402 IX. HEMATIN ENZYMES, II 



certain enzymes the hematin nature of which cannot yet be con- 

 sidered as established, such as hydrogenase, and with other biologi- 

 cally important catalyses in which hematin compounds probably play 

 a role. 



1.2. Model Reactions with Simple Hematin Compounds 



1.2.1. Catalatic Activity. Hematin compounds are the only por- 

 phyrin metal complexes to catalyze the decomposition of hydrogen 

 peroxide {1168); iron phthalocyanine is also active. Studies on the 

 catalatic action of hematin and hematin compounds have been carried 

 out by Kuhn, von Euler, Zeile, Langenbeck and Stern {328, 7 23, 7 2 Ji, 

 16U,1615,1620,16U,'S157). The affinity of hematin for hydrogen 

 peroxide is not smaller than that of catalase {721), but the hematin- 

 hydrogen peroxide complex breaks down much more slowly, and the 

 whole reaction is therefore much slower. Haldane {1098) calculated 

 that at 0° C. the activity in moles per liter per second is 10~^ for 

 ionic iron, 10~^ for hematin, but 10^ for catalase. 



A comparison of hematin with mesohematin, coprohematin, and 

 deuterohematin showed that the nature of the porphyrin side chain exerts a 

 certain influence on the catalatic activity of the hematin. The pH optimum 

 {1614-) and the stability of the hematin to hydrogen peroxide {3157) are also 

 influenced by the side chains, which explains contradictory statements with 

 regard to the relative efficiency of various hematins {cf. 1614- and IGJfJi). 

 Hematins are much more rapidly destroyed by hydrogen peroxide than is 

 catalase. 



The state of dispersion and adsorption of the hematin has a considerable 

 influence on its catalatic activity. Adsorption to charcoal increases, adsorp- 

 tion to alumina decreases, it {1620). No similar effect of adsorption has been 

 found with hem/chromes {264.9). Cyanide inhibits the activity at pH 7-9 

 under conditions under which dicyanide ferriporphyrin is formed, but does 

 not inhibit it at pH 6, where it even activates {3157); the solutions at the 

 latter /jH are probably colloidal. Smaller concentrations of cyanide (0.001 M) 

 at pH 8-9 have the remarkable effect of increasing the initial activity, but 

 cause rapid destruction of the catalyst. These observations of Zeile deserve 

 further study. Imidazole hem/chroraes are slightly more active catalysts 

 {1644) for the destruction of hydrogen peroxide than is hematin, but other 

 hemj'chromes are not. Hemoglobin has a catalatic activity of the same order 

 as that of free hematin {1161,2215,2650). 



1.2.2. Peroxidative Activity. Peroxidative activity is also restricted to the 

 iron compounds of porphyrins and to the related iron phthalocyanine {1168, 

 1169). For more than one reason it is difficult to compare the peroxidative 

 activities of different hematin compounds. First, the activity depends on 

 the substrate: although one hematin compound is a more active peroxidase 

 than another with one substrate, it may nevertheless be weaker with a 



