Acetyl CoA carboxylase shares control of fatty acid synthesis with fatty acid synthase in bovine mammary homogenate

The objectives of this research were to determine the flux control coefficients for acetyl CoA carboxylase and fatty acid synthase using an in vitro preparation of bovine mammary homogenate. For an enzyme to be considered rate limiting with the use of metabolic control analysis, its control coefficient would be equal to unity. The hypothesis for this experiment was that the control coefficient for acetyl CoA carboxylase was not equal to unity, and that this enzyme was not, therefore, the rate-limiting step. Mammary tissue was isolated from lactating Holstein cows at slaughter and frozen in liquid nitrogen. Tissue was ground, homogenized, and centrifuged to obtain a postmitochondrial supernatant for use in in vitro incubations containing labeled acetate. Specific inhibitors for acetyl CoA carboxylase and fatty acid synthase were used to fractionally inhibit de novo synthesis for the calculation of flux control coefficients. The composition of fatty acids synthesized in the absence of enzyme inhibitors was similar to the composition of fatty acids in the presence of inhibitors. Calculations following avidin inhibition of acetyl CoA carboxylase determined the flux control coefficient was 0.63 ± 0.15, which means that 63% of the control of fatty acid synthesis is exerted by acetyl CoA carboxylase. The remaining control (37%) was from fatty acid synthase, which indicates a significant degree of control over the flux of acetate in de novo synthesis resides with this enzyme. The rate-limiting status ascribed to acetyl CoA carboxylase was not supported, because the flux control coefficient was less than unity. Metabolic control analysis, through its use of pathway product measurements, allows for potential interactions in the pathway such as feedback inhibition contribution to the flux control coefficients, which would not otherwise be considered in studies measuring enzyme kinetics with purified enzymes.     

TEM and HREM study on the fine structure and the interfacial structure of bainite

The fine structure of bainite,the morphology and distribution of carbide in steels and the morphology of bainite in Cu-Zn-Al alloys have been investigated with TEM.The interfacial structure ledges and interfacial crystal lattice images of Cu-Zn-Al alloys have also been investigated with HREM.The addition of alloying microelements can fine the structure of bainitic ferrite markedly.The bainitic ferrite is composed of subunits or subchunks.The carbides differ in morphologies and are distributed in between laths,inside the plates and on the boundaries of subunits.There are abundant fine structures in bainitic ferrite.In the primary bainite of Cu-Zn-Al alloy there are interfacial structure ledges,the height of which is about 3 -40 nm,equal to 15-200 atomic layers.The phase transformation mechanism of bainite is discussed.     

Reconstitution of single molecular species from isolated subunits of glycinin

Ion-exchange HPLC systems using POROS 20 HQ and Mono Q HR 10/10 columns were applied to isolate glycinin subunits under denaturing conditions. Analyses by SDS-PAGE, N-terminal amino acid sequence, and sucrose density gradient centrifugation showed that the pseudoglycinin from the highly purified homo-subunit, A₃B₄, was reconstituted. The A₃B₄ pseudoglycinin was similar to the native glycinin with respect to molecular size, subunit structure, and secondary structure. The hexameric pseudoglycinin dissociated into trimers after long storage at pH 7.6.     

面团搅拌过程中麦谷蛋白大聚体的变化Ⅰ.麦谷蛋白大聚体中亚基组分的变化

利用3种不同品质性状(高,中,低筋)小麦为原料,研究了面团搅拌过程中麦谷蛋白大聚合体(GMP)分子亚基组分的变化.结果表明:经十二烷基硫酸钠-聚丙烯凝胶电泳(SDS-PAGE)分离后,GMP分子的亚基组成可以分为A区、B区和C区,经Scanimage软件扫描处理后发现,在面团搅拌过程中GMP分子中的亚基组成没有变化,但亚基的含量发生了明显的变化,其中A/(B C)比值出现有规律的变化.     

Cu-Zn-Al合金贝氏体的亚单元及其激发形核、台阶生长

利用透射电镜(TEM)观察到Cu-Zn-Al合金贝氏体的亚单元结构，其形状、大小及分布规律与扫描隧道显微分析(STM)的结果一致，亚单元上有台阶存在，表明它们是通过台阶机制生长的，亚单元是通过激发形核的，观察到三种类型的激发方式：面-面激发、边-边激发及边-面激发，Cu-Zn-Al合金的贝氏体相变为激发形核-台阶生长过程。     

Condensed tannins from Lotus corniculatus and Lotus pedunculatus exert different effects on the in vitro rumen degradation of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) protein

