N-Acetylneuraminic acid (NeuAc) (CAS: 131-48-6), also known as sialic acid, is an acetyl derivative of the amino sugar neuraminic acid. It occurs in many glycoproteins, glycolipids, and polysaccharides in both mammals and bacteria. The most abundant sialic acid, NeuAc, is synthesized in vivo from N-acetylated D-mannosamine (ManNAc) or D-glucosamine (GlcNAc). NeuAc and its activated form, CMP-NeuAc, are biosynthesized in five consecutive reactions that form the intermediates UDP-N-acetylglucosamine (UDP-GlcNAc), N-acetylmannosamine (ManNAc), ManNAc 6-phosphate, NeuAc 9-phosphate, and CMP-NeuAc. CMP-NeuAc is transported into the Golgi apparatus and, with the aid of specific sialyltransferases, added onto nonreducing positions on oligosaccharide chains of glycoproteins and glycolipids. NeuAc is widely distributed throughout human tissues and found in several fluids, including serum, cerebrospinal fluid, saliva, urine, amniotic fluid, and breast milk. It is found in high levels in the brain, adrenal glands, and the heart. Serum and urine levels of the free acid are elevated in individuals suffering from renal failure. Serum and saliva Neu5Ac levels are also elevated in alcoholics. A genetic disorder known as Salla disease or infantile NeuAc storage disease is also characterized by high serum and urine levels of this compound. The negative charge is responsible for the slippery feel of saliva and mucins coating the body's organs. This particular sialic acid is known to act as a "decoy"" for invading pathogens. Along with involvement in preventing infections (mucus associated with mucous membranes mouth, nose, GI, respiratory tract), Neu5Ac acts as a receptor for influenza viruses, allowing attachment to mucous cells via hemagglutinin (an early step in acquiring influenzavirus infection). NeuAc is also becoming known as an agent necessary for mediating ganglioside distribution and structures in the brain. Sialic acid (SA) is an N-acetylated derivative of neuraminic acid that is an abundant terminal monosaccharide of glycoconjugates. Normal human serum SA is largely bound to glycoproteins or glycolipids (total sialic acid (TSA): 1.5-2.5 mmol/L), with small amounts of free SA (1-3 umol/L). Negatively charged SA units stabilize glycoprotein conformation in cell surface receptors to increase cell rigidity. This enables signal recognition and adhesion to ligands, antibodies, enzymes, and microbes. SA residues are antigenic determinant residues in carbohydrate chains of glycolipids and glycoproteins, chemical messengers in tissue and body fluids, and may regulate glomeruli basement membrane permeability. Sialic acids are structurally unique nine-carbon keto sugars occupying the interface between the host and commensal or pathogenic microorganisms. An important function of host sialic acid is to regulate innate immunity. Sialic acid is the moiety most actively recycled for metabolic purposes in the salvage pathways in glycosphingolipid metabolism. Sialic acid is indispensable for the neuritogenic activities of ganglioside constituents which are unique in that a sialic acid directly binds to the glucose of the cerebroside, they are mutually connected in tandem, and some are located in the internal parts of the sugar chain. Sialylation (sialic acid linked to galactose, N-acetylgalactosamine, or another sialic acid) represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family.