Catechol, a bioactive degradation product of salicortin, reduces TNF-α induced ICAM-1 expression in human endothelial cells

The phenolic glucoside salicortin was isolated from a Willow bark extract, and its ability to reduce the TNF- α induced ICAM-1 expression (10 ng/mL, 30 min pretreatment with salicortin) was tested IN VITRO on human microvascular endothelial cells (HMEC-1). After 24 h, 25 µM salicortin decreased the TNF- α induced ICAM-1 expression to 65.9 % compared to cells which were treated only with TNF- α. In parallel, the stability of 25 µM salicortin under assay conditions was determined by HPLC. Within 24 h, the salicortin concentration decreased to 3.1 µM whereas catechol, a known NF- κB inhibitor, rose as a metabolite. After 8 h the catechol concentration was relatively constant and varied between 8.2 and 10.9 µM. Considering this degradation in the IN VITRO test system, 10 µM catechol was added 8 h after TNF- α stimulation, and 16 h later the ICAM-1 expression was determined. In this setting, the ICAM-1 expression was reduced to 74.8 %. This is comparable to the effect obtained from 25 µM salicortin and indicates that its activity is related to the generation of catechol, as salicin, saligenin, and salicylic acid are only marginally active or inactive in this test system in a concentration up to 50 µM. These results indicate catechol as an important bioactive metabolite from salicortin.

Structure-Based Engineering of Strictosidine Synthase: Auxiliary for Alkaloid Libraries

The highly substrate-specific strictosidine synthase (EC 4.3.3.2) catalyzes the biological Pictet-Spengler condensation between tryptamine and secologanin, leading to the synthesis of about 2000 monoterpenoid indole alkaloids in higher plants. The crystal structure of Rauvolfia serpentina strictosidine synthase (STR1) in complex with strictosidine has been elucidated here, allowing the rational site-directed mutation of the active center of STR1 and resulting in modulation of its substrate acceptance. Here, we report on the rational redesign of STR1 by generation of a Val208Ala mutant, further describing the influence on substrate acceptance and the enzyme-catalyzed synthesis of 10-methyl- and 10-methoxystrictosidines. Based on the addition of strictosidine to a crude strictosidine glucosidase preparation from Catharanthus cells, a combined chemoenzymatic approach to generating large alkaloid libraries for future pharmacological screenings is presented.

Correlation between processing efficiency for ribonuclease P minimal substrates and conformation of the nucleotide -1 at the cleavage position

It is demonstrated that acceptor stem duplexes derived from native tRNAs which contain a three-nucleotide extension at the 5'-terminus of mature tRNA are minimal substrates for ribonuclease P from both Escherichia coli and Bacillus subtilis. Variants with a cytidine at position -1 are most efficiently processed whereas the G -1 variant represents a comparatively poor substrate. An A -1 acceptor stem variant is a slightly better substrate than the G -1 variant though generally distinctly less efficient than the C -1 duplex. This is in qualitative agreement with the frequency of the occurrence of the corresponding nucleotides at position -1 in natural substrates, which is highest for pyrimidines and least for G. NMR analyses of the corresponding acceptor stems reveal that the conformation of the nucleotides at position -1 correlates with the substrate preferences of Ribonuclease P: Whereas C -1 adopts a conformation characterized by a glycosidic angle in the anti range (close to high-anti), the G -1 is clearly in syn conformation, and that of A -1 is intermediate between high-anti and syn. The riboses of nucleotides -1 are in all cases predominantly 2'-endo puckered.

The C-A mismatch base pair and the single-strand terminus in the E. coli initiator tRNA(fMet) acceptor stem adopt unusual conformations

Acceptor stem variants of tRNA(fMet) (Escherichia coli) have been characterized by nuclear magnetic resonance. The wild type contains a C1-A72 mismatch pair which is crucial for its biological function. For comparison, the mismatch was replaced by regular pairs U1-A72 and C1-G72. Further variants contain an altered discriminator base, G73, or a G1-C72/U73 combination. The stems of variants U1-A72/A73 and C1-G72/A73 have A-RNA geometry, which extends essentially to the single-strand terminus. C1-A72/G73 variant and wild type are structurally almost identical. C1 and A72 adopt peculiar conformations with C1 being largely destacked with respect to G2, while A73 stacks upon C1. The unique arrangement of the mismatch causes a distinctly different orientation of the single-strand terminus compared to variants with regular 1-72 base pairs, and to formyltransferase-complexed tRNA(fMet).

