IIEMATIN PIGMENTS OF UNKNOWN FUNCTION 653 



valency change. This protection is not absolute; dihydroxymaleic acid in 

 the case of peroxidase, and to a minor degree other reducing substances in 

 the case of catalase cause reduction and destruction. Destruction of per- 

 oxidase by dihj'droxymaleic acid and oxygen probably does not occur 

 biologically, less dangerous hydrogen acceptors normally being bound in the 

 region of the hematin. The destruction of catalase by coupled oxidation 

 occurs in the mammalian liver (cf. Chapter IX). 



Such protection is obviously impossible for substances which are reactive 

 in the ferrous form, such as hemoglobin, or whose function depends on a 

 change of valency, such as cytochrome c. In the ca.se of hemoglobin we have 

 discussed a variety of protective mechanisms (cf. Chapters VII and X). 

 Either the region in which reducing substances are bound by the protein is 

 removed from proximity to the heme group, or hemoglobin is combined with 

 substances which block the dangerous zone. Nevertheless the protection is 

 only partial, as the large catabolism of hemoglobin shows. This is counter- 

 balanced by the powers of the organism for porphyrin and hemoglobin 

 synthesis. 



In the case of the cytochromes the heme protein -hj-drogen donor inter- 

 action is an essential part of the mechanism of its function and hence unavoid- 

 able. Here the protection is through prevention of direct combination of 

 the heme iron with oxygen by the peculiar type of linkage between protein 

 and hematin; this, on the other hand, necessitates the assistance of an oxidase 

 for the catalytic function. 



The increase of size of organisms makes the supply of oxygen by 

 diflfusion impossible, and necessitates the development of special 

 oxygen carriers and oxygen stores. The adaptation for these func- 

 tions has been discussed in Chapter VH. 



5. HEMATIN PIGMENTS OF UNKNOWN FUNCTION 



In addition to the hematin compounds the function of which is 

 known, the hematin enzymes, and the oxygen carriers, a number of 

 hematin compounds have been observed which do not appear to 

 possess any metabolic function, or whose function is still unknown. 



It has been mentioned that yeast and other cells contain a hematin 

 compound which on reduction does not give a hemochrome, while on 

 reduction in the presence of pyridine it yields jnridine protohemo- 

 chrome. This compound has been called "free cell hematin" by 

 Keihn {lJf7J/.), although it is probably al.so a hemoprotein. Its evolu- 

 tionary role is uncertain. It may be a relic .of the time before the 

 hematin compounds became adapted to their specific enzymic or 

 metabolic functions, or it may be a reserve store of hematin for the 

 formation of metabolically active compounds, or have a still 

 unknown function. 



