Studies on the Soluble and Lysosomal Neuraminidases of Rat Liver

The presence of two neuraminidases in rat liver, one strongly bound to the lysosomes and another that occurs in soluble form in the cytosol has been established. The properties of these two enzymes differ markedly. The pH optimum, with neuramin lactose as the substrate, is 5.8 for the soluble neuraminidase (5.2 after partial purification) and 4.4 for the lysosomal enzyme. Soluble neuraminidase is inhibited by Cu2+, Hg2+, and Zn2+ which have no effect on the lysosomal enzyme. Conversely, Na+, K+, and Li+ inhibit the lysosomal but not the soluble neuraminidase. The soluble enzyme hydrolyzes low molecular weight substrates such as neuramin lactose, neuramin lactose sulfate, and a sialoglycopeptide fraction isolated from a Pronase digest of ovine submaxillary glycoprotein but no activity can be detected when brain gangliosides and intact ovine submaxillary glycoprotein are used as substrates. The lysosomal neuraminidase, on the other hand, hydrolyzes all these substrates very efficiently.