Arabinose labeled

The related 3-deoxy-D-waHnooctulosonic acid aldolase (KDO aldolase F.C 4.1.2.23) likewise suffers from an unattractive equilibrium constant but allows a simple synthesis of specifically labelled KDO from D-arabinose and labelled pyruvate28. 

With the same synthetic sequence, labeled ribose molecules produced AIRs labeled on the ribose moiety. From D-erythrose and (l3C)NaCN, the Fischer-Kiliani synthesis, as modernized by Serianni et al.59 produced D-(l-l3C)ribose and D-(l-l3C)arabinose. The labeled arabinose was transformed into D-(2-l3C)ri-bose in the presence of dioxobis(2,4-pentanedionato)-0-0 -molybdenum(VI) in... 

When grown in a mineral medium containing KDO as the only source of carbon, cells of Aerobacter cloacae can be induced to produce an enzyme that catalyzes the cleavage of KDO to give D-arabinose and pyruvic acid.89 This enzyme was purified 60-fold by Ghalambor and Heath.154 It has a pH optimum of 7, a KM = 6 mM, and an equilibrium constant of 77 mM. The reversible nature of the enzyme reaction can be utilized to synthesize 14C-labelled KDO from D-arabinose plus 14C-pyruvic acid. Cleavage of KDO as catalyzed by KDO aldolase has... 

The cyanohydrin synthesis of higher sugars, which involves intermediate aldonolactones, allows the introduction of a 14C label in the sugar chain. Thus, for example, L-[5-l4C]arabinose was synthesized (12) from D-xylose, which was first converted, by addition of K14CN and hydrolysis, into D-[ 1-... 

Although small proportions of other products are formed when D-xylose is exposed to rather high acid concentrations, arabinose, lyxose, and ribose form considerably more of alternative products (generally reductic acid) than of 2-furaldehyde under these conditions. Reductic acid (2,3-dihydroxy-2-cyclopenten-l-one, 47) has been detected as a product after acid exposure of D-xylose or its major dehydration product, 2-furalde-hyde. Further work performed with D-[l- C]xylose and [a- C]2-fural-dehyde showed that reductic acid having identical label distribution was obtained from both starting materials. This indicated that a common primary source was involved, probably 2-furaldehyde, as it is readily formed from D-xylose under acidic conditions. 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

The synthesis of " C-labelled o-glucose starts with the pentose o-arabinose and " C-labelled potassium cyanide, which react together to form a cyanohydrin (see Section 7.6.1). Since cyanide can attack the planar carbonyl group from either side, the cyanohydrin product will be a mixture of two diastereoisomers that are epimeric at the new chiral centre. The two epimers are usually formed in unequal amounts because of a chiral influence from the rest of the arabinose structure during attack of the nucleophile. 

Note how the process may be modihed to extend its versatility. Thus, using " C-labelled potassium cyanide with D-erythrose yields a mixture of [l- " C]-D-ribose and [l- " C]-D-arabinose. The sequence could then be repeated on the latter product, using unlabelled KCN, to give [2- " C]-D-glucose. 

Particulate preparations from com shoots readily incorporate 14C-labeled D-xylose from UDP-D-xylose-14C into a polysaccharide in which the D-xylose residues are combined by / -l,4-D-xylosyl bonds (31). It was shown that this polysaccharide, similar to natural plant xylan, contains a small proportion of L-arabinose units which have the furanose configuration. 

Physical or chemical modification of a substrate may additionally selectively affect transformation or uptake Keil and Kirchman (1992) compared the degradation of Rubisco uniformly labeled with 3H amino acids produced via in vitro translation to Rubisco that was reductively methylated with 3H-methane. Although both Rubisco preparations were hydrolyzed to lower molecular weights at approximately the same rate, little of the methylated protein was assimilated or respired. The presence of one substrate may also inhibit uptake of another, as has been demonstrated for anaerobic rumen bacteria. Transport and metabolism of the monosaccharides xylose and arabinose were strongly reduced in Ruminococcus albus in the presence of cellobiose (a disaccharide of glucose), likely because of repression of pentose utilization in the presence of the disaccharide. Glucose, in contrast, competitively inhibited xylose transport and showed noncompetitive inhibition of arabinose transport, likely because of inactivation of arabinose permease (Thurston et al., 1994). 

Fig. 5. Gel-permeation chromatography (on Bio-Gel P-2) of soluble extracellular material from spinach cell suspension cultures that had been fed L-[l- H]arabinose [32]. (a). Total H-labelled material in the culture filtrate - mainly [ Hjpolysaccharides (K. =0.00) and unincorporated [ HJarabinose (K,=0.96). (b) The intermediate fractions (shown in black in Fig. a) were pooled, concentrated and le-chromatographed on the same column and the fractions were assayed specifically for [ HjXGOs.

Fig. 5 HPLC separation of the water-soluble sugars extracted from wheat bran, MCC and cellulose powder CFll. Based on known standards, the elution peaks labeled A through I contained hextose, pentose, tetraose, xylotriose, xylobiose, glucose, xylose and arabinose, respectively...

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