Crystal structure of acceptor stem of tRNA(Ala) from Escherichia coli shows unique G.U wobble base pair at 1.16 A resolution

The acceptor stem of Escherichia coli tRNA(Ala), rGGGGCUA.rUAGCUCC (ALAwt), contains the main identity element for the correct aminoacylation by the alanyl tRNA synthetase. The presence of a G3.U70 wobble base pair is essential for the specificity of this reaction, but there is a debate whether direct minor-groove contact with the 2-amino group of G3 or a distortion of the acceptor stem induced by the wobble pair is the critical feature recognized by the synthetase. We here report the structure analysis of ALAwt at near-atomic resolution using twinned crystals. The crystal lattice is stabilized by a novel strontium binding motif between two cis-diolic O3'-terminal riboses. The two independent molecules in the asymmetric unit of the crystal show overall A-RNA geometry. A comparison with the crystal structure of the G3-C70 mutant of the acceptor stem (ALA(C70)) determined at 1.4 A exhibits a modulation in ALAwt of helical twist and slide due to the wobble base pair, but no recognizable distortion of the helix fragment distant from the wobble base pair. We suggest that a highly conserved hydration pattern in both grooves around the G3.U70 wobble base pair may be functionally significant.

Structural and dynamic helix geometry alterations induced by mismatch base pairs in double-helical RNA

A ribooligonucleotide microhelix derived from the acceptor stem of Escherichia coli tRNA(Ala) having a C3-A70 mismatch in place of the G3-U70 wobble pair in the wild-type tRNA(Ala), and a sequence variant with a regular U3-A70 base pair have been investigated by NMR. In vivo, suppressor tRNA(Ala) variants with C3-A70 (as well as several other) mismatch pairs are substrates for alanyl-tRNA synthetase (ARS), supporting the hypothesis of an 'indirect' recognition of the identity element 3-70 mismatch pair via structural modifications caused by the mispair in comparison to canonical A-RNA helices. It is demonstrated that the C-A mismatch likewise induces helix geometry alterations, in particular with respect to base stacking in the vicinity of the mismatch. However, with reference to the 'wild-type' G3-U70 microhelix, destacking in the C3-A70 acceptor stem duplex occurs in the opposite direction from the mismatch pair. Therefore it is concluded that the locally enhanced conformational flexibility or dynamics associated with the structural changes induced by the mismatch pairs could be an essential prerequisite for optimal adaptation of the tRNA(Ala) acceptor stem to the contact region of the ARS.

Novel glucoalkaloids from Rauwolfia cell cultures--acetylrauglucine and related glucosides

From cell suspension cultures of Rauwolfia serpentina grown in an optimized production medium for the glucoalkaloid raucaffricine, a novel glucoalkaloid was isolated and identified as 17-O-acetyl-21-O-beta-D-glucopyranosyl-ajmaline (acetylrauglucine). This alkaloid is formed in very small amounts (less than 5 x 10(-4)%). The biogenetically related N alpha-demethylated base (acetyl-nor-rauglucine) and the deacetyl product rauglucine have also been detected in culture extracts. In addition 21(R)-(beta-D-glucopyranosyl)-hydroxy-sarpagan-17-al has been isolated and identified as an artifact which originates from raucaffricine.

Characterization of 2 beta (R)-17-O-acetylajmalan: acetylesterase--a specific enzyme involved in the biosynthesis of the Rauwolfia alkaloid ajmaline

A novel enzyme was isolated, partially purified (217-fold) and characterized from cell suspension cultures of Rauwolfia serpentina Benth. The enzyme catalyzes one of the late biochemical reactions in the biosynthesis of ajmaline by hydrolysis of 17-O-acetylated alkaloids of the ajmalan group forming the appropriate deacetylated compounds. This esterase exhibits an unusually high substrate selectivity and exclusively accepts acetylated ajmaline derivatives with the naturally occurring 2 beta (R)-configuration. The properties of the enzyme were determined showing an optimum pH at 7.5, an isoelectric point of pH 4.9 and a relative molecular weight of 33 +/- 2 kDa. Inhibition studies of enzyme activity point to the necessity of SH-groups. The esterase seems not to be inhibited by ajmaline, the end product of the pathway. The highest enzyme activities were observed in leaves and cell suspension tissues of the tribe Rauwolfieae which are known to synthesize ajmaline and its congeners. The specific function of the esterase in the biosynthesis of the later alkaloids was established.

RLCC-isolation of Raucaffricine from its most efficient source - cell suspension cultures of Rauwolfia serpentina Benth

Raucaffricine (vomilenine-galactoside) was shown to be the major indole alkaloid of Rauwolfia serpentina Benth. cell suspension cultures grown in AP-medium (alkaloid production medium). Several grams of this glycoalkaloid can conveniently be isolated by RLCC (Rotation Locular Countercurrent Chromatography). A newly discovered enzyme efficiently converts the glycoalkaloid to its aglycon, vomilenine, which occupies a key function in the biosynthesis of ajmaline. This is the first demonstration of the occurrence of raucaffricine in Rauwolfia serpentina.