Identification of Intrinsic Sialidase and Sialoglycoprotein Substrates in Rat Brain Synaptic Junctions

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Abstract
Sialidase activity associated with rat brain synaptic junctions (SJ) and synaptic membranes (SM) was determined. Both fractions released sialic acid from exogenous glycopeptides and gangliosides. SJ accounted for 5-10% of the total sialidase activity recovered from SM following extraction with Triton X-100, and the specific activity of SJ sialidase was 60% that of the parent SM fraction. Intrinsic SJ sialidase hydrolyzed 12-15% of the sialic acid associated with endogenous SJ glycoproteins. Sialic acid residues associated with SJ glycoproteins were labeled with sodium borotritide and SJ proteins fractionated by affinity chromatography on concanavalin A[Con-A]-agarose. SJ glycoproteins that reacted with Con A (Con A+ glycoproteins) accounted for 25% of the total SJ [3H]sialic acid. Intrinsic SJ sialidase hydrolyzed 20% of the [3H]sialic acid associated with these glycoproteins. Each MW class of Con A+ glycoprotein previously shown to be a specific component of the postsynaptic apparatus contained sialic acid and was acted on by intrinsic SJ sialidase.