Cyclooxygenase 2 (COX-2) catalyzes the first step in the biosynthesis of prostaglandins (PGs), thromboxanes, and prostacyclins: The conversion fo arachidonic acid (Cay-90010) to PGH2 (Cay-17020). Discoveries of the induction of COX expression by a variety of stimuli such as phorbol esters, lipopolysaccharides, and cytokines led to the hypothesis that the inducible form of COX, COX-2, is responsible for the biosynthesis of PGs under acute inflammatory conditions. Thus, COX-2 has become the focus of attention for the nonsteroidal anti-inflammatory drug (NSAID) development. Human recombinant COX-2 contains a six residue histidine sequence (His-tag) near the amino terminus. The His-tag enzyme, which has a Km value for arachidonate of 6.5 µM, exhibits enzyme activity and sensitivity to NSAIDs similar to the non-tagged enzyme.