Disaccharides, Blood

An important source of galactose in the diet is the disaccharide lactose present in milk. Lactose is hydrolyzed to galactose and glucose by lactase associated with the brush border membrane of the small intestine. Along with other monosaccharides, galactose reaches the liver through the portal blood. 

Fructose is found in honey and fruit and as part of the disaccharide sucrose (common table sugar). Sucrose is hydrolyzed by intestinal brush border sucrase, and the resulting monosaccharides, glucose and fructose, are absorbed into the portal blood. The liver phosphorylates frurtose and cleaves it into glyceraldehyde and DHAP. Smaller amounts are metabolized in renal proximal tubules. The pathway is shown in Figure 1-12-7 important enzymes to remember are ... 

An exotic function of glycoproteins is to act as antifreezes. Specifically, a number of Antarctic fish live in water cooled to about -1.9°C, a temperature below the freezing point of water and below that where the blood, mostly water, of these fish is expected to freeze. Clearly, this would be a disaster for these fish. They are saved from this fate by antifreeze glycoproteins. These proteins contain about 50 repeats of the tripeptide Ala-Ala-Thr. To each of these threonine residues is hooked a specific disaccharide. 

Trifluoroacetic acid is volatile, and thus readily removed. This acid was used by Albersheim and coworkers for the hydrolysis of plant cell-walls,39 and has since been employed for cell walls,40-43 plant mucilages,44 blood-group oligosaccharides,45 peptidogalactoman-nans,46 heparin,47 and disaccharides in blood and urine.48,49 It has also been suggested as an alternative to 6 M hydrochloric acid in the determination of amino sugars,50 and for the hydrolysis of polyalcohols produced by periodate oxidation of polysaccharides.503 Lee... 

In model studies,224,225 oligosaccharides have been separated as their trifluoroacetates, and Vilkas and coworkers224 found that a tetra-saccharide so esterified had a retention time of six minutes at 250°. Similar results were obtained by Nakamura and Tamura,49 who showed that, on a column of OV-1 at 220°, the retention time of per-0-(trimethylsilyl)sucrose was 12 min, whereas, at 140°, the per(tri-fluoroacetate) required only 7 min. Other comparative data were provided for the common disaccharide alditols. A study of the concentration of lactose in blood was made by using this method 48 it was also shown48 that trifluoroacetylation of disaccharides proceeds more rapidly and satisfactorily in N,N-dimethylformamide than in ethyl acetate. 

D-Galactose, a product of hydrolysis of the disaccharide lactose (milk sugar), passes in the blood from the intestine to the liver, where it is first phosphorylated at C-l, at the expense of ATP, by the enzyme galactokinase ... 

Fischer performed a few experiments with the natural disaccharide trehalose. It does not react with phenylhydrazine. A diastase from. green malt has no action on it Froliberg yeast has a weak action (1895). Carp blood hydrolyzes it rapidly, in contrast to the blood of other fishes. Extracts of the mucous membrane from the small intestines of horses and cattle are also active (1906). A. Kalanther, in Fischer s laboratory, found, in 1898. that wine yeasts also hydrolyze this disaccharide. 

Under the experimental conditions used earlier for blood-group substances (0.2M NaOH, 0.26M NaBH4, room temperature, but for only 40 min) about 35% cleavage of the 1 — 3 bond occurred. With M NaBH4 and 0.05M NaOH at 50 °C for several minutes, complete reduction of the hexosamine residue occurred without cleavage of the disaccharide. 

Glucose is by far the most abundant monosaccharide it occurs free in fruits, plants, honey, in the blood of animals, and combined in many glycosides, disaccharides, and polysaccharides. The structure and properties of glucose will be considered in greater detail than those of the other monosaccharides, not only because of its importance, but because much of what can be said about glucose also can be said about the other monosaccharides. 

Fig. 8. Solid-phase synthesis sequence to fully protected blood group determinant tetrasaccharide 17 starting from disaccharide acceptor resin 13. Reagents and conditions (a) 7b, DMTST, DCM, MS 4 A (b) NaOMe, MeOH, THF (c) 12, DMTST, DCM, MS 4 A (d) (i) 5% hydrazine hydrate, DMF, (ii) Ac20, Py (e) NaOMe/MeOH.

Paulsen and co-workers [191] synthesised the trisaccharide (263) representing the afucosyl end group of the blood-group A (type 1) determinant from the disaccharide (264). This was acetylated and deacetonated and the product converted into the acetate (265) via the 3, 4 -orthoacetate. Condensation of (265) with the azido-chloride (237) in the presence of silver perchlorate gave the a-linked trisaccharide (266) which was deprotected to give (263). 

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