[ Special thanks to The Vampire Network for allowing us to re-share this article! ]
All nucleated cells are covered with a dense and complex coat of sugar chains (glycans) or protein-sugar chains (proteoglycans) (1). Sialic acids are nine-carbon acidic sugar in which the individual sugar moieties (monosaccharides) are linked together to form multi-sugar chains (polysaccharides) that occur at the end of most sugar chains attached to the surfaces of cells (2).
They comprise a family of 43 naturally occurring derivatives. One branch of the sialic acid family are the N-acetylneuraminic acids, which are the most widespread form of sialic acid and almost the only form found in humans (2). However, different forms of sialic acids are found in other animal species and in human pathologies (ex. cancer). Furthermore, sialic acids can be modified on many of the nine carbon moieties resulting in remarkable structural diversity. These altered forms may confer altered function (2, 3).
Sialic acid is synthesized endogenously (by the cells of the individual organism itself) from simple sugars (glucose and fructose) and the essential amino acid glutamine in a complex multi-step enzymatic reaction (2). The rate-limiting enzyme in the synthesis of sialic acid in humans and all mammals is UDP-Nacetylglucosamine-2-epimerase, which is expressed in the liver. Its expression is low at birth and early in development. Consequently, studies have shown that sialic acid must be supplemented from external sources early during postnatal life (2). Failure to do so has been associated with cognitive retardation and increased susceptibility to infection as well as faulty development of the immune system (2).
Sialic acid is expressed in nearly all bodily tissues, organ systems and secretions where it plays major roles in their function. Several of these and their significance to vampirism are highlighted in this review.