PEROXIDASES AND DIHYDROXYMALEIC ACID OXIDASE 419 



at least against irreversiljle oxidation — in the red cell, although Bingold's 

 evidence is of little value. 



The inability of intracellular catalase to protect bacteria against hydrogen 

 peroxide formed in the medium does not prove that the enzyme is unable to 

 protect the cell against hydrogen peroxide formed internally. While in a 

 sense it is true that the catalatic destruction of hydrogen peroxide is a wa.ste 

 of energy, so is every safety valve; the oxygen is not lost l)ut can recombine 

 with hemoglobin or the respiratory enzyme. 



In spite of these reservations it appears likely that catalase functions 

 in vivo as an oxidative catalyst, in a manner still unknown. This may at 

 least be so under special conditions. Stannard (JGIO) considered it possible, 

 although not proven, that catalase functions as an oxidase in the .system in 

 frog muscle which oxidizes carbon monoxide to carbon dioxide. 



3. PEROXIDASES AND DIHYDROXYMALEIC 

 ACID OXIDASE 



3.1. Introduction 



One of the richest sources of peroxida.se is horse-radish. Horse- 

 radish peroxidase was investigated in the pioneer enzyme studies of 

 Willstatter (3089,3093). A relationship to iron was suspected, but 

 no parallelism of activity and iron content was found. In 1931 Kuhn 

 and collaborators (1123,1616) discovered that the activity of the 

 enzyme was proportional to its light absorption at 420 lufi (Soret 

 band) and concluded from this that the enzyme was a hematin com- 

 pound. This was first doubted by Elliot and Keilin (670), who 

 found, in peroxidase solutions containing far more hematin than those 

 used by Kuhn, no proportionality between activity and the proto- 

 hemochrome bands which appeared upon reduction with sodium 

 dithionite. They mistook the 640 mju ab.sorption band of the enzyme 

 for that of acid hematin. Later, however, Keilin and Mann (1502) 

 found that the strength of this band was always proportional to the 

 activity of the enzyme. They isolated hemin crystals and found that 

 the spectroscopic properties of peroxida.se and of its compounds 

 resembled those of other hemoproteins. Meanwhile a similar per- 

 oxidase containing 1% hematin had been isolated from fig .sap by 

 Sumner and Howell (2703). 



Horse-radish peroxidase and the peroxida.ses of milk and leucocytes 

 were further purified and the first two were crystallized by Theorell 

 and his collaborators. Horse-radish peroxidase can be split into 

 protohemin and a protein, and resynthesized from them (2772,2788). 

 There is little doubt that the prosthetic group of j)eroxidase is proto- 

 hematin IX, the same as in hemoglobin and catala.se. 



