CHARACTERIZATION OF A PLANT PARAPEROXIDASE FROM CURCURBITA-PEPO USINGMAGNETIC CIRCULAR-DICHROISM - DIRECT EVIDENCE FOR CYANIDE LIGATION INTHE FERRIC RESTING STATE

A series of ferrous, ferric, and oxoferryl derivatives of two plant pe roxidase isozymes isolated from zucchini peels has been examined using electronic absorption and magnetic circular dichroism (MCD) spectrosc opy. Isozyme Al has spectroscopic properties that are very similar to those of a typical peroxidase enzyme such as horseradish peroxidase. I sozyme C-1, on the other hand, has abnormal spectroscopic properties i n the ferric resting state and has been termed a paraperoxidase. In ge neral, the ligand adducts of both isozymes in all oxidation states hav e spectral properties that compare favorably with the corresponding li gand derivatives of horseradish peroxidase (HRP). Thus, the central he me iron of both isozymes is likely coordinated to a proximal histidine ligand. The unusual spectral properties of the native low-spin ferric state of isozyme Ct appears to be due to the presence of cyanide as a n exogenous sixth ligand. The cyanide can be formed by the hydrogen pe roxidase dependent oxidation of thiocyanate. Oxidation of thiocyanate results in formation of a product, hypothiocyanite (OSCN-), which can be further oxidized by excess H2O2 to form cyanide. Therefore we propo se the most likely cause of the unusual spectral properties of isozyme C-1, the paraperoxidase, is the presence of cyanide as an exogenous l igand in the ferric resting state.