OXIDATION OF FERRIC PORPHYRINS *

Characterization of heme oxidation products falls naturally into two categories : species resulting from electron transfer, and in vivo and in vitro heme degradation products formed by oxidative addition or cleavage. In the former, we propose to include the intermediates appearing during the catalytic cycles of catalase (Cat), horseradish peroxidase (HRP), and cytochrome c peroxidase; the latter class consists of the bile pigments formed by catabolism of heme proteins as well as chemical intermediates such as oxophlorin and verdoheme. Salient features of the catalytic cycles of the enzymes have been discussed previously;' briefly, these are the appearance of a green species (compound I) when the enzyme is treated with HzOz or an organic hydroperoxide. The enzymatically active compound I is a two-electron oxidation product of the ferriheme protein and upon reduction forms first a one-electron oxidation product (Compound 11) which may be reduced further to the parent ferriheme. Compound I of cytochrome c peroxidase differs from these general observations inasmuch as its optical spectrum is quite similar to Compounds I1 of HRP and Cat and it displays an EPR2 signal at g 2.0. Although no EPR signal has been observed for HRP I or HRP 11, recent studies on the methyl hydroperoxide complex I1 of catalase show the existence of a complicated EPR spectrum. Other physical properties of interest are magnetic susceptibility measurements4 on Cat I and HRP I, which show that these species have an effective magnetic moment consistent with three unpaired electrons. The Mossbauer spectras-' of Compounds I and I1 of HRP and Cat have similar isomer shifts which, in turn, differ from those of the parent enzymes. A number of suggestions have been advanced concerning the chemical constitution, source of oxidizing equivalents, and electronic formulation of Compounds I