Combinatorial synthesis of carbohydrates.

Combinatorial chemistry principles have been applied to the generation of oligosaccharide libraries, both in solution and on the solid phase, with a view to producing inhibitors of carbohydrate-protein binding. The rich stereochemistry and high degree of functionalization of sugars has also resulted in their increasing use in the synthesis of glycomimetics and as scaffolds for the presentation of pharmacophore groupings to receptors that are noncarbohydrate-recognizing proteins.     

The formation of malonaldehyde in irradiated carbohydrates

Pulse radiolysis and 60Co γ-radiolysis have been used to elucidate the mechanism of formation of malonaldehyde in irradiated carbohydrates. Reaction of HO at C-5 and C6 of d-glucose may produce 3 mol of malonaldehyde. Substitution of C-2, C-3, and C-6 decreases the yield of malonaldehyde, but substitution of C-1 by phosphate (as in α-d-glucopyranosyl phosphate) has no effect. Mechanisms are proposed for the formation of malonaldehyde in irradiated pentoses, disaccharides,and polyhydroxy alcohols.     

The chemistry of insect hemolymph. II. Trehalose and other carbohydrates

α,α-Trehalose, a sugar previously regarded as a product characteristic of certain lower plants, has been identified as a major blood sugar of insects. Trehalose has been isolated in pure form from the blood of pupae of the silk moth, Telea polyphemus, and has been recognized chromatographically in all the insects examined, which comprise 10 species belonging to 5 different orders. Trehalose has been determined quantitatively with anthrone after either chromatographic separation or chemical degradation of other sugars. In larvae and pupae of 4 species of Lepidoptera it ranges from 0.2 to 1.5 gm. per 100 ml. of blood and makes up over 90 per cent of the blood sugar; in larvae of a sawfly, about 80 per cent of the blood sugar is trehalose. In Bombyx mori and Platysamia cecropia, the pupal blood trehalose level is about half that in the mature larva, suggesting utilization of trehalose for glycogen synthesis during pupation. Small amounts of glucose and apparent glycogen are also present in the plasma of these insects. In Bombyx larval plasma there is also 0.04 to 0.12 gm. per 100 ml. of glucose-6-phosphate and smaller amounts of an apparent ketose phosphate.     

The kinetics of periodate oxidation of carbohydrates 2. Polymeric substrates

A study of the kinetics of periodate oxidation on a series of dextran oligomers and polymers is carried out by isothermal microcalorimetry. In addition to these substrates, some dimeric carbohydrates and hyaluronan were studied. Rate constants were calculated from the calorimetric decay curves, which, properly corrected for calorimetric response, are proportional to the rate of periodate conversion. The dependence of the kinetic rates on the molecular weight of dextran samples and on the substrate concentration, is described in terms of the much higher rates of terminal reducing units. The presence of two sites with comparable reaction rates makes the analysis of the calorimetric curves difficult, even in the simple overall pseudo-first-order condition. The suitability of a phenomenological treatment of kinetic data is explored.     

Cell surface carbohydrates in cell adhesion.

Carbohydrates are ubiquitous constituents of cell surfaces, and possess many characteristics that make them ideal candidates for recognition molecules. In many systems where cell adhesion plays a critical role, carbohydrate binding proteins have been shown to bind to cell surface carbohydrates and participate in cell-cell interactions. Such systems include fertilization, development, pathogen-host recognition and inflammation. In particular the recent discovery of the LEC-CAMs and their importance in leukocyte biology has refocused attention on lectin-mediated cell adhesion. The LEC-CAMs offer good targets for the development of therapeutics based on carbohydrate structures.     

The carbohydrates of the isoforms of three avian riboflavin-binding proteins.

The carbohydrate chains of nine isoforms of chicken egg-white riboflavin-binding protein (RfBP) and six isoforms each of quail egg-white and yolk RfBP have been structurally characterized. The two N-glycosylation sites, Asn36 and Asn147, of the most abundant isoform of each of the three proteins were analyzed in further detail leading to the identification of different glycosylation patterns. In both chicken and quail egg-white RfBP the carbohydrates attached to position 36 had a lower degree of branching and, in the case of the quail protein, this site was only partially glycosylated. A very heterogeneous mixture of complex structures was characteristic of the other glycosylation site. Analysis of the two sites in quail yolk RfBP confirmed this result which agrees with what has been established for hen yolk RfBP. The presence in the three proteins of a highly heterogeneous mixture of differently branched glycans suggests that the differences in isoelectric points, which is a peculiarity of the different isoforms, are probably indeed due to differences in carbohydrate structure.     

