Non-redundant functions of cyclooxygenases: oxygenation of endocannabinoids

Abstract

The two cyclooxygenase (COX) enzymes catalyze the oxygenation of arachidonic acid to prostaglandin endoperoxides, which are the common intermediates in the biosynthesis of the bioactive lipids prostaglandins and thromboxane. COX-1 and COX-2 are approximately 60% identical in amino acid sequence, exhibit highly homologous three-dimensional structures, and appear functionally similar at the biochemical level. Recent work has uncovered a subtle functional difference between the two enzymes, namely the ability of COX-2 to efficiently utilize neutral derivatives (esters and amides) of arachidonic acid as substrates. Foremost among these neutral substrates are the endocannabinoids 2-arachidonoylglycerol and arachidonoylethanolamide. This raises the possibility that COX-2 oxygenation plays a role in a novel signaling pathway dependent on agonist-induced release of endocannabinoids and their selective oxygenation by COX-2. Among the products of COX-2 oxygenation of endocannabinoids are glyceryl prostaglandins, some of which (e.g. glyceryl prostaglandin E(2) and glyceryl prostaglandin I(2)) exhibit interesting biological activities in inflammatory, neurological, and vascular systems. These compounds are produced in intact cells stimulated with physiological agonists and have been isolated from in vivo sources. Important concepts relevant to the hypothesis of a COX-2-selective signaling pathway are presented.

FIGURE 1.

COX-2-dependent oxygenation of AA and its neutral derivatives. Oxygenation of neutral AA derivatives produces the analogous PG metabolites in every case except for thromboxane A₂ (TxA₂).

FIGURE 2.

Comparison of the structures of the COX-1 and COX-2 active sites. The blue surface represents solvent-accessible space. The larger space in the COX-2 active site is due to a side pocket that is poorly accessible in COX-1 because of the added steric bulk at position 523 (Ile in COX-1 and Val in COX-2).

FIGURE 3.

Agonist-dependent oxygenation of AA and 2-AG by the COX enzymes. The interaction of an agonist with its cell-surface receptor leads to the release of free AA from membrane phospholipids via the action of PLA₂. The free AA is subject to oxygenation by both COX isoforms, leading to the formation of PGs, or it may be reincorporated into the membrane. 2-AG is formed via a PLCor PLD-mediated pathway. It may be oxygenated by COX-2, leading to the formation of PG-Gs. Alternatively, it may be hydrolyzed to free AA or reincorporated into the membrane. Hydrolysis of PG-Gs leads to their conversion to PGs. PGs formed from hydrolysis of PG-Gs are indistinguishable from PGs formed by direct oxygenation of AA. 2-PG-Gs can isomerize to 1-PG-Gs with a half-life of 4–10 min. Many of the same elements of this scheme may pertain to AEA as well as 2-AG.