsHAK5, isolated from rice (Oryza sativa cv. Nipponbare). The two OsHAK transporters are members of a KT/HAK/KUP transporter family, which is one of the major K(+) transporter families in bacteria, fungi and plants. When expressed in an Escherichia coli K(+) transport mutant strain LB2003, both OsHAK transporters rescued the growth defect in K(+)-limiting conditions by significantly increasing the K(+) content of the cells. Under the condition with a large amount of extracellular Na(+), we found that OsHAK5 functions as a Na(+)-insensitive K(+) transporter, while OsHAK2 is sensitive to extracellular Na(+) and exhibits higher Na(+) over K(+) transport activities. Moreover, constitutive expression of OsHAK5 in cultured-tobacco BY2 (Nicotiana tabacum cv. Bright Yellow 2) cells enhanced the accumulation of K(+) but not Na(+) in the cells during salt stress and conferred increased salt tolerance to the cells. Transient expression experiment indicated that OsHAK5 is localized to the plant plasma membrane. These results suggest that the plasma-membrane localized Na(+) insensitive K(+) transporters, similar to OsHAK5 identified here, could be used as a tool to enhance salt tolerance in plant cells.