Phospholipids Are The Main Lipid Constituents Of Membranes

Phospholipids may be regarded as derivatives of phos-phatidic acid (Figure 14-8), in which the phosphate is esterified with the — OH of a suitable alcohol. Phos-phatidic acid is important as an intermediate in the synthesis of triacylglycerols as well as phosphoglycerols but is not found in any great quantity in tissues.

Phosphatidylcholines (Lecithins) Occur in Cell Membranes

Phosphoacylglycerols containing choline (Figure 14-8) are the most abundant phospholipids of the cell mem-

* According to the standardized terminology of the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Biochemistry (IUB), the monoglycerides, diglyc-erides, and triglycerides should be designated monoacylglycerols, diacylglycerols, and triacylglycerols, respectively. However, the older terminology is still widely used, particularly in clinical medi-

Figure 14-6. Triacylglycerol.

brane and represent a large proportion of the body's store of choline. Choline is important in nervous transmission, as acetylcholine, and as a store of labile methyl groups. Dipalmitoyl lecithin is a very effective surface-active agent and a major constituent of the surfactant preventing adherence, due to surface tension, of the inner surfaces of the lungs. Its absence from the lungs of premature infants causes respiratory distress syndrome. Most phospholipids have a saturated acyl radical in the sn-1 position but an unsaturated radical in the sn-2 position of glycerol.

Phosphatidylethanolamine (cephalin) and phos-phatidylserine (found in most tissues) differ from phosphatidylcholine only in that ethanolamine or serine, respectively, replaces choline (Figure 14-8).

Phosphatidylinositol Is a Precursor of Second Messengers

The inositol is present in phosphatidylinositol as the stereoisomer, myoinositol (Figure 14-8). Phosphatidylinositol 4,5-bisphosphate is an important constituent of cell membrane phospholipids; upon stimulation by a suitable hormone agonist, it is cleaved into diacylglycerol and inositol trisph osphate, both of which act as internal signals or second messengers.

Phosphatidic acid

Phosphatidic acid

Choline

Choline

Ethanolamine

NH3+

Serine

Serine

OH

OH

/|2

3|\

O

H

HH

\1

4/

H

H

OH OH

\| 6

5|/

OH

Myoinositol

Cardiolipin Is a Major Lipid

O-1

of Mitochondrial Membranes

1 II 1 II

Phosphatidic acid is a precursor of phosphatidylglyc

E

1 II 1 II H — C — OH O O H —C — O — C-R3 1 11 1

erol which, in turn, gives rise to cardiolipin (Figure

1 ii 1 — O-ch2 r4—C — O—ch2

14-8).

H2C-O-C-R3

Phosphatidylglycerol

Figure 14-8. Phosphatidic acid and its derivatives. The O- shown shaded in phosphatidic acid is substituted by the substituents shown to form in (A) 3-phos-phatidylcholine, (B) 3-phosphatidylethanolamine, (C) 3-phosphatidylserine, (D) 3-phosphatidylinositol, and (E) cardiolipin (diphosphatidylglycerol).

Figure 14-7. Triacyl-sn-glycerol.

Lysophospholipids Are Intermediates in the Metabolism of Phosphoglycerols

These are phosphoacylglycerols containing only one acyl radical, eg, lysophosphatidylcholine (lysoleci-thin), important in the metabolism and interconversion of phospholipids (Figure l4-9).It is also found in oxidized lipoproteins and has been implicated in some of their effects in promoting atherosclerosis.

Plasmalogens Occur in Brain & Muscle

These compounds constitute as much as 10% of the phospholipids of brain and muscle. Structurally, the plasmalogens resemble phosphatidylethanolamine but possess an ether link on the sn-1 carbon instead of the ester link found in acylglycerols. Typically, the alkyl radical is an unsaturated alcohol (Figure 14-10). In some instances, choline, serine, or inositol may be substituted for ethanolamine.

Sphingomyelins Are Found in the Nervous System

Sphingomyelins are found in large quantities in brain and nerve tissue. On hydrolysis, the sphingomyelins yield a fatty acid, phosphoric acid, choline, and a complex amino alcohol, sphingosine (Figure 14-11). No glycerol is present. The combination of sphingosine plus fatty acid is known as ceramide, a structure also found in the glycosphingolipids (see below).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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