Uri Zehavi

Enzymic synthesis of oligosaccharides on a polymer support. Light-sensitive, substituted polyacrylamide beads

Abstract Two light-sensitive, gluco and cellobio polymers, were synthesized by formation of an amide bond between 4-carboxy-2-nitrobenzyl β- d -glucopyranoside or 4-carboxy-2-nitrobenzyl 4- O -β- d -glucopyranosyl-β- d -glucopyranoside, respectively, and aminoethyl-substituted, polyacrylamide-gel beads. Subsequently, the polymers were used as acceptors of glycosyltransferase ( d -galactosyltransferase, EC 2.4.1.22) and transglycosylase (lysozyme, EC 3.4.1.17) activity. As 2-nitrobenzyl derivatives, the polymers did undergo intramolecular oxidation-reduction upon irradiation (

Adenylate cyclase responses to sucrose stimulation in membranes of pig circumvallate taste papillae

1. Typical adenylate cyclase (AC) responses to guanine nucleotides were found in membranes of pig circumvallate (CV) taste papillae. 2. Sucrose stimulated AC activity in the CV membranes and this stimulation was GTP dependent and tissue specific. 3. The stimulatory effect of sucrose in the CV membranes was dependent on the concentration of membranes used in the AC assay. 4. This study provides the first biochemical data on cellular transduction of taste in the pig, compares positively to preliminary results in cattle and supports recent suggestions for a role of cAMP in sweet taste transduction.

Applications of Photosensitive Protecting Groups in Carbohydrate Chemistry

Publisher Summary This chapter discusses the applications of photosensitive protecting groups in carbohydrate chemistry. The use of selectively removable protecting groups is indispensable in carbohydrate synthesis, being concerned with polyfunctional molecules that bear many sites to be protected. Usually, protecting groups are removed by thermal chemical reactions. However, the photochemical removal of protecting groups normally achieved by thermal reactions, as well as of those specially designed for photochemical cleavage, has become a very useful complementary approach. In general, photosensitive protecting groups are removed by photosolvolysis, making use of the different reactivities of the excited and ground states, or by intramolecular, photoactivation reactions. The selective protection of hydroxyl groups is most frequent in carbohydrate synthesis and photosensitive protecting groups have been used for this effect in oligosaccharide synthesis, nucleotide synthesis, and saccharide modification. The protection of amino groups of amino sugars benefits particularly from the use of new blocking groups introduced for peptide synthesis. In this context, light-sensitive urethans and amides that can be utilized for the protection of amino groups in amino sugars are of particular interest in saccharide synthesis and modification.

Taste responses to neohesperidin dihydrochalcone in rats and baboon monkeys

Preference-aversion behavior to solutions containing neohesperidin dihydrochalcone (NHDHC) was studied rats and baboon monkeys. Electrophysiological responses evoked by application of NHDHC solutions to taste receptors innervated by the chorda tympani and the glossopharyngeal nerves were also measured. As a group, rats were indifferent to solutions containing up to 1.2 x 10(-3) M NHDHC in short and long-term preference tests. A solution containing the very high concentration of 8.2 x 10(-3) M NHDHC was consumed less than water by all rats. The aversive behavior of rats to the 8.2 x 10(-3) M NHDHC solution appeared to be due to taste quality rather than olfaction. When percent preferences were calculated on an individual basis for the long-term preference tests, 59% of the rats were indifferent to solutions containing up to 1.2 x 10(-3) M NHDHC, 33% of the animals found this solution aversive and less than 8% showed preference. Behavioral responses to a solution of 3.4 x 10(-4) M aspartame also varied considerably among rats. The electrophysiological data were in line with the behavioral responses suggesting weak taste responses for NHDHC in rats. More pronounced responses observed in the glossopharyngeal nerve as compared to the chorda tympani. Baboon monkeys showed a strong preference for solutions containing 1.6 x 10(-5) M-1.6 x 10(-3) M NHDHC. A solution of 1.6 x 10(-2) M was consumed to a lesser extent than water. It is concluded that baboon monkeys present a better experimental model than rats for investigating the sweetness of NHDHC.

Enzymic glycosphingolipid synthesis on polymer supports.

Two synthetic approaches leading toN-4-carboxymethyl-2-nitrobenzyloxycarbonyl derivatives of model compounds and of glycosylsphingosines are described. The carboxymethyl group can be converted into a hydrazido derivative and the resulting products react with an active ester polymer yielding light-sensitive polymeric products that may subsequently serve as acceptors in glycosyltransferase reactions.

