A family of mammalian Na+-dependent L-ascorbic acid transporters.

Vitamin C (L-ascorbic acid) is essential for many enzymatic reactions, in which it serves to maintain prosthetic metal ions in their reduced forms (for example, Fe2+, Cu+), and for scavenging free radicals in order to protect tissues from oxidative damage. The facilitative sugar transporters of the GLUT type can transport the oxidized form of the vitamin, dehydroascorbic acid, but these transporters are unlikely to allow significant physiological amounts of vitamin C to be taken up in the presence of normal glucose concentrations, because the vitamin is present in plasma essentially only in its reduced form. Here we describe the isolation of two L-ascorbic acid transporters, SVCT1 and SVCT2, from rat complementary DNA libraries, as the first step in investigating the importance of L-ascorbic acid transport in regulating the supply and metabolism of vitamin C. We find that SVCT1 and SVCT2 each mediate concentrative, high-affinity L-ascorbic acid transport that is stereospecific and is driven by the Na+ electrochemical gradient. Despite their close sequence homology and similar functions, the two isoforms of the transporter are discretely distributed: SVCT1 is mainly confined to epithelial systems (intestine, kidney, liver), whereas SVCT2 serves a host of metabolically active cells and specialized tissues in the brain, eye and other organs.     

Contents and distribution of rare earth elements in wheat seeds

Contents of 15 rare earth elements (REEs) in the seeds of 60 breeds of wheat have been analyzed by the inductively-coupled plasma mass spectrometry (ICP-MS). The distribution pattern of contents of REEs in wheat seeds has been observed and compared with that in soils. Comparison with literature data has also been made. The results show that the background of REEs in wheat seeds is 10-11-10-8 g.g-1, 3-4 levels lower than in soils. The distribution pattern is light REEs higher in contents and slight Eu-anomaly, similar to that in soils. The data obtained in this study can accurately represent the background content of REEs in wheat seeds.     

铸造铝青铜的摩擦特性与机械性能

针对高压齿轮泵侧板的工作特点，熔制了五种铸造铝青铜并分别进行了拉伸性能、摩擦特性和耐磨性实验，为该侧板国产化提供了必要的准备。     

煤炭企业成本主要影响因素的分析

在对煤炭企业成本主要影响因素定性分析的基础上，采用人工神经网络BP模型对其进行定量研究，并将它们应用到煤炭企业实际，取得了良好的效果。     

利用计算机求解实际应力应变方程

本文从理论上分析了可用最小二乘法替代经验方法近似求解实际应力应变方程的可行性，并用Ｑｕｉｃｋ Ｂａｓｉｃ语言编制程序，求出方程的解，提出了拉伸试验的一体化设想。     

岩石孔底断裂控制机理研究

在保证垂直槽孔断开明成型并全面贯通的孔压下，对孔底水平截面Ｉ－Ⅱ型复合裂纹断裂与装药结构的关系等进行了分析，建立了断裂判据，初步确定了裂纹扩展的方向及预应力对断裂的贡献。其结果与工程实践相吻合。     

二元函数柯西中值定理的一个注记

给出了二元函数柯西中值定理的“中间点”，当点Ｂ（ｘ０＋△ｘ，ｙ０＋△ｙ）沿直线段ＡＢ向点Ａ（ｘ０，ｙ０）逼近时的渐过性质，获得了两个在较弱条件下的渐近性质。     

颅内胶质瘤术后放疗43例临床分析

目的：回顾总结４３例脑胶质瘤术后放疗的病例，分析影响预后的因素。方法：１９８８年３月至１９９２年１２月，４３例经术后病理证实为脑胶质瘤的病例。其中手术部分切除３７例（８６％），肉眼全切６例（１４％），术后放疗采用＾６０Ｃｏγ线。放射剂量为３０￣６０Ｇｙ，常规分割。结果：４３例总５年生存率２５．６％。高分级组１、３、５年生存率分别为７２％、２０％、１２％，低分级组的１、３、５年生存率分别为７２．２％     

化学实验中非预期效应分析及思索

在详细地分析了化学实验中产生非预期效应的原因的基础上。提出了解决办法 ,结合实验中非预期效应的解决 ,阐明了培养学生创造性能力、科学观察能力、信息的获取、转化、传递、加工能力等综合能力的方法     

亚强停点及二参数过程的弱停止

本文引入亚强停点及弱局部鞅的概念,得到了一组相关的结果。