Catalyzes the removal of O-acetyl ester groups from position 9 of the parent sialic acid, N-acetylneuraminic acid. H2O + N-acetyl-9-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate H2O + N-acetyl-4-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate Inhibited by diisopropyl fluorophosphate and diethyl-P-nitrophenyl phosphate. Disulfide-linked heterodimer of a small subunit and a large subunit. Widely expressed. The two subunits are derived from a single precursor by proteolytic cleavage. Glycosylated.