The Action of Sialidases on Substrates ContainingO-Acetylsialic Acids

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Abstract
O-Acetyl substitution of sialic acids in glycoconjugates reduces the rate of action of sialidases on these substrates. A plasma glycoprotein fraction and an erythrocyte ganglioside containing 4-0-acetylsialic acids were isolated and characterized from equine blood, and a sialyllactose preparation with Neu5,9Ac2 was purified from rat urine. Using the novel substrates II3 Neu4Ac5Gc-LacCer and II3Neu5,9Ac2-Lac the influence of individual mono-0-acetylated sialic acids on bacterial and viral sialidases could be clearly shown. This extends and clarifies observations with glycoproteins containing mixtures of mono-, di- and higher 0-acetylated sialic acids with substitution at the hydroxyls on carbons 4, 7, 8 and 9. A 4-0-acetyl substitution in sialic acids blocks the action of bacterial sialidases for substrates containing these derivatives, while viral enzymes show low but significant activity, reflected in Km and Vmax values. A small reduction in bacterial sialidase activity was observed for II3Neu5,9Ac2-Lac relative to II3Neu5Ac-Lac in agreement with kinetic analysis. Newcastle disease virus sialidase showed a 50% reduction in hydrolysis rate for the 9-0-acetylated substrate and ten-fold reductions of both Km and Vmax values.