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91.
Cys-scanning mutagenesis has been applied to the remaining 45 residues in lactose permease that have not been mutagenized previously (from Gln100 to Arg144 which comprise helix IV and adjoining loops). Of the 45 single-Cys mutants, 26 accumulate lactose to > 75% of the steady state observed with Cys-less permease, and 14 mutants exhibit lower but significant levels of accumulation (35-65% of Cys-less permease). Permease with Phe140-->Cys or Lys131-->Cys exhibits low activity (15-20% of Cys-less permease), while mutants Gly115-->Cys, Glu126-->Cys and Arg144-->Cys are completely unable to accumulate the dissacharide. However, Cys-less permease with Ala or Pro in place of Gly115 is highly active, and replacement of Lys131 or Phe140 with Cys in wild-type permease has a less deleterious effect on activity. In contrast, mutant Glu126-->Cys or Arg144-->Cys is inactive with respect to both uphill and downhill transport in either Cys-less or wild-type permease. Furthermore, mutants Glu126-->Ala or Gln and Arg144-->Ala or Gln are also inactive in both backgrounds, and activity is not rescued by double neutral replacements or inversion of the charged residues at these positions. Finally, a mutant with Lys in place of Arg144 accumulates lactose to about 25% of the steady state of wild-type, but at a slow rate. Replacement of Glu126 with Asp, in contrast, has relatively little effect on activity. None of the effects can be attributed to decreased expression of the mutants, as judged by immunoblot analysis. Although the activity of most of the single-Cys mutants is unaffected by N-ethylmaleimide, Cys replacement at three positions (Ala127, Val132, or Phe138) renders the permease highly sensitive to alkylation. The results indicate that the cytoplasmic loop between helices IV and V, where insertional mutagenesis has little effect on activity [McKenna, E., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11954-11958], contains residues that play an important role in permease activity and that a carboxyl group at position 126 and a positive charge at position 144 are absolutely required. 相似文献
92.
Composites based upon 3D textile preforms have found broad structural application. This paper presents an analytical methodology for functional composites using piezoceramic fibers in a 3D braided preform. The effective elastic, piezoelectric and dielectric properties of 2-step braided composites with a polymeric matrix have been investigated. In the analytical approach, the effective properties of the braider and axial yarns of the unit cells are determined first using a 3D connectivity model. Then, the effective properties of the 2-step braided composite are predicted using an averaging technique. Results of a numerical example illustrating the variation of elastic, piezoelectric and dielectric constants with the braider yarn angle are provided. Textile preforming technique in general offers the potential of near net shape forming and 3D fiber placement. The present work provides the analytical basis for 3D piezoceramic textile composites. 相似文献
93.
The muscle and brain creatine kinases of giant panda have been isolated and purified. The purified muscle and brain enzymes (MM and BB) are homogeneous on both the polyacrylamide gel electrophoresis in the presence and absence of SDS. Both enzymes are dimers, consisting of two identical subunits each with a molecular weight of 42,000 daltons. The characteristics of muscle and brain enzymes have been studied, respectively. The hybridized enzyme, MB, was prepared by hybridization of MM and BB. The kinetic parameters of MM, BB and MB were determined, respectively. The results from modification of SH groups show that the SH groups of panda creatine kinases are essential for their activity and among the all SH groups in the enzyme only one per subunit is essential for enzymatic activity. 相似文献
94.
Serological testing for immunoglobulin G (IgG) antibodies to Helicobacter pylori has proven useful in supporting the diagnosis of infection with this organism, but the clinical value of IgA antibodies in H. pylori-related gastritis remains controversial. The purpose of our study was to determine the frequency of IgA-positive IgG-negative patients with symptoms of gastrointestinal (GI) disorders, thus assessing the clinical utility of IgA testing for H. pylori-related gastritis. It was found previously that the frequency of infected individuals in this category (IgA positive and IgG negative) is about 2%, but a large number of IgG-negative patients with GI disorders suggestive of H. pylori infection have not been investigated until now. 相似文献
95.
To investigate the effects of a new nonNMDA antagonist on the trisynaptic pathways in the hippocampus, the author examined kainate(KA)-induced generalized seizures in rats. A novel nonNMDA antagonist, YM90K, showed the blockade of the Schaffer collaterals in 2-deoxyglucose study (2-DG) and that the CA1-2 pyramidal cells of the hippocampus were preserved seven days after the KA injections. On the other hand, the control and MK-801 (NMDA-antagonist) treated rats did not depress the Schaffer collaterals and showed persistent hypermetabolism of glucose in the CA1 pyramidal cell layer, where neurons were not preserved seven days later. 2-DG was useful to reveal the effects of nonNMDA antagonist on the KA-induced generalized seizures. This suggests that YM90K is a potent nonNMDA antagonist and that it has a neuroprotective effect in rats. 相似文献
96.
TA Serel G Akhan HR Koyuncuo?lu A Oztürk K Do?ruer S Unal K Celik 《Canadian Metallurgical Quarterly》1997,31(6):537-539
The treatment and favorable outcome of a bitch with uterine torsion and two retained fetuses are described. The condition was corrected surgically by ovariohysterectomy. Complications (i.e., septic shock, peritonitis, and hemostatic abnormalities) were managed with aggressive medical therapy. Torsion of the gravid uterus in dogs is a life-threatening condition which can have a successful outcome if medical complications encountered in the pre- and postoperative periods are treated quickly and effectively. 相似文献
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HR Brand 《Canadian Metallurgical Quarterly》1985,31(5):3454-3456