Rapid kinetics of tyrosyl radical formation and heme redox state changes in prostaglandin H synthase-1 and -2.
Hydroperoxide-induced tyrosyl radicals are putative intermediates in cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2. Rapid-freeze EPR and stopped-flow were used to characterize tyrosyl radical kinetics in PGHS-1 and -2 reacted with ethyl hydrogen peroxide. In PGHS-1, a wide doublet tyrosyl radical (34-35 G) was formed by 4 ms, followed by transition to a wide singlet (33-34 G); changes in total radical intensity paralleled those of Intermediate II absorbance during both formation and decay phases. In PGHS-2, some wide doublet (30 G) was present at early time points, but transition to wide singlet (29 G) was complete by 50 ms. In contrast to PGHS-1, only the formation kinetics of the PGHS-2 tyrosyl radical matched the Intermediate II absorbance kinetics. Indomethacin-treated PGHS-1 and nimesulide-treated PGHS-2 rapidly formed narrow singlet EPR (25-26 G in PGHS-1; 21 G in PGHS-2), and the same line shapes persisted throughout the reactions. Radical intensity paralleled Intermediate II absorbance throughout the indomethacin-treated PGHS-1 reaction. For nimesulide-treated PGHS-2, radical formed in concert with Intermediate II, but later persisted while Intermediate II relaxed. These results substantiate the kinetic competence of a tyrosyl radical as the catalytic intermediate for both PGHS isoforms and also indicate that the heme redox state becomes uncoupled from the tyrosyl radical in PGHS-2.
Division of Hematology, Department of Internal Medicine, University of Texas Health Science Center, Houston, Texas 77030, USA.
A l
Al
Tsai
G
G
Wu
G
G
Palmer
B
B
Bambai
J A
JA
Koehn
P J
PJ
Marshall
R J
RJ
Kulmacz
D000818:Animals; D001051:Apoenzymes / Q000737:chemistry / Q000378:metabolism; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000378:metabolism; D006418:Heme / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008565:Membrane Proteins; D010084:Oxidation-Reduction; D011451:Prostaglandin-Endoperoxide Synthases / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012669:Seminal Vesicles / Q000201:enzymology; D012756:Sheep; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000378:metabolism*
D001051:Apoenzymes; D005609:Free Radicals; D007527:Isoenzymes; D008565:Membrane Proteins; D011994:Recombinant Proteins; D014443:Tyrosine; D006418:Heme; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; C496511:PTGS1 protein, human; C496540:PTGS2 protein, human; D011451:Prostaglandin-Endoperoxide Synthases