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Since the discovery in 1991 of a second isoform of prostaglandin endoperoxide H synthase (PGHS, or cyclooxygenase), there has been considerable interest in the question of why two isoforms of this enzyme are necessary and what roles they might play. PGHS-1–deficient (1) and PGHS-2–deficient (2, 3) mice and isoform-specific inhibitors have been developed and used to investigate the physiological functions of PGHS-1 and PGHS-2. These studies suggest that there are processes in which each isozyme is uniquely involved (e.g., platelet aggregation for PGHS-1, ovulation for PGHS-2) and others in which both isozymes function coordinately (e.g., carcinogenesis, inflammation). There are also physiological events in which one PGHS isozyme normally functions but for which the other can compensate when the first is lacking (e.g., parturition and remodeling of the ductus arteriosus). Biochemical studies indicate that each isoform can function independently; namely, that there are distinct PGHS-1 and PGHS-2 prostanoid biosynthetic pathways. Thus, the unique physiological roles for each isozyme can be rationalized by what is known about the biochemistry of the enzymes.  To facilitate discussion of the physiological functions of PGHS-1 and PGHS-2 and to point out those functions for which PGHS-1 and PGHS-2 can substitute for one another, we describe the following in sequence: (a) physiological processes that depend solely or primarily on PGHS-1, (b) physiological processes that depend solely or primarily on PGHS-2, and (c) processes in which both PGHS-1 and PGHS-2 are involved and act coordinately. We then summarize the biochemical evidence for distinct PGHS-1 and PGHS-2 biosynthetic pathways.

Why there are two cyclooxygenase isozymes