It is not well appreciated that a growing number of enzymes utilize amino acid radicals to catalyze chemical reactions (see Chem. Rev., 1998, 98, 705-762). Enzymes that fall into this category include many proteins that are targets of clinically used drugs such as ribonucleotide reductase and prostaglandin H synthase (PGHS). Both enzymes use a tyrosyl radical in catalysis. PGHS is the focus of our research as well as lipoxygenases, which like PGHS utilize arachidonic acid as substrates. PGHS is a major target for anti-inflammatory drugs, and lipoxygenases are involved in many diseases including cancer, atherosclerosis, asthma, and Alzheimer’s disease. Using synthetic organic chemistry, we have prepared isotopically labeled arachidonic acids as well as arachidonic acid analogs that serve as enzyme inhibitors.

Prostaglandin H Synthase.

PGHS, sometimes called cyclooxygenase (COX), has received enormous interest from the pharmaceutical industry as the target of anti-inflammatory drugs such as aspirin and ibuprofen. In the 1990s, it was discovered that in humans two genes are present for COX. One of the enzymes encoded by these genes, COX-1, is not involved in inflammation, while the second isozyme (COX-2) clearly is. Both isozymes catalyze the conversion of arachidonic acid into prostaglandin H2 (See Scheme 1). Most anti-inflammatory drugs including aspirin and ibuprofen inactivate both COX-1 and COX-2, which is undesirable. Celebrex® selectively inactivates COX-2.

Despite extensive investigations of both proteins, the actual mechanism of catalysis is still incompletely understood. We have recently synthesized a number of isotopically labeled arachidonic acids to further investigate this intriguing reaction. This has allowed us in collaboration with the laboratories of Ah-Lim Tsai and Richard Kulmacz to identify substrate radical intermediates that are formed during the conversion of arachidonic acid into PGG2. We are currently preparing arachidonic acid analogs to further probe the mechanism of the enzyme.