Plasma Membrane-Bound Sialidase

Recent studies have suggested that certain plasma membrane-bound siali-dases are linked to the membranes via a GPI anchor, as observed with the sialidase from T. cruzi (Rosenberg et al., 1991). Phosphatidylinositol-specific phospholipase C (PIPLC) released sialidase activity (28% of total) from purified synaptosomal membranes of pig brain, but not from lysosomal membranes (Chiarini et al., 1990). The presence of GPI-anchored sialidase was also shown in human erythrocyte membranes (Chiarini et al., 1993). On the other hand, the sialidase activity in rat brain myelin could not be solubilized with PIPLC despite a concomitant release of alkaline phosphatase activity (about 40% of total), suggesting a different mode of association of the enzyme with the membranes (Saito and Yu, unpublished data). 

Sialidase activities directed toward ganglioside substrates in plasma membrane and lysosomal fractions can be distinguished based on their responses to detergents and inhibitors. The requirement of detergents significantly differs between both enzyme activities (see Section 5.1). Heparin or heparin sulfate almost completely inhibits the GM3 sialidase activity in the plasma membrane preparation, but only slightly affects the lysosomal enzyme activity. The inhibition of the plasma membrane enzyme, but not the lysosomal enzyme, can be 

Substrate Specificity of Plasma Membrane-Bound Sialidases  

Substrates Rat liver Human brain synaptosomes Rat brain myelin Human erythrocytes  

Recently, a specific antibody against the purified cytosolic sialidase from rat skeletal muscle was prepared and its immunoreactivity with other sialidases examined (Miyagi et al., 1990b). The antibody completely immunoprecipitates the cytosolic enzyme in rat liver and skeletal muscle, but not the synaptosomal and lysosomal enzymes in rat brain and liver. These results suggest that a single cytosolic sialidase species may exist in rat tissues which is immunologically distinct from the lysosomal and plasma membrane enzymes. 

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R. W. Egan, and M. M. Billah, Induction of lysosomal and plasma membrane-bound sialidases in human T-cells via T-eell receptor, Biochem. J., 380 (2004) 425 33. 

In studies with other systems, Glick et al. (1971) found sialidase, active toward fetuin, to be enriched in the lysosomal fraction of L cells Tulsiani and Carubelli (1971) found sialidase, active toward sialyllactose, not only enriched in lysomes isolated from rat mammary glands, but also in the soluble fraction Gielen et al. (1973) found a membrane-bound sialidase in leukocytes that was very active toward glycoproteins in the leukocyte homogenate and Bosmann (1974) found sialidase activity associated with the plasma membranes of human erythrocytes. Recently, Kishore et al. (1975) have reported the presence of a sialidase in Golgi isolated from rat liver. 

The activity of several lytic enzymes, including sialidase, is necessary for fertilization. While sperm are in the seminal plasma, inhibitors of these enzymes are present in the decapacitation factor (DF). Sialidase was found to be a component of sperm acrosomes by Srivastava et aL (1970). They found sialidase activity and a sialidase-like factor which rendered bound sialic acid reactive in the thiobarbituric acid assay (Warren, 1959) but did not release it from the molecule and, upon purification, appeared to be converted to sialidase. An inhibitor of the sialidase-like factor was found in a partially purified DF preparation. The DF is lost during capacitation, which can be broadly defined to include the changes sperm undergo to enable them to penetrate the vitelline membrane of the egg. Gould et aL (1971) have proposed that the role of sialidase in fertilization is to alter the tertiary and quaternary structure of sialoglycoprotein present in the zona pellucida to cause the zona pellucida to be less penetrable by spermatozoa and serve to reduce or block polyspermy. 

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