342 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



{3127) observed that cystine did not take up oxygen in the presence of 

 hematin, but did so if cyanide also was present. Haurowitz (1171), however, 

 found also no evidence for a valency change in the hematin catalysis of 

 hydrogen sulfide oxidation. The hemochrome-catalyzed oxidation of benz- 

 aldehyde is cyanide sensitive {1617). 



The catalysis of fatty acid or linseed oil oxidation by hematin is still more 

 obscure. It takes place in a heterogeneous system (oil-water emulsion). 

 There is no evidence of a valency change; and the destruction which the 

 hematin undergoes in the process points to a peroxidative reaction. It is 

 often not easy to distinguish between a true oxidative and a peroxidative 

 process. There is, however, in this case no independent evidence for peroxide 

 formation. 



Barron and Lyman {187) observed that cyanide inhibited this catalysis 

 completely only in a narrow pH range (9.2 to 9.5). They tried to explain 

 this by assuming that the inhibitor is free hydrocyanic acid (pK of 9.1) and 

 that it inhibits the catalytic action of the cyanide ferroporphyrin-cyanide 

 ferriporphyrin system, not that of the heme-hematin system. This explana- 

 tion does not appear to be in accord with later findings on the pH stability 

 of cyanide ferroporphyrin and cyanide ferriporphyrin (Chapter V, 6.3.). 

 Hematin also catalyzes the oxidation of ergosterol {1929). Bergel {219) 

 assumes the formation of peroxides of tertiary amino acids in the hematin- 

 catalyzed oxidation of these substances. 



Combination of hematin with bases to form hemochromes does not 

 raise the oxidative efficiency to a very marked extent. 



In hematin-catalyzed fatty acid oxidation, the effect of the bases is small 

 and nicotine even inhibits the oxidatioji {179). The effect is also small in 

 glutathione oxidation {179 Ji) but somewhat larger in the oxidation of cysteine. 

 Here pilocarpine hemochrome is about four times as active as free hematin 

 (i6^^), pyridine hemochrome eleven times, and nicotine hemochrome twenty- 

 eight times {1579). The catalytic effect of pyridine hemochrome on benz- 

 aldehyde oxidation was found to be about fifty times as strong as that of 

 hematin {1927). Hemochrome systems are far more effective catalysts than 

 free hematin for the oxidation of ascorbic acid, but this is due to the fact that 

 they have the necessary high oxidation-reduction potential {179). As 

 catalysts of the oxidation of ascorbic acid, hemochromes are only about 

 half as effective as copper. 



As is to be expected, hemoglobin has not been found to be a very 

 effective oxidative catalyst {1253,2289,2563). It has been observed 

 to catalyze the oxidation of phospholipides {2563)*; this reaction is 

 not accelerated by cytochrome c. In Chapter X other reactions will 

 be discussed in which hemoglobin acts as oxidative catalyst, being 

 destroyed in the process (coupled oxidation). 



* This is also true for unsaturated fats (Watts and Peag, SOOlto). 



