METABOLISM OF PYRROLE PIOMEN'TS 7 



In this way are bound, first, other nitrogenous substances, such as 

 pyridine or ammonia, or the protein globin in hemoglobin; second, 

 the molecules on which hemoglobin or the hematin catalysts act 

 (oxygen, hydrogen peroxide); and, third, the specific inhibitors used 

 in the study of the hematin enzymes (e.g., cyanide, carbon mon- 

 oxide). 



(6) Finally, the nature of the compound is profoundly modified 

 by the specific combination of the hematin with a specific protein 

 (cf. Chapters VI, VIII, and IX). The nature of the protein not only 

 determines whether the particular compound is suitable for the 

 function of carrying oxygen, or as an oxidative, catalatic, or peroxi- 

 dative catalyst; it is also the basis of the observed specificity of, for 

 example, hemoglobins in different species, genera, and phyla, and 

 also of the ontogenetic differences found, for example, between fetal 

 and adult hemoglobins. 



4. METABOLISM OF PYRROLE PIGMENTS 



The cells of most aerobic organisms have a considerable potentiality 

 for synthesizing the porphyrin nucleus, and onlj'^ in a few species 

 must this be added in the food as a vitamin. We shall show in 

 Chapter XIII that the synthesis of the porphyrins is closely linked 

 with carbohydrate metabolism. Even with the scanty evidence avail- 

 able at present, it is possible to devise a satisfactory theory for their 

 formation from intermediate products of carbohydrate metabolism, 

 although the final experimental proof is still lacking. 



Like all other substances the pyrrole pigments undergo catabolic 

 changes, the rate of which in the case of hemoglobin is of considerable 

 magnitude. We still know very little about the metabolism of the 

 chlorophylls, the hematin enzymes, or myohemoglobin, but the 

 principle of the breakdown of hemoglobin to bile pigments has been 

 elucidated by the work of Lemberg. This will be discussed in 

 Chapters X and XI. 



