Arabinosylation

The L-arabino-(4-0-methyl-D-glucurono)xylans are found in softwoods and annual plants. The L-arabinose is present primarily as a-L-arabinofuranosyl units, although P-L-arabinopyranosyl units may also be present. In either case, the arabinosyl units are often, but not always, present as single-unit side chains, as are the uronic acid units. 

Stereoselective Strecker reactions with galactosylamine 1 can also be achieved with sodium cyanide and acetic acid in 2-propanol. The reactions, however, proceed slowly and with a lower stereoselectivity, giving diastereomeric ratios of the products between 3 1 and 7 1. The scope of the method can be extended to other glycosylamines, e.g., 2,3,4-tri-O-pivaloyl-a-D-arabinosyl-amine which allows the stereoselective synthesis of (A )-amino nitriles61,62. 

The heteroxylans (CHX) present in cereals, seeds, gum exudates, and mucilages are structurally more complex [6]. They have a (1 4)-)0-d-xylopyranose backbone decorated, except of the single uronic acid and arabinosyl residues with various mono- and oligoglycosyl side chains. Reinvestigations of CHX isolated from corn bran [52] have confirmed that the xylan backbone is heavily substituted (at both positions 2 and 3) with y0-D-Xylp, y0-L-Ara/, of-D-GlcpA residues and oligosaccharide side chains (1), (3) and (4). 

The substitution pattern of arabinosyl side chains in AX from cereal flours and bran, based on the structural analysis of ohgomer fragments produced by xylan-degrading enzymes of known mode of action, was described by several authors [60-63], and various structural models were created [39,60]. fii a recent study [64] on the fine structure of wheat flour AX, a method was... 

The antimicrobial action of ethambutol, like that of isoniazid, is specific for mycobacteria, suggesting a target in the unique components of the mycobacterial cell wall. Cells treated with ethambutol accumulate an isoprenoid intermediate, decaprenyl-arabinose which is the source ofarabinose in the arabinogalactan polymer. This suggests that ethambutol blocks assembly of the arabinogalactan through inhibition of an arabinosyl transferase enzyme. 

The low amounts in 3- and 3,6-linked galactosyl, in 3- and 5-linked arabinosyl residues may be due to the presence of contaminating arabinogalactans and arabinans in apple and tomato juice RG-II preparations. 

On incubation of endo-arabinanase with reduced arabino-oligosaccharides, the rate of hydrolysis increases up to the hexasaccharide indicating that the enzyme binds over 5-6 arabinosyl residues 11), Further information on these requirements has been gained by characterising the... 

This is approximately equal to 0.46 mM terminal arabinosyl residues. 

Certain derivatives of 6-mercaptopurine, such as 6-(methylthio)purine, 6-mercaptopurine-3-oxide [448a], and 6-mercaptopurine ribonucleoside and its acylated derivatives apparently owe their activity to their in vivo conversion to 6-mercaptopurine [11,13]. It would appear, however, that the 9-alkyl derivatives of 6-mercaptopurine, and its arabinosyl and xylosyl derivatives, are not metabolized-except in the case of the 9-alkyI derivatives, to a limited extent to their 5-glucuronides—and that their mechanism of action is quite different from that of 6-mercaptopurine. 

The role of arabinosyl substituents and the need for a-arabinosidase in the production of xylose from arabinoxylan was also deduced from the results in Table II. When the purified 20 kDa xylanase and / -xylosidase were used in the hydrolysis, the xylose yield was only 66% of that produced by the whole culture filtrate at the same activity levels, and no arabinose was produced. Addition of a-arabinosidase increased the yields of both xylose and arabinose. 

Ethambutol is a water-soluble, heat-stable compound that acts by inhibition of arabinosyl transferase enzymes that are involved in cell wall biosynthesis. Nearly all strains of M tuberculosis and M. kansasii and most strains of Mycobacterium avium-intracellulare are sensitive to ethambutol. Drug resistance relates to point mutations in the gene (EmbB) that encodes the arabinosyl transferases that are involved in mycobacterial cell wall synthesis. 

Vidarabine is administered only as a topical ophthalmic ointment. It has relatively limited solubility and is not significantly absorbed after application to the eye. Within the tissues, it is rapidly deaminated to its principal metabolite, arabinosyl hypoxanthine, which retains some degree of antiviral activity. 

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