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. Isoform 1 is widely expressed. Isoform 2 shows a more restricted distribution with highest expression in brain and ovary and lower levels in liver and thymus. The two subunits are derived from a single precursor by proteolytic cleavage. The lysosomal isoform is glycosylated.