Although the many structures in this group of compounds may seem bewildering at first (Figure 16-2), there are some simple generalizations that can be set forth to allow a facile understanding of structure-function relationships. PGs arise from a cyclic endoperoxide generated by the enzyme system, PG synthetase. This is a complex of enzymes, including cyclooxygen-ase, required to produce the key intermediate, the cyclic endoperoxide derivative of arachidonic acid or other fatty acids. There is a constitutive (COX1) and an inducible cyclooxygenase (COX2). COX2 is induced under conditions of inflammation, and the induction of this enzyme is inhibited by glucocorticoids representing one of its pathways of anti-inflammatory action. The cyclic endoperoxide intermediate is acted on by various isomerases to produce PG subclasses. The cyclic endoperoxide is also a precursor of PGI2 and TX. On the other hand, other groups of compounds in this class, LT and lipoxins (LP), are derived directly from arachidonic acid without the mediation of a cyclic endoperoxide (Figure 16-2). Figure 16-3 repeats Figure 16-2 but shows individual structures. The one-double-bond members of some of the classes, such as PGEj or PGFla, are not shown here, but are illustrated elsewhere in the chapter. Of the many groups of compounds, the PGs are composed of only three abundant groups: PGF, PGE, and PGA. F stands for phosphate buffer-soluble (phosphate begins with an F in Swedish) structures, which are the most hydrophilic or polar of the PGs. The F series has the general structure shown in Table 16-1. All of the PGs have the five-membered ring shown here. The F compounds (most polar) have two hydroxyls in the C-9 and C-ll positions, constituting the most hydrophilic of the classical PGs. The E series (PGE) is intermediate in solubility (is ether-soluble, thus, E) and has one hydroxyl and one ketone substituted on the five-membered ring (Table 16-1). The A
Cell Membrane Phospholipid
Phospholipase A 2 ♦
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