Phosphoglycerides major classes

Phosphoglycerides such as lecithin and cephalin are closely related to fats. The glycerol backbone in these molecules is esterified to two fatty acids (one saturated and one unsaturated) and to one phosphate ester. Sphingolipids, another major class of phospholipids, have an amino alcohol such as sphingosinc for their backbone. These compounds are important constituents of cell membranes. 

Of the two major phosphohpid classes present in membranes, phosphoglycerides are the more common and consist of a glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol (Figure 41-2). The fatty acid constiments are usually even-numbered carbon molecules, most commonly containing 16 or 18 carbons. They are unbranched and can be saturated or unsamrated. The simplest phosphoglyceride is phosphatidic acid, which is... 

Each phospholipid class in a given tissue has a characteristic fatty acid composition. Though the same fatty acid may be present in a number of lipids, the quantitative fatty acid composition is different for each class of lipids and remains fairly constant during the growth and development of the brain. A typical distribution profile of the major fatty acids in rat brain phospholipids is given in Table 3.1. Not only do the phosphoglycerides differ in the structure of the polar head groups, or phospholipid... 

A necessary prerequisite for eicosanoid formation is the availability of an appropriate unesterified fatty acid. In mammals this is usually arachidonic acid. The concentration of unesterified arachidonic acid in cells is well below the Km for prostaglandin H synthetase. Thus, the first stage for eicosanoid formation will normally be an activation of the release of arachidonate from the sn-2 position of phosphoglycerides which contain it. Two classes of phospholipids are thought to play major roles as sources of arachidonate in cells - phosphatidylcholine (the major membrane constituent) and phosphoinositides (by virtue of the high enrichment of arachidonate at their sn-2 position). Thus, hydrolysis of phosphoinositides not only produces two second messengers directly (section 8.8) but may also initiate an... 

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