X I. INTRODUCTION 



known for a long time that the red blood pigment, hemoglobin, serves 

 as carrier of oxygen to the tissues in all vertebrates (with one excep- 

 tion, Amphioxus) and in some invertebrates, and that a closely related 

 substance, myohemoglobin, plays a role in storing oxygen in the red 

 muscles of vertebrates, and in a few muscles of invertebrates. It has 

 been only during the last twenty years, however, that the more 

 general and fundamental importance of hematin compounds for the 

 process of cell respiration has become evident — through Warburg's 

 studies on the respiratory ferment and Keilin's rediscovery of the 

 cytochromes, previously observed spectroscopically by MacMunn. In 

 the process of cellular respiration, which occurs in all aerobic organ- 

 isms, the chemical energy of the organic substances is made avail- 

 able for a variety of energy requirements of the living cell. While 

 the details of these transformations have still to be worked out, it is 

 evident that the energy is used much more efficiently in the process 

 of respiration than in fermentation processes. In the former, the 

 total energy of carbohydrate (674 kcal. per mole of glucose) which is 

 gained from light energy in the photosynthetic process is released 

 again : 



6 CO2 + 12 H2O -f 674 kcal. ^ CeHisOe + 6 O2 + 6 H2O 



In fermentation processes, on the contrary, only part of this energy 

 is released, the remainder being left in the compounds formed by 

 these fermentations, such as alcohol, lactic acid, and butyric acid. 

 In addition to the respiratory ferment (cytochrome oxidase) and a 

 variety of cytochromes, other hematin enzymes (catalase, peroxi- 

 dases) are found in the cells of the aerobic organisms. The role of 

 the latter enzymes in the respiratory process is not yet fully under- 

 stood, but they certainly possess functional importance. The hematin 

 enzymes are discussed in Chapters VIII and IX. 



We thus find pyrrole pigments as essential biological catalysts (in 

 the wider sense of the term) of most fundamental biological processes, 

 and their close study is obviously required if we want to understand 

 these processes. 



Recently it has become likely that another enzyme, hydrogenase, 

 may belong to the hematin compounds. If this is correct, pyrrole 

 pigments may have played a role at an early evolutionary stage of 

 life on earth (c/. Chapter XIV) before photosynthesis and respiration 

 developed. 