Isolation, structure determination, synthesis, and antifungal activity of a new native alfalfa-root saponin

Abstract An alfalfa-root saponin (compound F) was isolated and its chemical structure determined as 3β-[ O -α- d -glucopyranosyl-(1→4)-β- d -glucopyranosyl]-2β-hydroxy-Δ 12 -oleanene-23,28-dioic acid (3β-medicagenic acid β-maltoside). It was synthesized via a modified Koenigs-Knorr reaction employing light-sensitive esters as protective groups for the aglycon. The saponin has antimycotic activity against some plant pathogenic fungi similar to that of its aglycon and a previously isolated homologous saponin, the percent inhibition being 90, 57, 67, 89, 62, and 68 for Sclerotium rolfsii, Fusarium oxysporum f. sp. lycopersici, Rhizoctonia solani, Trichoderma viride, Aspergillus niger , and Pythium aphanidermatum , respectively, at a concentration of 40 μg/mL.

Probing acceptor specificity in the glycogen synthase reaction with polymer-bound oligosaccharides

Polymers having maltose and maltotriose side-chains were synthesized by attaching 4-carboxy-2-nitrobenzyl 4-O-alpha-D-glucopyranosyl-beta-D-glucopyranoside or 4-carboxy-2-nitrobenzyl O-alpha-D-glucopyranosyl-(1----4)-O-alpha-D-glucopyranosyl-O-beta-D- glucopyranoside, respectively, to aminoethyl-substituted polyacrylamide gel beads. Subsequently, the two polymers, and analogous polymers having D-glucose and cellobiose side-chains, served in a comparative study as acceptors in the glycogen synthase (UDP-D-glucose: glycogen 4-alpha-glucosyltransferase, EC 2.4.1.11) reaction. Highest transfer (4.2%) was observed for the polymer bearing maltotriose groups. The bound saccharides were then removed by irradiation (greater than 320 nm), and examination of them demonstrated that alpha-D-glucosyl oligomerization in the glycogen synthase reaction had occurred.

Citrate Uptake into Tonoplast Vesicles from Acid Lime (Citrus aurantifolia) Juice Cells

Abstract. Citrate transport into the vacuoles of acid lime juice cells was investigated using isolated tonoplast vesicles. ATP stimulated citrate uptake in the presence or in the absence of a ΔμH+. Energization of the vesicles only by an artificial K+ gradient (establishing an inside-positive Δψ) also resulted in citrate uptake as was the case of a ΔpH dominated ΔμH+. Addition of inhibitors to endomembrane ATPases showed no direct correlation between the inhibition to the tonoplast bound H+/ATPase and citrate uptake. The data indicated that, although some citrate uptake can be accounted for by Δψ and by a direct primary active transport mechanism involving ATP, under in vivo conditions of vacuolar pH of 2.0, citrate uptake is driven by ΔpH.

Mode of Action of the Antimycotic Agent G2 Isolated from Alfalfa Roots

The mode of action of the antimycotic alfalfa root saponin, medicagenic acid 3-O-beta-D-glucopyranoside (compound G2), which possesses a pronounced antifungal activity against medically important yeasts and dermatophytes, was studied in Saccharomyces cerevisiae. Compound G2 caused lethal leakage of ions out of the yeast cells. Exposure of S. cerevisiae to compound G2 resulted in a disappearance of the main sterol, ergosterol, from the cell membranes, suggesting that compound G2 was highly specific for ergosterol. Independently, chemical data indicated that compound G2 forms stable complexes with both ergosterol and cholesterol. Addition of cholesterol or ergosterol protected the cells of S. cerevisiae and several pathogenic yeasts from the inhibitory activity of compound G2 by producing a higher ratio of sterols (mainly ergosterol) to phospholipids in the membranes. The fact that an amphotericin B-resistant Candida tropicalis was susceptible to G2 suggested that its mode of action was different from that described for polyene antibiotics. This was also confirmed by the finding that 0.2 M KCl did not protect S. cerevisiae cells against ion leakage with G2, but did so with amphotericin B.

Enzymic glycosphingolipid synthesis on polymer supports.: II. Synthesis of lactosyl ceramide

In a first example of a polymer-supported enzymic synthesis of glycosphingolipids, synthetic (2S,3R,4E)-2-amino-1-(β-d-glucopyranosyloxy)-3-hydroxy-4-octadecene (glucosylsphingosine) was converted to theN-4-carboxymethyl-2-nitrobenzyloxycarbonyl derivative and was subsequently attached to a water-soluble polymer. The material served as an acceptor in thed-galactosyltransferase reaction (36% transfer yield) and further photolysis, acylation and chromatography afforded (2S,3R,4E)-1-[4-O-(β-d-galactopyranosyl)-β-d-glycopyranosyloxy]-3-hydroxy-2-octadecanoylamino-4-octadecene (lactosylceramide, 54% yield).