PHENYLHYDRAZONES AS NEW GOOD SUBSTRATES FOR THE DIOXYGENASE AND PEROXIDASE REACTIONS OF PROSTAGLANDIN SYNTHASE - FORMATION OF IRON(III)-SIGMA-PHENYL COMPLEXES

Phenylhydrazones of various aromatic and aliphatic aldehydes or ketones act as good substrates of the dioxygenase reaction of prostaglandin synthase (PGHS). Corresponding alpha-azo hydroperoxides are formed as intermediates with maximum initial rates of O2 consumption between 8 and 230 mol (mol of PGHS)-1 s-1 for benzophenone and hexanal phenylhydrazone, respectively. The K(m) values for these reactions vary from 100 to 300 muM. These alpha-azo hydroperoxides are then converted to the corresponding alpha-azo alcohols by the peroxidase reaction of PGHS. During such oxidations of phenylhydrazones by PGHS, a new complex of this hemeprotein characterized by peaks at 438 and 556 nm is formed. This complex was obtained both by direct reaction of PGHS Fe(III) with phenyldiazene and by reaction of PGHS Fe(III) with phenylhydrazine in the presence Of O2. By analogy to results previously reported for hemoglobin, myoglobin, catalase, and cytochrome P450, this species should be a sigma-phenyl PGHS Fe(III)-Ph complex. The PGHS Fe(III)-Ph complex should derive from an oxidation of the intermediate alpha-azo alcohol by PGHS Fe(III), cleavage of the resulting alkoxy radical with formation of a ketone (or aldehyde) and Ph., and combination of PGHS Fe(II) with Ph.. Such an oxidation of alpha-azo alcohols by lipoxygenase-Fe(III) with formation of Ph. was reported previously. The formation of Ph. and of PGHS Fe(III)-Ph is likely the cause of the inhibitory effects previously reported for arylhydrazones toward PGHS.