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 ketone s 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 a nd 230 mol (mol of PGHS)-1 s-1 for benzophenone and hexanal phenylhydr azone, 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. D uring such oxidations of phenylhydrazones by PGHS, a new complex of th is hemeprotein characterized by peaks at 438 and 556 nm is formed. Thi s complex was obtained both by direct reaction of PGHS Fe(III) with ph enyldiazene and by reaction of PGHS Fe(III) with phenylhydrazine in th e presence Of O2. By analogy to results previously reported for hemogl obin, myoglobin, catalase, and cytochrome P450, this species should be a sigma-phenyl PGHS Fe(III)-Ph complex. The PGHS Fe(III)-Ph complex s hould derive from an oxidation of the intermediate alpha-azo alcohol b y PGHS Fe(III), cleavage of the resulting alkoxy radical with formatio n of a ketone (or aldehyde) and Ph., and combination of PGHS Fe(II) wi th 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 p reviously reported for arylhydrazones toward PGHS.