Sialyltransferases glycolipid

Glycolipid Sialyltransferase Activities in Mouse and Human Tumor Cells... 

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.

Kinetic Properties of Sialyltransferases. The sialyl-transferase activities with the endogenous glycoprotein and glycolipid acceptors in the standard assays (15) were linear with time for at least 60 min, while those with the exogenously added GMi and DS-fetuin were linear with time only for about 30 min (Figure 1). Activities were directly proportional to the amount of enzyme added up to 0.75 mg protein/assay (Figure 2). 

The clear distinction between the glycoprotein and glycolipid sialyltransferases suggests that the extent of sialylation of the two classes of complex carbohydrates in vivo may be under different metabolic controls. This notion is supported by the observation that the developmental profiles of the glycoprotein and glycolipid sialyltransferases are distinctly different. 

Whereas the activities of CMP-NeuNAc lactosylceramide sialyltransferase increase postnatally (38), the activities of the CMP-NeuNAc glycoprotein sialyltransferase decline dramatically during early development (17). Separate regulatory controls on the glycoprotein and glycolipid metabolism have also been implicated in a study on the brain tissues of Tay-Sachs disease patients. 

Figure 4. Effects of nonionic detergents on sialyltransferase activities. X. Endogenous glycoproteins A, exogenous DS-fetuin O, endogenous glycolipids O, exogenous GM, (16).

More recently, studies on Campylobacter jejuni have demonstrated how specific bacterial carbohydrate antigens can mimic the fine structure of human cell surface glycolipid structures and elicit autoimmune reactivity. Yuki et al.61 first identified that the lipooligosaccharide (LOS) of C. jejuni mimicked human G(M)1 ganglioside and subsequently extended these studies to demonstrate that specific single amino acid variants of the C. jejuni Cst-II sialyltransferase protein are responsible for the synthesis of LOS variants that in turn, mimic different... 

The substrate requirements, linkage specificity, and kinetic mechanism of an q -2,3-sialyltransferase have been compared with an a-2,6-sialyltransferase from porcine submaxillary mucin.The former enzyme will specifically modify glycoproteins and glycolipids which contain 2-acetamido-2-deoxy-3-O S-D-galactosyl-D-galactose sequences. The only acceptor substrates reported for the a-2,6-sialyltransferase are those glycoproteins containing the structure (58). ... 

Sialyltransferases. In eukaryotic cells, addition of terminal sialic acid to glycoconjugates is carried out by specific enzymes called sialyltransferases (STs). STs catalyze the transfer of a sialic acid from the activated nucleotide sugar donor CMP-N-acelylneuraminic acid (CMP-NeuSAc) to the terminal nonreducing position of oligosaccharide chains either of glycoproteins or glycolipids. 

---