Sialic acid is a generic term for the
N- or
O-substituted derivatives of
neuraminic acid, a
monosaccharide with a nine-
carbon backbone. It is also the name for the most common member of this group,
N-acetylneuraminic acid (Neu5Ac or NANA). Sialic acids are found widely distributed in animal tissues and to a lesser extent in other organisms, ranging from plants and fungi to yeasts and bacteria, mostly in
glycoproteins and
gangliosides (they occur at the end of sugar chains connected to the surfaces of cells and soluble proteins). That is because it seems to have appeared late in evolution. However, it has been observed in
Drosophila embryos and other insects and in the capsular polysaccharides of certain strains of bacteria. In humans the brain has the highest sialic acid concentration where they have an important role in neural transmission and ganglioside structure in
synaptogenesis. In general, the amino group bears either an acetyl or a glycolyl group, but other modifications have been described. These modifications along with linkages have shown to be tissue specific and developmentally
regulated expressions, so some of them are only found on certain types of
glycoconjugates in specific cells. The hydroxyl substituents may vary considerably; acetyl, lactyl, methyl, sulfate, and phosphate groups have been found. The term "sialic acid" (from the Greek for
saliva,
σίαλον/
sialon) was first introduced by
Swedish biochemist
Gunnar Blix in 1952.