Phosphosphingolipids

Sphingomyelin is probably synthesized by an exchange reaction in which the phosphorylcholine moiety of phosphatidylcholine is transferred to ceramide  

Synthesis of glycosphingolipids and sulfoglycosphin-golipids involves the addition of sugar and sulfate residues to ceramide from UDP-sugar derivatives or the activated sulfate donor 3 -phosphoadenosine-5 -phosphosulfate (Chapter 17), and appropriate transferases. These pathways are discussed in Chapter 16. Catabolism of sphingolipids is by specific lysosomal hydrolases. Several inherited disorders associated with the deficiencies of these enzymes are discussed below. 

Pulmonary Surfactant Metabolism and Respiratory Distre.ss Syndrome 

Pulmonary surfactant is a complex of lipids and proteins with unique surface active properties that is synthesized exclusively in alveolar type II cells. The composition of surfactant is 90% lipids and 5-10% surfactant-specific proteins. The lipid component is made up of dipalmitoylphosphatidylcholine (also called lecithin, 70-80%) and another major phospholipid, phosphatidyl-glycerol (PG, 10%). The remainder of the phospholipids of surfactant are phosphatidylinositol (PI), phos-phatidylethanolamine (PE), and phosphatidylserine (PS). Immature surfactant contains higher amounts of PI compared to PG. Thus, a low ratio of PG to PI indicates lung immaturity. Cholesterol, a neutral lipid, is also a constituent of the lipid component of surfactant. 

After synthesis in the various compartments of endoplasmic reticulum of alveolar type II cells, surfactant components are assembled in the cytosol into lamellar bodies. In the process of formation of lamellar bodies, the transfer of phospholipids between membranes is facilitated by phospholipid transfer proteins, which are nonenzymatic proteins found in all eukaryotic cells and which play an important role in lipid metabolism. There are three well-characterized phospholipid transfer proteins  

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Phospholipids (a) glyoerol backbone and phosphoglyoerols, (b) sphingosine backbone and phosphosphingolipids. 

Phosphosphingolipids Phosphonosphingolipids Neutral glycosphingolipids Acidic glycosphingolipids... 

Figure 1.6. Typical structures of sphingolipids (phosphosphingolipid and glycosphingolipid classes), based on a dl8 l ceramide (R = fatty acid group).

The primary hydroxyl group is substituted in one of two ways to give two classes of sphingolipid these are the phosphosphingolipids and glycosphingolipids. 

In phosphosphingolipids, the primary hydroxyl group is esterified with choline phosphate. The lipid is known as sphingomyelin. It has the structure ... 

Question Which of the following lipids are amphiphilic fatty acids acylate ions TAGS cholesterol phosphoglycerides phosphosphingolipids glycosphingolipids ... 

Membranes contain lipids, proteins, and small amounts of carbohydrate. The mass ratios of these vary considerably according to the type of membrane. The carbohydrate is present as glycoglycerolipid, glycosphingolipid, and glycoprotein. The most common types of lipid found in all membranes are phosphoglycerides and phosphosphingolipid (sphingomyelin). Cholesterol is found... 

The vitamin also activates serine palmitoyltransferase, the first enzyme of phosphosphingolipid synthesis, and in bacteria it can, together with inorganic phosphate, replace part of the ATP requirement of galactocerebroside sulfo-transferase (Tsaioun, 1999). In animals, lipid sulfatides are decreased in vitamin K deficiency and increased with higher intakes (Sundaram et al., 1996). 

The major subclasses include phosphosphingolipids, with head groups attached through phosphodiester linkages. 

Phosphosphingolipid Lipid containing a sphingoid base and a phosphorous. 

SM is the major phosphosphingolipid in mammalian tissues and lipoproteins, and has been studied extensively with respect to its role in the formation of sterol-enriched ordered membrane domains and cell signaling (J. Slotte, 2006). Like Cers (and most other complex sphingolipids), SMs typically have high phase-transition temperatures (>37°C), which is a factor in their tendency to associate with rafts. 

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