Glycoproteins desialylated

Fig. 4 Stereoselective binding study of verapamil to AGP and desialylated AGP using frontal analysis. Electropherograms of 50 /aM racemic verapamil solution (A), 50 /U.M verapamil solution in 50 jjM human al-acidic glycoprotein solution (B), and 50 /jM verapamil solution in 50 /jM asiaolo al-acidic glycoprotein solution (C). (Reprinted with permission from Ref. 54. Copyright 2001 Elsevier Science.)...

These studies demonstrating a protective effect of sialic acid residues on serum glycoproteins provide an explanation for earlier, conflicting observations about the biological effect of, for example, desialylated erythropoietin, which stimulates erythropoiesis only after direct application to bone-marrow cell-cultures, and not after injection into the blood stream.469 In the latter experiment, only the native, sialylated hormone was active. Rapid clearance and inactivation of follicle-stimulating hormone,470 or interferon,471 after treatment with sialidase may be explained by uptake into liver cells. 

Sialic acid seems to be involved not only in regulation of the lifetime of soluble, serum glycoproteins but also of mammalian blood-cells. It was observed by Woodruff and Gesner474 that desialylated lymphocytes are reversibly trapped in liver they recirculate to the blood stream after about 24 h. This phenomenon was confinned with Listeria-specific, mouse T lymphocytes, which accumulated in the liver for one day, in contrast to the control cells.60 Reappearance of these cells in the circulation after one day may be explained by re-sialylation of their membrane glycoconjugates. This time period is in the range observed for the turnover of sialic acid in cell membranes, lasting, for example, for 33 h in rat-liver hepatocytes.475... 

These, and other, mechanisms possibly involved in erythrophagocytosis will have to be elucidated in the future. Also shoidd be investigated the matter of whether antigenic sites (proteins or lipids), other than the desialylated glycoproteins and glycolipids having penulti-... 

Figure 6. Effect of CMP-NeuAc concentration (V/S), of pH (V/pH), of enzymatic protein concentration (V/protein), and of incubation time (V/t) on the activity of synaptosomal membrane-bound sialyltransferase. Calf brain cortex. Acceptor substrates for sialyltransf erase (if) lactosylceramide ( ) desialylated fetuin (%) endogenous glycoprotein (endogenous glycolipids.

The Km value for exogenously added GM is 0.2 mM, which is identical to the value obtained with the chick embryonic brain (24) but is lower than the value reported for the rabbit neurohypophysis (25). Substrate inhibition at high glycolipid concentration as observed with other glycosyltransferases (22,26,27) was also noted in this study when the concentration of GM was above the Km (15). The Km value of the exogenously added DS-fetuin is 0.15 mM or 1.2 mM in terms of acceptor sites (15). This value is only one third of that obtained for desialylated a -acid glycoprotein which is a much less efficient acceptor than DS-fetuin (23). 

Compound 8 is a diantennary, asialo glycopeptide that can be isolated from various desialylated glycoproteins.1-4 The 500-MHz, H-n.m.r. spectrum of this nonreducing compound, obtained from cq-acid glycoprotein,66-70 is given in Fig. 8, and the n.m.r. parameters are listed in Table V. Comparison of the spectrum of 8 with that of 7 demonstrates that, for the glycopeptide, the spectrum is less complex, despite the additional residues. 

Removal of senescent red blood cells from the circulation has been attributed to desialylation of the membrane glycoproteins. In vitro removal of sialic acid from human red blood cells and introduction of the modified cells into the circulatory system result in drastic shortening of their life span. However, aging and removal of red blood... 

Over 95% of plasma iron is in the Fe + state bound to the glycoprotein transferrin, a monomeric /I i-globulin (M.W. 80,000). Electrophoretic studies have revealed the existence of 21 genetic variants. In some, single-amino-acid substitutions account for variation in electrophoretic mobility. Transferrin is synthesized primarily in the liver and appears at the end of the first month of fetal development. Its half-life in humans is about 8 days. Desialylation may be a requirement for its removal from plasma by the liver, as it is for other plasma proteins (Chapter 10). In fact, asialotransferrin is more rapidly cleared from plasma than transferrin. It is not required for intestinal absorption of iron. 

The concentration of urinary glycoproteins may play an important role in stone formation. Therefore, the level and activity of urinary proteases, for example, urokinase and plasmin, might be of great significance. According to this hypothesis, low activities or decreased production of urinary urokinase or plasmin may increase the urinary uromucoid concentrations, as well as stone formation. Increased levels of a urokinase inhibitor, namely urinary trypsin inhibitor (67,000 Da) and decreased urokinase activity in the urine of stone formers are well established (T2). Urinary urokinase activity was therefore studied, as well as the desialylation of urinary glycoproteins. (Fig. 4). 

In contrast to urinary uromucoid, which consists of about 9% sialic acid (B3), stone uromucoid contains no sialic acid (K1,M1). This desialylation of mucosub-stances to a mineralizable matrix may occur in the urine, because sialidase is one of the renal enzyme systems regularly present in urine (Ml). Consequently, significant higher urinary sialidase activities are present in stone patients than are found in healthy males (V4). The age of the patient also correlates with the sialidase activity. The sialidase activity increases with age. Therefore, it is expected that elderly patients may have more desialylated glycoproteins, which increases the risk for stone formation. 

The cDNA of the sialic-acid-specific lectin of S. sanguis codes for a polypeptide of 1435 residues (calculated mw of 158.4 kDa) with three unique domains, two of which consist of repetitive amino acid sequences [66]. The third, which resides near the earboxy terminus, contains 48% proline. The lectin bound to a single salivary glycoprotein of mw 400 kDa [66]. Binding was inhibited by sialic acid and was abolished by desialylation of the glycoprotein the best inhibitor was A -acetylneuraminyllactose. 

Glycosyl transferases are quite powerful for stepwise addition of glycosyl units to existing (neo)glycoproteins. Paulson et al. [36] used a-2,3- and a-2,6-specific sialyl transferases to resialylate red blood cells that had been extensively desialylated enzymatically. Since... 

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