Synaptosomes from guinea‐pig cerebral cortex contain a fetuin: sialyl glyco‐protein: glycosyl transferase; evidence is presented which indicates that both a sialyl transferase; evidence is presented which indicates that both a sialyl transferase and endogenous acceptors were located in the synaptosome ‘ghost’ fractions. Following solubilization of synaptosomes with Triton X‐100 and the use of fetuin minus NANA as acceptor, 25 per cent of the transferase was recovered after centrifugation and column chromatography on Sephadex G‐100 and G‐200 with a 64·0‐fold purification. The enzyme had a pH optimum of 6·3, required no divalent metal cation for activity, and exhibited high activity with either fetuin minus sialic acid, prothrombin minus sialic acid, Tamm‐Horsfall glycoprotein minus sialic acid, or orosomucoid minus sialic acid as acceptor; neither BSM nor PSM minus NANA functioned as an effective acceptor. The fetuin:sialyl transferase using fetuin minus sialic acid and CMP‐sialic acid as substrates a and b, respectively, gave the following kinetic constants when using the Cleland bisubstrate model: Ka= 35μM; Kb= 3 μM; Kia, = 25 μM; Kib= 25μM; and V1= 92 pmoles. min−1.mg−1 of protein. The following divalent cations inhibited the reaction: Ba2+ > Hg2+ > Pb2+ > Cu2+.
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H B Bosmann (1973) studied this question.
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