Condensed tannins (CT) or proanthocyanidins (PA), which occur in a restricted range of forages, have the ability to interact with proteins and enzymes and can influence the digestion of plant protein in the rumen. We compared the effects of CT extracts from Lotus corniculatus and pedunculatus on degradation of the principal leaf protein, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), by rumen microorganisms. Total soluble leaf protein extracted from white clover (Trifolium repens ) was incubated with fresh rumen fluid from sheep and a range of concentrations of each CT extract. The rate of degradation of the large (LSU) and small subunit (SSU) of Rubisco was quantified by fractionating the proteins in samples taken from in vitro rumen incubations using sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS‐PAGE) and imaging densitometry. To deduce the effects of the CT extracts, experiments were performed in the presence (CT inactive) and absence (CT active) of polyethylene glycol (PEG; MW 3350). The two CT extracts differed markedly in their effects on the degradation of the LSU and SSU of Rubisco. At concentrations of 0.89 and 1.79 mg CT mg ⁻¹ total soluble leaf protein, the CT extract from L pedunculatus was more effective at preventing the degradation of the LSU and SSU by rumen microorganisms than the CT extract from L corniculatus. At a concentration of 1.79 mg CT mg ⁻¹ total soluble leaf protein, the CT extracts from L corniculatus and pedunculatus prevented about 0.75 and 0.83 of the LSU and about 0.69 and 0.86 of the SSU, respectively, from being degraded. Addition of PEG removed the inhibition and almost complete degradation of these proteins occurred, as was the case in incubations without CT extracts. The results of this study suggest that the concentration of CT in the diet and the chemical structure which affects the activity of the CT needs to be considered when assessing the effects of CT on protein metabolism in ruminants. © 1999 Society of Chemical Industry     

Sequence Analysis of Candida albicans Phosphoribosyl-aminoimidazole Carboxylase (ADE2) Gene

The nucleotide sequence of the Candida albicans ADE2 gene, which encodes phosphoribosylaminoimidazole carboxylase, has been determined. The sequence possesses an uninterrupted open reading frame of 1704 nucleotides corresponding to 568 amino acid residues. The deduced amino acid sequence shares a high degree of homology with ADE2 homologues in other fungal species including Saccharomyces cerevisiae, Pichia methanolica, Schwanniomyces occidentalis and Schizosaccharomyces pombe. Three regions of amino acid sequence were highly conserved among all reported ADE2 genes. The hexanucleotide TGACTC characteristic of genes involved in purine and amino acid biosyntheses is located in front of putative TATA boxes in the promoter region. The GenBank Accession Number of this gene is U75582. © John Wiley & Sons, Ltd.     

脉冲噪声环境中鲁棒的自适应波束形成方法

本文提出一种脉冲噪声环境中的自适应波束形成方法.方法假定噪声服从对称 α 稳定(S α S:Symmetric α -stable)分布,首先定义分数低阶阵列响应,然后根据最小方差无畸变响应波束形成器(MVDR)提出分数低阶最小方差无畸变响应波束形成器(FrMVDR).理论上证明了当阶数小于噪声特征指数的一半时,分数低阶阵列输出功率有界.计算机仿真实验证明了本文提出的FrMVDR波束形成器在高斯噪声和非高斯脉冲噪声环境中性能都优于MVDR和其他有关的基于分数低阶矩的波束形成器,是一种鲁棒的自适应波束形成器.     

叶绿素α的激光光致发光机理及其浓度测量新方法研究

介绍了一种以红光半导体激光器作为荧光激发光源,结合光纤光谱技术测量水体中叶绿素α浓度的新方法。通过分析叶绿素α的激光光致发光机理,得出了水体中叶绿素α荧光发射光谱的相对荧光峰值强度与叶绿素α浓度的近似线性关系。实验结果表明,在红光半导体激光器激励下根据叶绿素α的荧光发射光谱直观地判断叶绿素α的浓度这种方法完全可行,由此为研制叶绿素α荧光仪提供了一种新的光源选择,并为实现现场实时监测海水中叶绿素α浓度提供了实验依据。     

Effect of condensed tannins prepared from several forages on the in vitro precipitation of ribulose-1,5-bisphosphate carboxylase (Rubisco) protein and its digestion by trypsin (EC 2.4.21.4) and chymotrypsin (EC 2.4.21.1)

A series of in vitro experiments was undertaken to determine the extent to which Sephadex LH-20 treated extracts from a range of temperate forages precipitated ribulose-1,5-bisphosphate carboxylase (Rubisco) and affected the enzymatic hydrolysis of Rubisco protein by trypsin and chymotrypsin at a range of pH values. Rubisco was chosen because it represents the principal dietary protein for ruminants fed fresh forages. Condensed tannins (CT) or proanthocyanidins (PA) are routinely purified by chromatography using Sephadex LH-20 as a matrix. However, these extracts contained non-CT phenolics together with PA so the term ‘CT extract’ was preferred to ‘PA’ to describe the extracts. The in vitro precipitation of Rubisco provided a means to compare the reactivity of the CT extracts. The amount of CT extract required to precipitate all the Rubisco in 10 μg of total soluble leaf protein from white clover (Trifolium repens) when this protein was incubated with CT extracts of Lotus corniculatus, L pedunculatus and sainfoin (Onobrychis viciifolia) was similar, with between 25 and 50 μg of extract required. The CT extract of sulla (Hedysarum coronarium) also precipitated all the Rubisco, however this only occurred with 50 μg of the extract. The CT extract of dock (Rumex obtusifolius) precipitated all the Rubisco when 5 μg of extract or greater was incubated with total soluble leaf protein. However, the differences between the reactivity of all these CT extracts at a range of pH values appeared to be small. Condensed tannin extracts of L corniculatus and L pedunculatus partially inhibited the hydrolysis of Rubisco by trypsin and chymotrypsin to a similar extent, but the extent of the inhibition was affected by pH. The inhibition was greater at pH 6·0 than 7·0, whilst at pH 8·0, CT extracts had little or no affect on trypsin and chymotrypsin. It was concluded that, although the precipitation of Rubisco provided an ideal method for comparing CT extracts, reactivity alone was unlikely to account for the differences in nutritive value that occur with forages containing CT. © 1998 SCI.