Sialyltransferases exogenous acceptors

The requirements for the rat and pork liver sialyltransferases are shown in Table VI. The need for exogenous acceptor is almost absolute and neuraminidase-treated i-acid glycoprotein is far more eflFective than other derivatives, indicating that a terminal galactose residue is required for acceptor activity. The sialyltransferases show no requirements for cations, and the pH optima are 5.7 and 7.0 for the rat and pork liver enzymes, respectively. The Km values for the pork liver enzyme are 0.19 mM for the nucleotide sugar and 0.9 mM for the glycoprotein acceptor (calculated on the basis of available acceptor sites). 

The sialyltransferase transfers H-sialic acid from CMP-N- H-acetylneuraminic acid to endogenous IgG acceptor and to exogenous acceptor prepared by treating rabbit IgG with neuraminidasa Both the transferase and the endogenous product are bound to the microsome fraction. 

Cho, J. W., and Troy, F. A., 1989, Gangliosides as exogenous acceptors to map the acceptor sugar requirements of the poly-a2,8-sialyltransferase in Escherichia coli kl, Proc. Xth Int. Symp. Glycoconjugates, p. 143. 

Total sialic acid content was found to be lower in several SV40-transformed clones of Swiss 3T3 mouse cells (Ohta et aL, 1968). The plasma membrane as well as other membrane fractions of similar cell lines had reduced amounts of sialic acid (Wu et aL, 1969). Isolated plasma membranes from Py-BHK cells had 60% less sialic acid than membranes from normal BHK cells (Makita and Seyama, 1971). These changes appear to be associated with reduced sialyltransferase activities in the transformed cells. Grimes (1970) determined the levels of sialic acid and sialyltransferase activity in normal and SV40-transformed Swiss and Balb 3T3 mouse cells. There was a close correlation between sialic acid content and sialyltransferase activity in each cell line even though enzyme levels were measured with exogenous acceptors such as desialylated fetuin or bovine submaxillary mucin. This correlation has been extended to a number of other cell lines (Grimes, 1973). 

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). 

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