Inhibition of cyclooxygenases 1 and 2 by the phospholipase‐blocker, arachidonyl trifluoromethyl ketone

Background and purpose: Arachidonyl trifluoromethyl ketone (ATK) is widely used as an inhibitor of cytosolic group IV phospholipase A 2 (cPLA 2 ) and calcium‐independent group VI phospholipase A 2 (iPLA 2 ). ATK thus reduces arachidonic acid (AA) substrate for cyclooxygenase (COX; also known as prostaglandin H synthase) and attenuates prostaglandin (PG) synthesis. It has been shown previously, that ATK blocks thromboxane B 2 production induced by exogenous AA in human platelets. It remains, however, unknown whether ATK also directly modulates the activity of cyclooxygenase (COX). Experimental approach: Time courses for inhibition of COX by ATK was obtained using osteoblast‐like MC3T3‐E1 cells, with exogenous AA as substrate and the pure enzymes COX‐1 and COX‐2. PGE 2 was measured by GC‐MS. Key results: ATK was a potent inhibitor of COX‐1 and COX‐2 with IC 50 values of 0.5 and 0.1 μ M in MC3T3‐E1 cells and of 1.7 and 2.6 μ M using the pure enzymes. Inhibition was reversible, with slow‐ and tight‐binding characteristics. The arachidonyl carbon chain was essential, as the saturated palmitoyl analogue had no effect. Conclusions and implications: Attenuation of PG synthesis by ATK is taken to be the consequence of PLA 2 inhibition and the findings of many studies are interpreted on that basis. If there are, however, alternative routes for AA liberation (such as phospholipase C/diacyl glycerol lipase or phospholipase D), this interpretation can lead to false conclusions. As ATK is a widely used and important pharmacological tool in eicosanoid research, knowledge of its interactions with other major enzymes of the cascade is of considerable importance. British Journal of Pharmacology (2008) 155 , 731–737; doi: 10.1038/bjp.2008.304 ; published online 21 July 2008