Substrate-induced free radicals in prostaglandin H synthase.

Reaction of ovine PGH synthase with arachidonic acid or hydroperoxides produces several tyrosine radical species that can be distinguished by electron paramagnetic resonance (EPR) spectroscopy. We have correlated the temporal sequence of the EPR signals with optical changes of the heme center, and with product formation. The synthase was reconstituted with either heme (Fe-PGHS) or Mn protoporphyrin IX (Mn-PGHS). Incubation of Fe-PGHS with equimolar arachidonate resulted in rapid appearance of a wide doublet tyrosyl radical EPR signal (34 G peak-to-trough); the intensity was near maximal by 7 s. The doublet gave way over the next 10 s to a wide singlet (32 G peak-to-trough) which peaked at 46 s and then decayed slowly. Electronic absorbance spectra indicated that formation of peroxidase Compound I was complete within 1 s; accumulation of peroxidase Compound II paralleled accumulation of the wide doublet tyrosyl radical. PGG2 and PGH2 accumulated rapidly during the first 5 s of reaction; little arachidonate remained after 12 s. The tyrosyl radical giving the wide doublet EPR signal is thus the best candidate for the oxidizing species postulated to abstract the 13S hydrogen atom from arachidonate during cyclooxygenase catalysis by Fe-PGHS. Incubation of Mn-PGHS with arachidonate also led to rapid generation of an oxidized peroxidase cycle intermediate, a protein-linked free radical, and prostaglandins. The radical signal seen with Mn-PGHS (singlet, 36 G peak-to-trough) was distinct from those observed with Fe-PGHS, but the kinetics of the Mn-PGHS radical were consistent with participation in cyclooxygenase catalysis.