Replacement of the Catalytic Nucleophile Aspartyl Residue of Dextran Glucosidase by Cysteine Sulfinate Enhances Transglycosylation Activity

Dextran glucosidase from Streptococcus mutans (SmDG) catalyzes the hydrolysis of an α-1,6-glucosidic linkage at the nonreducing end of isomaltooligosaccharides and dextran. This enzyme has an Asp-194 catalytic nucleophile and two catalytically unrelated Cys residues, Cys-129 and Cys-532. Cys-free SmDG was constructed by replacement with Ser (C129S/C532S (2CS), the activity of which was the same as that of the wild type, SmDG). The nucleophile mutant of 2CS was generated by substitution of Asp-194 with Cys (D194C-2CS). The hydrolytic activity of D194C-2CS was 8.1 × 10⁻⁴ % of 2CS. KI-associated oxidation of D194C-2CS increased the activity up to 0.27% of 2CS, which was 330 times higher than D194C-2CS. Peptide-mapping mass analysis of the oxidized D194C-2CS (Ox-D194C-2CS) revealed that Cys-194 was converted into cysteine sulfinate. Ox-D194C-2CS and 2CS shared the same properties (optimum pH, pI, and substrate specificity), whereas Ox-D194C-2CS had much higher transglucosylation activity than 2CS. This is the first study indicating that a more acidic nucleophile (-SOO−) enhances transglycosylation. The introduction of cysteine sulfinate as a catalytic nucleophile could be a novel approach to enhance transglycosylation

Quasiparticle resonance in decay spectrum of unbound nuclei near neutron drip-line

Background: The pairing correlation in weakly bound nuclei causes a mixing among bound and unbound configurations. A remarkable consequence is emergence of the quasiparticle resonance, which has been predicted with the coordinate space Hartree-Fock-Bogoliubov (HFB) theory, but not yet observed experimentally. Purpose: We discuss possible observation of quasiparticle resonances in decay spectrum of unbound nuclei near the neutron drip-line. We deal with an example of unbound nucleus ${}^{21}$C which disintegrates to ${}^{20}$C and a neutron. Method: We describe a scattering state consisting of ${}^{20}$C and a neutron in the framework of the HFB formalism. We assume that a nucleon knockout reaction produces a doorway state of the decay, and we evaluate the decay spectrum by taking an overlap of the doorway state and the scattering state of ${}^{20}\mathrm{C}+n$. A numerical calculation was performed with the Woods-Saxon potential and a density-dependent effective pairing interaction. Results: We show that the quasiparticle resonance appears as low-lying peaks in the decay spectrum of ${}^{21}$C. They originate from the weakly bound single-neutron orbits $2s_{1/2}$ and $1d_{5/2}$, but emerge as unbound resonant quasiparticle states under the influence of the neutron pairing correlation. The resonance energy and the width of the calculated quasiparticle resonances are consistent with an experimental observation whereas they are sensitive to the neutron pairing correlation. Conclusion: The results suggest that nucleon knockout reactions populating the unbound nucleus ${}^{21}\mathrm{C}$ provide realistic opportunity of experimentally observing the quasiparticle resonance and of disclosing the pairing correlation in neutron-rich nuclei

Effects of l-tryptophan on gastric emptying evaluated by breath test in relation to gastric accommodation evaluated by Barostat in rats

AbstractGastric emptying has been known to correlate the pyloric sphincter contractile function and distention-induced gastric relaxation (gastric accommodation). In the present study, the effects of l-tryptophan on the gastric emptying and accommodation were evaluated by breath test using [1-13C]acetic acid and Barostat study, respectively, in rats. l-Tryptophan significantly decreased Cmax and AUC120min and delayed Tmax, indicating the inhibition of gastric emptying. l-Tryptophan significantly enhanced the gastric accommodation. These findings show that l-tryptophan may inhibit the gastric emptying through the enhanced gastric accommodation. Therefore, l-tryptophan may be useful for the therapy of postprandial dyspepsia, especially for early satiety