Sialic acids influency

As terminal constituents of glycoconjugates, sialic acids are involved in a wide range of biological phenomena. Different approaches and strategies have been developed to improve yield and stereoselectivity in sialylation reactions. Recently it has been reported that a substituent at the C-5 amino group in sialic acid influences the reactivity of both sialyl donors and acceptors. This review sununarizes the latest accomplishments in sialylations using different N-substitued sialyl derivatives. 

The availability of, and analytical data for, these standards arc prerequisites for quantitative and qualitative analysis of the yield of modified sialic acids from periodate-borohydride-treated cells, and, correspondingly, for evaluation of the influence of such modifications on the biological behavior of cells. In such an experiment, related to a study of the life expectancy of rabbit erythrocytes, the simultaneous analysis of Neu5Ac, Neu5Gc, and their C7 and C8 analogs from rab-... 

Little is known of any regidation of the metabolism of sialic acid on a level higher than the enzymic level just described. The few experiments performed with animals point to an influence exerted by hormones. Thus, increase of the synthesis of sialoglycoeonjugate (mucus) has been described as occurring in mouse vaginal-epithelium under the influence of estrogen.240 Similarly, insulin therapy restored the sialic acid content of rat hepatocyte-membrane which was decreased in streptozotocin-induced diabetes.241... 

Several observations have been made with regard to an influence of age on the sialic acid content of tissues. A marked diminution of sialic... 

Nonphysiological compounds have also been described as influencing the overall metabolism of sialic acid. Administration of ethanol (2 g/kg) to rats significantly decreases the sialic acid content of brain tissue.246 Convulsions induced by pentylenetetrazole (6,7,8,9-tetrahy-dro-5/f-tetrazoloazepine) are accompanied by a diminution in the rate of biosynthesis of polysialogangliosicles GT, and GQn, in rat brain.227 Such ManNAc analogs as 2-acetamido-l,3,4,6-tetra-0-acetyl-2-deoxy-D-mannopyranose or the 2-(trifluoroacetamido) derivative lead to a marked lowering of the incorporation of radioactivity from labelled ManNAc into glycoconjugate sialic acids of murine, erythroleukemia (Friend) cells.247... 

The mucous secretions are vital, as they act as lubricants and defensive agents, in cavities of the body communicating with the environment, or on body surfaces.229 415 418 452 The influence of sialic acids on the macromolecular conformation seems to be the reason for the proteolytic resistance of several sialo-glycoproteins. The first example of such a role for sialic acids came from studies by Faillard and Pribilla453 that demonstrated a loss of the... 

The frequent occurrence of sialylated enzymes, or even of multiple forms, which are sometimes tissue-dependent, with a varying number of sialyl residues as, for example, in y-glutamyltranspeptida.se (EC 2.3.2.2),456,457 is not yet fully understood. Although the activity of most of these enzymes is not influenced by removal of sialic acid,454 the activity of monoamine oxidase A (EC 1.4.3.4) of outer mitochondrial membranes of rat liver has been shown to be destroyed by treatment with sialidase438 the substrate specificity of acetylcholinesterase (EC 3.1.1.7) is altered,459 the kinetic properties of human acid and alkaline phosphatases (EC 3.1.3.1 and 3.1.3.2) are changed, and the stability of a-D-galactosidase (EC 3.2.1.22) is drastically lowered.415 In these cases, an influence of sialyl residues on the conformation of the enzyme is assumed, but awaits firm evidence. 

Knowledge concerning structural features of sialic acids necessary for their anti-recognition effect in erythrocytes is still scanty. The sialic acid side-chain seems not to be involved in this function, because shortening by periodate-borohvdride treatment does not significantly influence the viability of the erythrocytes.142 It is considered that the carboxyl group of Neu plays the main role in the protective effect of sialic acid residues. 

Dang, C. V., Shin, C. K., Bell, W. R., Nagaswami, C., and Weisel, J. W. (1989). Fibrinogen sialic acid residues are low affinity calcium-binding sites that influence fibrin assembly./. Biol. Chem. 264, 15104-15108. 

In 60, the additional N-acetyllactosamine unit is terminated by NeuAc in a-(2—>3) linkage to Gal-c this can be inferred from comparison of the set of chemical shifts of the NeuAc structural-reporter groups for 60 with those reported to be characteristic for the aNeuAc-(2—>3)/3Gal( 1—>4 )/3GlcN Ac( 1—> ) sequence (compare Table XIV). As the influences of sialic acid in a-(2- 3) linkage to Gal are well described (see Table XIV), the values of chemical shift of the structural-reporter-group signals for an additional, asialo N-acetyllactosamine unit can be predicted. 

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