The INDIVIDUALIST

A Wiki about biochemical individuality

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> * [[Siglecs]]


Immunology

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Description

It should be noted that the term "sialic Acids" refers to the group of sugars including neuraminic Acid and its derivatives, not to a specific sugar.

Serum neuraminic (sialic) acid is a protein-bound carbohydrate considered to be a mono-saccharide and occurs in combination with other mono- saccharides like galactose, mannose, glucosamine, galactosamine and fucose. Sialic acid is the group name for the acetylated neuraminic acids, such as N-acetyl neuraminic acid (NANA), N-glycolyl neuraminic acid and Di-acetyl neuraminic acid.

http://glycores.ncifcrf.gov/mono/images/d-neup.jpg

Discussion

Neuraminidase is an enzyme that catalyzes chemical reactions of N acetylneuraminic acid and other neuraminic acids, which are members of a group of substances classed as sialic acids. Neuraminic acid is one of the major species of sugars found in humans and it is a component of practically all the glycoproteins. N-acetylneuraminic acid and other neuraminic acids are the predominate sugars in glycoproteins.

Tamio Yamakawa of the University of Tokyo suggested that dogs may possess a blood-group system specified by the sialic acid in red-blood- cell glycolipids. Whereas all -European dogs so far examined have glycolipids that incorporate acetylneuraminic acid, Yamakawa and his co-workers have shown that representative Japanese dogs such as the Kishu and Shiba breeds often have glycolyneuraminic acid instead and that this occurrence is genetically determined.

Akita and Hokkaido dogs from northern Japan seem to be exceptional in having only acetylneuraminic acid in their red-blood-cell glycolipids. The origin of the Japanese dog is still controversial, but since the glyconeuraminic acid glycolipid is inherited as a dominant trait, the findings suggest that the origins of the Akita and Hokkaido breeds are different from those of other Japanese dogs and that the Akita and Hokkaido breeds are related to European dogs.

Old red blood cells have less sialic acid on their surface than young ones, and so it has been postulated that the decrease of sialic acid is the sign responsible for the removal of the older red blood cells from the circulatory system. This hypothesis seemed to be further substantiated by the finding that when red blood cells are taken out of the circulation, and when the sialic acid is removed from their surface and they are re-injected into the blood, their life span is extremely short: only a couple of days out of the normal lifetime of 120. In spite of these striking correlations there is considerable doubt whether the removal of sialic acid and the exposure of galactose units on the surface of the red blood cell are responsible for the removal of senescent red cells from the blood under physiological conditions in vivo.

Sambucus (elderberry) lectin is highly specific for sialic acid.

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