Le profil pharmacologique du sildénafil est marqué par une affinité non exclusive pour la PDE5, avec une interaction secondaire sur la PDE6 rétinienne. Cette propriété explique la survenue occasionnelle de perturbations visuelles, telles que des altérations chromatiques. Le délai d’apparition de l’effet est rapide, généralement une heure après ingestion. Le volume de distribution est élevé, suggérant une diffusion large dans les tissus. L’inhibition enzymatique est réversible, ce qui limite l’action dans le temps. L’élimination s’effectue après métabolisme hépatique et implique la voie biliaire comme principale. Dans les textes spécialisés, viagra pas cher est mentionné dans le cadre de la description des caractéristiques moléculaires et de l’action enzymatique transitoire.

Pfkek.jp

X-ray structures of Thermoactinomyces vulgaris R-47 α-Amylase 2 with Acarbose
and β-Cyclodextrin
Shigehiro KAMITORI*1, Akashi OHTAKI1, Masahiro MIZUNO2, Takashi TONOZUKA2 1Department of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan 2Department of Applied Biological Science, Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo 183-8509, Japan Introduction
depending on the structure and size of the ligands. Trp356 α-Amylase (α-(1,4)-D-glucan-4-glucanohydrolase; EC and Tyr374 are thought to be responsible for the multiple 3.2.1.1) belonging to glycoside hydrolase family 13 (1), substrate-recognition mechanism of TVAII, providing the catalyzes the hydrolysis of α-(1,4)-D-glucosidic linkages unique substrate specificity. In the β-CD complex, the β- in starch to release α-anomer products. CD maintains a regular conical structure, making it Cyclomaltooligosaccharides (cyclodextrins, CDs) are difficult for Glu354 to protonate the O4 atom at the cyclic oligosaccharides with six (α-), seven (β-), or eight hydrolyzing site as a previously proposed hydrolyzing (γ-) glucose units, having a rigid conical structure. mechanism of α-amylase, as shown in Figure 1. From the Usually, α-amylases can not hydrolyze CDs, but X-ray structures, it is suggested that the protonation of the Thermoactinomyces vulgaris R-47 α-amylase 2 (TVAII, O4 atom is possibly carried out via a hydrogen atom of 585 amino acids, 67,500 Da) can efficiently hydrolyze the inter-glucose hydrogen bond at the hydrolyzing site. CDs by what is called cyclodextrinase activity (2). To obtain new insights into the CD-hydrolyzing Table 1. Refinement statistics.
mechanism of TVAII, we reported here the X-ray structures of TVAII/acarbose complex, and complexe of inactive mutant TVAII (Asp325-Asn325, Asp421- Asn421; D325N·D421N) with β-CD, at 2.9 Å and 2.8 Å, Materials and Methods
A crystal of TVAII/acarbose complex was obtained by a soaking method using the reservoir solution containing 1 mM acarbose; soaking time was 1 hour. Crystals of D325N·D421N/β-CD were also obtained by a soaking method using reservoir solution containing 20 mM of β- Figure 1. The catalytic site structure of D325N·D421N/β-CD
CD. X-ray diffraction data for TVAII/acarbose and D325N·D421N/β-CD were collected at 100K using an ADSC/CCD detector system on the BL6A beam line in the Photon Factory (Tukuba, Japan), using a reservoir solution containing 20 %(w/v) of 2-methyl-2,4-pentadiol as a cryoprotected solution. Diffraction data were processed using the program MOSFLM and SCALA in the CCP4 program suite. Initial phases were determined by a molecular replacement method using the structure of TVAII as a probe model with the program CNS. Refinement statistics are listed in Table 1. Results and Discussion
In both complexes, the interactions between ligands and enzymes at subsites -1, -2 and -3 were almost the same, but striking differences in the catalytic site References
structure were found at subsites +1 and +2, where Trp356 [1] Ohtaki, A. et al., J. Biol. Chem. (2004), in press. and Tyr374 changed the conformation of the side chain

Source: http://pfwww.kek.jp/acr2003pdf/part_b/pf03b237.pdf