436 IX. HEMATIN ENZYMES, II 



catalyze the iodination. It has therefore been assumed that peroxidase plays 

 an important role in the formation of thyroxine in the thyroid gland {1521, 

 ■30^0). Inhibitors of thyroxine formation in vivo, such as thiourea and 

 thiouracil, have indeed been shown to inhibit peroxidase and the in vitro 

 synthesis of thyroxine from diiodotyrosine {420,553,557,1011,2204,3040). 

 The inhibitory action of these substances appears to be due, not to a direct 

 effect on the enzyme, but rather to competition with tyrosine for hydrogen 

 peroxide or for iodine. While Glock {1011) did not detect peroxidase in the 

 thyroid gland, De Robertis and Grasso (557) found a heat-labile peroxidase 

 in rat thyroid. The possible role of cytochrome oxidase in thyroxine forma- 

 tion has been discussed in Chapter \TII. 



Hurst {1371) found that peroxidase plays a role in the hardening of the 

 insect cuticle. 



Peroxidase is said to appear before hemoglobin in the development of the 

 hen's egg (c/., however, 12^. It is widely distributed in eggs. In the devel- 

 opment of grasshopper eggs it appears only in the diapause and rises rapidly 

 after the diapause {307). Since these findings are, however, based on 

 histological evidence, it is not certain whether they prove the presence of a 

 true heat-labile peroxidase. 



4. MODE OF CATALATIC AND PEROXIDATIVE ACTION 



4.1. Peroxidase and Dihydroxymaleic Acid Oxidase 



4.1.1. Mode of Action of Peroxidase. The mode of the action of peroxidase 

 was explained by Haber and Willstatter {1080,3020) by assuming radical 

 chains: 



/■ 

 (Fe'+) + H2A -^ Fe'+ +HA (1) 



HA + H2O2 -» A + HoO + OH (2) ] 



/ / ^ 



OH + H2A -* H2O + HA (3) J 



Since the hydroxyl radical was also assumed to initiate the radical chain 

 causing catalatic destruction of hydrogen peroxide, while peroxidases do not 

 destroy hydrogen peroxide in the absence of hydrogen donors, Willstatter 

 had to make the assumption that reactions "i and 3 proceeded together in a 

 threefold collision: 



HA + H2O0 + H2A ^ A + 2 H2O -f HA 



This explanation is improbable, and later research has not found any 

 evidence either for a valency cliange of the hematin iron or for radical chains 

 taking part in/the mechanism of normal peroxidative action. 



According to Theorell (Section 3.2.6.), the hematin iron of perox- 

 idase at physiological />H carries a hydroxyl group. The hydrogen 

 peroxide compound probably has, therefore, the structure (FeOOH) 



