Soluble Serum Sialyltransferases

Soluble sialyltransferase activity has often been measured in the serum of tumour patients and significant elevation detected (Bernacki and Kim 1977, Ganzinger 1977, Henderson and Kessel 1977, Coombes et al 1978, Ip and Dao 

Fox et al 1979). Liver, platelets and erythrocytes have been proposed as sources of the enzyme under normal conditions (Kim et al 1971, Bosmann 1972, Mookeriea et al 1972). Neoplastic cells have been implicated as a source of the transferase in disease (Bosmann and Hall 1974, Bosmann and Hilf 1974, Bernacki and Kim 1977). At present the use of such measurements in diagnosis is not practicable, as too little characterization has been carried out. 

The catabolism of the sialic acids is inseparable from the biosynthesis of the monosaccharides themselves and the various sialyltransfer reactions described in the previous subsections. Most information is available in the field of sialidase research, as will be evident below. However, the complete catabolism of the sialic acids entails several components, shown in Fig. 2, which exhibit links to all parts of the metabolic routes involving sialic acids. 

The occurrence of an enzymic activity cleaving CMP-Neu5Ac to CMP and Neu5Ac has been described in sheep brain (Shoyab and Bachhawat 1967), rat liver and isolated hepatocytes (Kean and Bighouse 1974, van Dijk et al. 1977), calf kidney cortex (van Dijk et al. 1976) and human ovary and serum (Chatterjee et al. 1978). The existence in serum has allowed monitoring of this activity in patients with malignant disease, where an elevation of activity was observed in cases of ovarian cancer (Chatterjee et al. 1978). 

Careful analysis of the subcellular location has identified the plasma membrane as the predominant cellular site (Kean and Bighouse 1974). Van Dijk et al. (1977) have further demonstrated that the enzyme activity is regional and with its functional centre directed towards the outside of the cell. 

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Sialyltransferases can be solubilized from their subcellular site by using detergents, and be purified by affinity chromatography on, for example, CDP-6-aminohexanol-agarose,267 as already described. Solubilization of frog- and rat-liver sialyltransferases by means of Triton X-100 has been described.27 Soluble sialyltransferase occurs in colostrum, and is also present in small quantities in normal blood-serum. From the latter source, the enzyme was purified 300-fold by poly (acrylamide) gel-electrophoresis.2 ... 

The sialyltransferases are membrane-bound proteins located in the endoplasmic reticulum (ER) and in the Golgi apparatus. Information about their sequence homology is limited, but they do appear to share a common topography [35]. A catalytic domain resides at the C-terminus followed by an N-terminal segment that anchors the enzyme into the ER or Golgi membrane. Soluble, catalytically active sialyltransferases that lack the anchor segment have been isolated from milk, serum, and other body fluids, suggesting that this N-terminal anchor is not necessary for the enzyme to retain catalytic activity. However, the ability to obtain from natural sources quantities of most sialyltransferases that would be needed for synthesis applications is hampered by low tissue concentrations and difficult purifications. 

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