Usually, there is only oneO-acetyl group, mostly at
O-9, but di- and tri-O-acetylated Sia are known,
especially in mucins from the bovine submandibular
gland and the human colon (Corfield and Schauer,
1982a). Lactyl or phosphoryl residues may occur atO-9 and methyl or sulfate groups atO-8. All these different substituents may be combined, e.g., 8-O-methyl with 9-O-acetyl and N-glycolyl, yielding the manifold types of Sia found throughout the animal kingdom. Apart from these types, unsaturated Sia as well as anhydro and lactone forms have been identified in various biological sources.