HORSE-RADISH PEROXIDASE 



421 



donors passes readily into II, red compound; according to 

 Theorell, the former is the catalytically active hydrogen peroxide - 

 enzyme complex. Compounds II and III are both considered ferric 

 by Keilin, since carbon monoxide does not alter the position of the 



TABLE IV 



Spectroscopic and Magnetochemical Data on Compounds 

 of Horse-radish Peroxidase 



a According to Theorell (2775). 



absorption bands; compare, however, Abrams and co-workers {1). 

 Hydroquinone restores peroxidase. It is not impossible that III, 

 which arises with an excess of peroxide of at least 15-25 moles, is 

 ferrous, since its absorption spectrum happens to coincide with that 

 of the ferrous carboxy compound. The formation of compound III 

 may explain the inhibition of the enzyme activity by an excess of 

 hydrogen peroxide. It is of interest that only compound III has 

 been observed by Keilin {1502) to cause catalatic decomposition of 

 hydrogen peroxide {cf. Section 4.). According to Chance {J^2I|), the 

 Soret band is shifted to about 390 m/z by hydrogen peroxide com- 

 bination. Azide does not combine if peroxide is present {1502). In 

 weakly acid solution peroxidase also combines with phosphate and 

 picrate if these are present in high concentrations. The absorption 

 spectrum of the compounds resembles that of fluoride peroxidase 

 {2776). 



3.2.3. Inhibitors. Both cyanide and sulfide strongly inhibit per- 

 oxidase at concentrations of 10"* to 10-« M {989,1502,307 Jf). While 

 Wieland and Sutter {3074) found both inhibitions to be irreversible, 

 Keilin {1502) observed that hydrogen peroxide replaced cyanide 



