Xylose labeled

After proteolytic digestion of the xylose-labeled trichloracetic acid precipitate, a labeled compound was isolated which comigrated with autlientie 0- 8-D-xyIosyl-L-serine in electrophoresis and in paper chromatography in several solvent systems. Furthermore, the labeled substance... 

Xylose-rich pectic polysaccharide was extracted from defatted and protein-free cell wall preparation (5) using HCl solution (pH 1.6) at 85° C for 4 h. The extract was adjusted to pH 5.0 with ammonia, concentrated on a rotary evaporator under reduced pressure at 40°C, and precipitated with 5 volumes of 96% ethanol. After washing twice with 80% ethanol and drying in an air circulated oven at 40°C for 2 h, the pellet was ledissolved with distilled water and then precipitated with 4 vols 96% ethanol. Before the pellet was gently ground, the precipitated pellet was washed twice with 70% ethanol and dried at 40 ° in an air circulated oven for 16 h. The resultant white powder was labelled "xylose-rich pectic polysaccharide" and stored in a refrigerator. 

Weidenhagen46 obtained L-xylosone in 60% yield on oxidizing L-xylose by his modification of the copper acetate method.46 This method was employed by Salomon, Bums and King63 in the preparation of C14-labeled ascorbic acid and by Hamilton and Smith69 in the preparation of isoascorbic acid. 

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. 

D-[l- C]Xylose was subjected to boiling in 4 M aqueous sodium hydroxide. The resulting mixture contained 2,4-dihydroxybutanoic acid, lactic acid, and D-a,j8-xylometasaccharinic acid. The almost uniform distribution of the C label among the carbon atoms of 2,4-dihydroxybutanoic acid indicated that this acid is probably formed by the recombination of completely isomerized, two-carbon fragments. Fragmentation of D-xylose occurred mainly at one of the central bonds, C-2-C-3 or C-3-C-4. 

At the same time, in this laboratory, we detected the dimethyl acetals of D-xylose and D-glucose by chromatographic resolutions of the products of methanolsis of labelled free sugcirs, and we have, likewise, concluded that they are not primary products but are formed either concurrently with the furanosides or, more probably, from them 8). Fig. 3 illustrates the variations of the main components of the reaction of D-xylose as determined by radiochemical counting of the chromato-graphically resolved components (the pyranosides were vmresolved under the conditions used), and in Fig. 4 the concentration of the acetal is... 

Figure 1. Developing secondary thickened wall of the hypocotyl of Phaseolus vulgaris. The section was treated with an antibody specific for / 1 — 4 linked D-xylose units and stained with gold-labelled goat-antirabbit serum. The label is seen at the secondary thickening (st) and the vesicles of the Golgi apparatus (v).

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. 

As previously mentioned, Kauss (40) has shown that the methyl donor for the formation of 4-methyl-D-glucuronic acid of hemicellulose B proved to be S-adenosyl-L-methionine, the same as in pectin. A particulate preparation from immature com cobs containing hemicellulose B was found capable of transferring the 14C-labeled methyl group from S-adeno-syl-L-methionine to a macromolecular acceptor present in the particles. The radioactive product was shown to be hemicellulose B labeled in the 4-methyl-D-glucuronic acid residues. It was isolated chiefly as 4-methyl-glucuronosyl-( 1 - 2)-D-xylose. 

By adding l4C-labelled sodium cyanide to L-threo-pentos-2-ulose (9), L-[l-14C]ascorbic acid was prepared (see Scheme 2) having an activity592-596 of 2.1 x 108 counts, min-1, mg-1. L-Xylose was prepared by the sequence shown in Scheme 21. The resulting L-xylose was oxidized with cupric acetate to 9 in 40-50% yield. In the second procedure, [1-14C]1 (specific activity 0.1 /iCi. mg-1) was purified by way of 5,6-O-cyclohexylidene-L-ascorbic acid.596... 

Uniformly labelled 1 was prepared by the Reichstein-Griissner synthesis (see Scheme 4), starting with unifonnly labelled D-glu-cose.597-599 L-[2,3,4,5,6-14C]Ascorbic acid was prepared 800 from L-[U-14C]xylose by way of L-threo-pentos-2-ulose (9) (see Scheme 2). The L-[U-14C]xylose was prepared from D-[U-,4C]glucitol by using the route shown in Scheme 21. 

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). 

In other soil incubation studies Cheshire et al. (1974, 1979) found that some sugars in straw in labeled plant material decomposed rapidly, but others decomposed relatively slowly, and about 15% of these remained after five years. There was little indication that xylose or cellulose was synthesized by soil microorganisms. However, studies by Cheshire and Anderson (1975) showed that plant residues are essential to maintain the soil carbohydrate levels. Total carbohydrate contents of soils fallowed for 10 years fell by as much as 50%. Because there was no significant change in the compositions of the soil carbohydrate as the result of the fallow, it was concluded that the sugars were equally susceptible to metabolism regardless of the management system. 

Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material of wood. In hardwoods the primary hemi-cellulose is a xylan (polymer of xylose), whereas in softwoods the primary hemi-cellulose is a glucomannan, although both of these polysaccharides occur to some extent in both types of wood. The DP of the hemicelluloses is much less than that of cellulose, in the range of 100-200. 

It was proved by using labeled compounds that C-l of xylose becomes the formyl group of furfural.12 Further analogous preparative methods have been reported,13 14 and a pyrolytic preparation of furfural from xylose is known.15... 

To examine relative roles of hexose and uronic acid as AsA precursors, the possibility of generating labeled GlcUA from [2-3H]- or [2-14C]MI in situ in strawberry fruit was tested (Loewus, 1965 Loewus et al., 1962). In [2-3H] MI-labeled strawberry fruit, 40% of the 3H was recovered in free D-xylose (Xyl) and uronosyl and pentosyl residues of pectin. In [2-14C] Mi-labeled strawberry fruit, 33% of the 14C was recovered in these products. Only trace amounts of 3H or 14C appeared in AsA (Loewus et al., 1962). D-Galacturonosyl, D-xylo-syl, and L-arabinosyl residues of cell wall polysaccharides as well as free Xyl were degraded to establish the location of the radiolabel. In each instance, it was at carbon 5 (Loewus and Kelly, 1963). 

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