H2o

Diacylglycerol

-> Activation of protein kinase C

Regulation of other enzymes (by protein phosphorylation)

FIGURE 10-17 Phosphatidylinositols in cellular regulation. Phosphatidylinositol 4,5-bisphosphate in the plasma membrane is hy-drolyzed by a specific phospholipase C in response to hormonal signals. Both products of hydrolysis act as intracellular messengers.

This enzyme catalyzes the transfer of a phosphoryl group from ATP to a specific residue in one or more target proteins, thereby altering their activity and consequently the cell's metabolism. This signaling mechanism is described more fully in Chapter 12 (see Fig. 12-19).

Inositol phospholipids also serve as points of nu-cleation for certain supramolecular complexes involved in signaling or in exocytosis. Proteins that contain certain structural motifs, called PH and PX domains (for pleckstrin homology and Phox homology, respectively), bind phosphatidylinositols in the membrane with high specificity and affinity, initiating the formation of multienzyme complexes at the membrane's cytosolic surface. A number of proteins bind specifically to phos-phatidylinositol 3,4,5-trisphosphate, and the formation of this phospholipid in response to extracellular signals brings the proteins together at the surface of the plasma membrane (see Fig. 12-8).

Membrane sphingolipids also can serve as sources of intracellular messengers. Both ceramide and sphingomyelin (Fig. 10-12) are potent regulators of protein kinases, and ceramide or its derivatives are known to be involved in the regulation of cell division, differentiation, migration, and programmed cell death (also called apop-tosis; see Chapter 12).

Eicosanoids Carry Messages to Nearby Cells

TEicosanoids are paracrine hormones, substances that act only on cells near the point of hormone synthesis instead of being transported in the blood to act on cells in other tissues or organs. These fatty acid derivatives have a variety of dramatic effects on vertebrate tissues. They are known to be involved in reproductive function; in the inflammation, fever, and pain associated with injury or disease; in the formation of blood clots and the regulation of blood pressure; in gastric acid secretion; and in a variety of other processes important in human health or disease.

All eicosanoids are derived from arachidonic acid (20:4(A5,8,11,14)) (Fig. 10-18), the 20-carbon polyun-saturated fatty acid from which they take their gen-

Membrane

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