Two isoforms of cyclooxygenase enzyme, also known as prostaglandin endoperoxide H synthase, have been identified: cyclooxygenases 1 and 2 (COX-1, COX-2). The two isoforms generate the same prostanoids but are encoded by different genes, which are mapped on different chromosomes and are differently regulated. The genes for each isoform show considerable sequence homology between species (around 90%), whereas the genes for the two isoforms show only about 60% sequence homology within any one species. ventolin 100 mcg
The COX-1 isoform of the enzyme is presumed to be ubiquitously present, albeit in low concentrations, and to function as a ‘‘housekeeping’’ enzyme that is constitutively expressed. COX-1 isoform of the enzyme can produce prostanoids rapidly from both endogenous and exogenous precursors when need for prostanoids arises in the tissue. The gene for the COX-2 isoform is an immediate early gene, the transcription of which is induced by specific ligands that activate the normally quiescent gene. Mitogens, cytokines, and tumor promoters are such ligands. Except in the brain and kidney, which seem to express COX-2 constitutively, the concentration of COX-2 gene transcripts is normally below detection limit, and production of prostanoids by this isoform usually occurs with a certain delay.