428 



IX. HEMATIN ENZYMES, II 



that the properties of alkaline hematin are largely due to polymerization 

 which cannot occur in the alkaline peroxidase, and that Rawlinson's magneto- 

 chemical investigations (cf. Chapter V, Section 3.) indicate the presence of a 

 eovalently linked compound in alkaline hematin solutions. 



TABLE VI 

 DiflTerential Titration of Peroxidase (Enzyme) and Protein (Apoenzyme)" 



" According to Theorell (2776). 



* On the basis that apoenzyme has a pK value of 10.2 and peroxidase a pK value 

 of 11.0. 



It is rather unexpected that compounds II and III (Fig. 3) should have 

 the same catalytic activity, unless hydrogen peroxide replaces the hydroxy 1 

 group in form III very effectively. Finally, the combination of ferrous heme 

 compounds with carboxylic anions, which would have to be assumed for 

 ferroperoxidase, has so far not been demonstrated. 



While the evidence of Theorell cannot thus be considered as fully 

 conclusive and is even partly contradictory, the linkage of the hematin 

 iron to a carboxylic acid group of the protein* seems to be more likely 

 than the linkage to an imidazole group, and there is rather good 

 evidence for a second linkage between a propionic acid side chain and 

 a basic group in the protein, which may be a tyrosine-ester linkage. 



The different mode of linkage between hematin and protein in 

 peroxidase and hemoglobin may explain the great difference between 

 the pK value [(FeOH) -^ (Fe)+ + 0H-] of peroxidase (5.0) and 

 that of hemiglobin (8.1). The inactivity of the compounds of the 

 apoenzyme with rhodo- and pyrrohematin is explained by the lack 

 of a suitably placed carboxyl group in the side chains of these hema- 

 tins, which is able to combine with a basic group of the apoenzyme. 



3.3. Other Plant Peroxidases 



From the sap of the fig, which is an extremely rich source of peroxidase, 

 Sumner and Howell [2703) isolated a peroxidase of PZ 700-1000, which 



* This is no longer upheld in Theorell's recent review {2779a). 



