VPB 112: General Veterinary Biochemistry (1+1)

SPHINGOPHOSPHOLIPIDS AND GLYCOLIPIDS

• There are 3 subclasses of sphingolipids. All are derivatives of ceramides, but differing in their head groups. Examples: sphingomyelins, glycolipids, and gangliosides.
• Sphingomyelins (Sphingophospholipid)
  • The sphingomyelins contain amino alcohol, sphingosine (synthesized from serine and palmitic acid) rather than glycerol.
  • A fatty acid is attached to the amino group of sphingosine by an amide linkage, producing  a  ceramide. A structure also found in glycolipids
  • Ceramide can also serve as a precursor of glycolipids.
    The fatty acids found most frequently in sphingomyelins are palmitic, stearic, lignoceric and nervonic acid.
  • The alcohol group at carbon 1 of sphingosine is esterified to phosphorylcholine, producing sphingomyelin, an important constituent of myelin of nerve fibers, which insulates and protects the nerve fibres of CNS.
  • In Niemann-Pick disease, sphingomyelin accumulates in brain, liver and spleen, which are enlarged causing mental retardation and death occurs in early life.
  • On hydrolysis the sphingomyelin yields a fatty acid, phosphoric acid, choline and a complex amino alcohol sphingosine.

• Glycolipids
  • The glycolipids contain ceramide and one or more sugars.
  • They do not contain phosphate group. The sugar containing sphingolipids are also called as  “glycosphingolipids”. Eg., Cerebrosides. These are, ceramide attached to single sugar, which may be either a molecule of galactose or glucose. As they contain neutral sugars, they are called as "Neutral Glycosphingolipids".
  • Galactocerebrosides are present in cell membranes of the nervous tissue (particularly white matter of the brain) whereas glucocerebrosides are present in extra-neural tissues and only traces in brain.
  • In humans diseases called Taysach’s disease and Gaucher’s disease are seen. In these the lipids are not broken down properly and accumulate causing severe brain disorder.
  • It is involved in myelin formation, nerve impulse conduction, signal transduction and as receptor component for certain hormones / bacterial toxins. 
  • It is antigenic.
  • A common fatty acid component of cerebroside is cerebronic acid. In addition, it may contain nervonic or lignoceric acid.
  • Psychosin: When fatty acids are cleaved from cerebroside by alkaline hydrolysis with Ba(OH)2, the remaining glycoside sphiingosin is called as psychosin.

•  Gangliosides
  • They are also called as "Acidic glycosphingolipids", as they contain sialic acids.
  • These are most complex glycosphingolipids, that contain ceramide and one or more molecules of sialic acid (N- acetyl neuraminic acid). Gangliosides are identified as GM1, GM2, GM3, etc, where G stands for gangliosides and M for mono sialic acid. The subscript number is based on the chromatographic migration.
  • Similarly, GD -D for disialic acid, GT - T-trisialic acid and GQ - Q-quadra (four) sialic acids.

  

  • They are predominantly found in ganglions. They are the components of cell surface membrane. It constitutes 6% of the brain lipid. They act as specific receptors for some pituitary hormones and also involved in cell – cell recognition, growth and differentiation of tissues.
• Sulfatides
  • Sulfoglycosphingolipids (sulfatides) are cerebrosides that contain a galactosyl residue that has been sulfated and is therefore negatively charged at physiologic pH.
  • Sulfatides are found predominantly in nervous tissue and in other tissues like liver, kidney and testes. Ratio of cerebrosides to sulphatides in brain is normally 3:1.