Organ Distribution of Mammalian Sialidases

Other organs which have been reported to have sialidase activity are rat liver (Mahadevan et aL, 1967), rat and rabbit kidneys (Mahade-van et al., 1967 Kirschbaum and Bosmann, 1973 Kuratowska and Kubicka, 1967), ram testis (Roston et al., 1%6), human intestinal 

The preceding survey of mammalian organs which have sialidase activity indicates that it is broadly distributed and may be an integral component of all organs containing sialyl compounds. 

Sialidase has been reported to be a soluble enzyme, as well as to be associated with particulate cell fractions. The weight of evidence shows it mainly to be a cell-surface component. It is associated with its endogenous substrate in the plasma membrane of the cell with, nevertheless, some sialidase activity also present in lysosomes. The soluble sialidase may represent small membrane fragments. 

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. 

Whether there is one sialidase active toward the various substrates or several sialidases specific for certain substrates and localized in certain organelles has not been shown. It is not inconceivable that the same sialidase may be found in the plasma membrane and in lysosomes since part of the plasma membrane is taken up into lysosomes in the natural sequence of events during turnover. 

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