138 



THE CELL AND PROTOPLASM 



On the whole, we must admit that the 

 isolation of the most important hydrolytic 

 enzymes in a crystalline state has not yet 

 brought us any knowledge about the chemi- 

 cal mechanism of their action. And it is 

 probably not possible to clear up these ques- 

 tions before we know much more about the 

 structure of the protein molecule, the ther- 

 modynamics of the peptide linkages, the 

 protein synthesis in the living cells, and 

 last, but not least, the accurate amino acid 

 composition of protein. 



The Hemin Ferments 



Two metals have hitherto been known 

 as forming ferments with proteins, namely 

 iron and copper. The iron first forms com- 

 pounds with porphyrins, and the iron-por- 

 phyrins are then linked to protein to form 

 ferments, in which the iron may be regarded 

 as the "active" group and the iron-por- 

 phyrin as the prosthetic group. In the 

 copper ferments no porphyrin is present; 

 the copper atom is both the "active" and 

 the "prosthetic" group. 



We have been especially interested in the 

 iron-porphyrin ferments, because they 

 seem able to give more general information 

 on the interaction between "active" group, 

 ' * prosthetic ' ' group, and protein. The iron- 

 porphyrin ferments known to date are the 

 following: (a) Warburg's respiratory fer- 

 ment, (&) the cytochromes-a, -h and -c, (c) 

 the catalases, and (d) the peroxidases. 



If we add hemoglobin and myoglobin, 

 which from some points of view could be 

 regarded as ferments just as well as the 

 other compounds, we shall find that the 

 properties of the iron atom, which is the 

 "active group" in each one, have been 

 strongly modified under the influence of 

 various porphyrins and proteins. Some 



catalysts. The enzymes were those which could be 

 extracted without losing their activity, while fer- 

 ments were those which could not be separated 

 from the structure of cells without being destroyed. 

 There is little reason for maintaining this differ- 

 ence now, since it turns out to be merely a techni- 

 cal question whether or not it is possible to extract 

 the catalysts. Furthermore, even some of those 

 which have been successfully extracted are surely 

 linked to the cell structure under physiological 

 conditions. 



may add oxygen or carbon monoxide, as 

 Warburg 's respiratory ferment is supposed 

 to do, or make very rapid oscillations be- 

 tween the ferrous and the ferric state at a 

 suitable potential level, as in the cyto- 

 chromes, in catalase, in Warburg's respira- 

 tory ferment, and probably in the per- 

 oxidases in certain cases. Warburg's 

 ferment and the cytochromes-a and -h are 

 not yet accessible for preparative purifica- 

 tion, as they cannot be extracted, and so 

 at present only cytochrome-c from beef 

 heart, some catalases, and some peroxidases 

 have been prepared in a more or less puri- 

 fied form. 



Cytochrome-c, as well as the other cyto- 

 chromes, was discovered by Mac Munn in 

 the years 1886 to 1889. In spectroscopic 

 studies of different tissues, such as muscles, 

 he saw some absorption bands which he 

 believed could not possibly be due to hemo- 

 globin. He recognized the bands as belong- 

 ing to hematin compounds which are of im- 

 portance for tissue respiration, and even 

 succeeded in extracting part of his "histo- 

 hematin" or "myohematin" in the form of 

 "modified myohematin," which, as we now 

 can conclude, must have been some impure 

 cytochrome-c. Mac Munn was attacked by 

 Hoppe-Seyler and Levy, who declared his 

 new substances to be only hemoglobin, and 

 so his discovery was forgotten for 40 years, 

 until Keilin, in 1925, rediscovered these sub- 

 stances and demonstrated their general dis- 

 tribution in the living cells, their important 

 function in tissue respiration, and their 

 composition as three different types of 

 hemochromogens, the cytochromes-a, -6, 

 and -c. 



We obtained cytochrome-c in Stockholm 

 in 1935 in a highly purified form, and even 

 thought it was definitely pure, because it 

 had the same iron content as hemoglobin, 

 0.34 per cent. Keilin and Hartree con- 

 firmed our view two years later, after 

 reaching the same degree of purity by sev- 

 eral different methods. However, some 

 months ago we reinvestigated the prepara- 

 tions by means of Tiselius' electrophoretic 

 method and were able to remove some color- 

 less impurities, so that the iron content in- 



