Characterization of sialidase activity in mouse synaptic plasma membranes and its age-related changes.

Sialidase activity in synaptic plasma membranes (SPM) isolated from C57BL/6 mouse brain was examined using exogenous ganglioside substrates. The enzyme activity directed toward GM3 showed sharp pH dependency with optimal pH of 4.0, and was greatly enhanced by Triton CF-54, Nonidet P-40 or CHAPS. The apparent Km and Vmax values for enzyme activity in SPM were 11 microM and 164 pmol/mg protein/min, respectively. Examination of sialidase activities in subcellular fractions of brain tissues showed the enrichment of enzyme activity in SPM prepared from either young adult or senescent mice. Substrate specificity of SPM sialidase was compared with that of myelin sialidase using delipidated, solubilized enzyme preparations. The SPM sialidase hydrolyzed GD1a more effectively as compared with the myelin enzyme. While SPM sialidase could hydrolyze GM1, the hydrolytic rate by the SPM enzyme was significantly lower than that by the myelin enzyme. The sialidase activity in SPM decreased with increasing age; activity was highest between the ages of 4-7 months, decreased to a relatively constant level between 13-25 months, and reached its lowest level at 31 months. These results demonstrate that SPM contain a distinct sialidase activity which is regulated in an age-dependent manner.