Spectral analysis of the protein-derived tyrosyl radicals from prostaglandin H synthase

We have analyzed the low temperature EPR spectra of the protein-derived tyrosyl radicals detected upon addition of arachidonic acid or 5-phenyl-4-pentenyl-1-hydroperoxide (PPHP) to prostaglandin H synthase. With either arachidonic acid or PPHP the initial radical detected is a doublet (peak-to-trough = 35 Gauss) that disappears rapidly and is replaced by a broad singlet (peak-to-trough = 30 Gauss) followed by a narrow singlet (peak-to-trough = 26.5 Gauss). The relative amounts of these signals vary with time and concentration of arachidonic acid. The three tyrosyl radical signals were subjected to computer simulation and power saturation analysis. The data establish that there are only two distinct tyrosyl radical species, the doublet and the narrow singlet. The broad singlet seen at intermediate times and at low arachidonic acid concentrations is a composite of the doublet and the narrow singlet. The composition of the broad singlet in incubations of prostaglandin H synthase with 0.5 mM arachidonic acid is approximately 40% doublet and 60% singlet. The broad singlet signal does not represent a distinct tyrosyl radical species.