Hepatocyte heterogeneity in the metabolism of carbohydrates.

Periportal and perivenous hepatocytes possess different amounts and activities of the rate-generating enzymes of carbohydrate and oxidative energy metabolism and thus different metabolic capacities. This is the basis of the model of metabolic zonation, according to which periportal cells catalyze predominantly the oxidative catabolism of fatty and amino acids as well as glucose release and glycogen formation via gluconeogenesis, and perivenous cells carry out preferentially glucose uptake for glycogen synthesis and glycolysis coupled to liponeogenesis. The input of humoral and nervous signals into the periportal and perivenous zones is different; gradients of oxygen, substrates and products, hormones and mediators and nerve densities exist which are important not only for the short-term regulation of carbohydrate metabolism but also for the long-term regulation of zonal gene expression. The specialization of periportal and perivenous hepatocytes in carbohydrate metabolism has been well characterized. In vivo evidence is provided by the complex metabolic situation termed the 'glucose paradox' and by zonal flux differences calculated on the basis of the distribution of enzymes and metabolites. In vitro evidence is given by the different flux rates determined with classical invasive techniques, e.g. in periportal-like and perivenous-like hepatocytes in cell culture, in periportal- and perivenous-enriched hepatocyte populations and in perfused livers during orthograde and retrograde flow, as well as with noninvasive techniques using miniature oxygen electrodes, e.g. in livers perfused in either direction. Differences of opinion in the interpretation of studies with invasive and noninvasive techniques by the authors are discussed. The declining gradient in oxygen concentrations, the decreasing glucagon/insulin ratio and the different innervation could be important factors in the zonal expression of the genes of carbohydrate-metabolizing enzymes. While it is clear that the hepatocytes sense the glucagon/insulin gradients via the respective hormone receptors, it is not known how they sense different oxygen tensions; the O2 sensor may be an oxygen-binding heme protein. The zonal separation of glucose release and uptake appears to be important for the liver to operate as a 'glucostat'. Thus, zonation of carbohydrate metabolism develops gradually during the first weeks of life, in part before and in part with weaning, when (in rat and mouse) the fat- and protein-rich but carbohydrate-poor nutrition via milk is replaced by carbohydrate-rich food. Similarly, zonation of carbohydrate metabolism adapts to longer lasting alterations in the need of a 'glucostat', such as starvation, diabetes, portocaval anastomoses or partial hepatectomy.     

Synthesis of carbohydrates for applications in glycobiology.

The current status of synthetic carbohydrate chemistry for the provision of biologically active oligosaccharides is summarized. Examples are given to demonstrate that synthetic strategy and methodology are now sufficiently developed that carbohydrate chains containing 2-6 sugar residues can be synthesized with reasonable predictability. Such syntheses, however, remain extremely laborious, taking on average 7 weeks per monosaccharide residue for a trained individual to complete. The use of glycosyltransferases can dramatically speed up this process for the provision of small (mg) quantities of test compounds. It is proposed, and supported by examples, that the most rapid and efficient manner of preparing such quantities may be to chemically synthesize small di- or trisaccharide primers and elaborate these to the required complex oligosaccharides enzymatically.     

Dynamics of extractable carbohydrates in Pisum sativum. III. The effect of IAA and temperature on content and translocation of carbohydrates in pea cuttings during rooting

The number of roots formed in cuttings of pea ( Pisum sativum L. cv. Alaska) was regulated both by the temperature and by IAA, whereas the time to the appearance of the first roots was regulated only by the temperature. Cuttings treated with 10^-3 M IAA had a smaller content of extractable carbohydrates than the control ones irrespective of the temperature. In the bases of cuttings rooted at 25°C the content of extractable carbohydrates was lower than in those rooted at 15°C. Cuttings treated with IAA showed up to elevenfold increase of extractable carbohydrates in the bases at day 3. This increase of soluble sugars was not correlated with the number of roots formed or the speed of rooting. It is concluded that IAA affects the accumulation of carbohydrates, and this is not connected with the rooting ability of the cuttings.