SIMPLE HEMATIN COMPOUNDS AS OXIDATIVE CATALYSTS 341 



sufficiently vigorous, since some carbon monoxide heme compounds 

 may be less sensitive to irradiation than others. 



2. SIMPLE HEMATIN COMPOUNDS 

 AS OXIDATIVE CATALYSTS 



The oxidation of a great variety of substrates can be catalyzed by 

 simple hematin compounds. Among these are unsaturated fatty acids 

 and fats {187,UU9^6,1176,1288,1618,1620,1621,2289,2563,3137), car- 

 otenoids {9J^3), cysteine and glutathione (513,1135,1578,1579,16U, 

 179It), hydrogen sulfide (1171), ascorbic acid (1697), benzaldehyde 

 (1617,1927), pyruvic acid (1618,1928), and tertiary amino acids (219). 

 Carbon monoxide cannot be oxidized by protohematin or related 

 blood porphyrin iron compounds, but is oxidized to carbon dioxide 

 by pheophorbid or pheoporphyrin hematins (2020). 



As model experiments these reactions are very interesting, but 

 there is so far no stringent evidence that any of these systems play a 

 biological role. There is more likelihood that the oxidation of ascorbic 

 acid and sulfhydryl compounds does so than in the case of the other 

 substrates, but even here the issue is not yet clear (cf. Chapter XI). 

 The catah'tic effects of hemochromes on ascorbic acid and on cysteine 

 are probably due to a valency change of the hematin iron, hemt- 

 chromes oxidizing the substrate more rapidly than atmospheric 

 oxygen, and hemochromes being more rapidly oxidized by it than the 

 substrate (1579,1697). On the basis of cyanide inhibition experi- 

 ments Krebs (1579) claimed that heme can catalyze the oxidation of 

 cysteine by atmospheric oxygen without passing the ferric (hematin) 

 stage. He observed that the cyanide inhibition of the oxidative 

 action of hematin on ferrous cysteine was smaller than was to be 

 expected from the combination with cyanide measured spectrophoto- 

 metrically, and he concluded from this that part of the catalysis was 

 not due to a valency change of the iron. This conclusion is not 

 justified, since the potential of dicyanide iron porphyrin is higher 

 than that of the free hematin (Chapter V, 6.3.) and there is thus no 

 reason why dicyanide ferriporphyrin should not oxidize cysteine, 

 though more slowly than hematin. 



Krebs also used ferrocysteine as hydrogen donor, so that part of the 

 observed inhibition may have been due to the reaction of cyanide with this, 

 not with hematin. A further source of error may liave been reaction of 

 cyanide with the cystine, retransforming part of the latter into cysteine and 

 removing cyanide from the reaction mixture. Thus Wright and Alstyne 



