578 XII. HEMOGLOBIN CATABOLISM, II 



the synthesis of hemoglobin and will be discussed in the following 

 chapter. A large number of books and reviews are available on this 

 suhject(322,367406,603,981,982,1070,1822,3255,£46J^,2506,28i8,2850, 

 2908,2983,3028). 



From a physiologic point of view, porphyrins have perhaps received 

 an unduly large amount of attention. This is due to the fact that their 

 characteristic spectroscopic properties and their strong fluorescence 

 make their detection and estimations in tissues and excreta relatively 

 easy. This has made many workers overlook the fact that the occur- 

 rence and excretion of porphyrins is measured in micrograms, while 

 the metabolism of the prosthetic group of hemoglobin and the forma- 

 tion and excretion of bile pigments is measured in milligrams. For 

 example, Watson {2989, p. 2498) and Kench and co-workers {1515) 

 have rightly stressed the improbability of a direct causal connection 

 between the anemia and the increased porphyrin excretion in lead 

 intoxication. In this condition many grams of hemoglobin disappear 

 from the blood, while the excess excretion of porphyrin amounts only 

 to a few micrograms. With the exception of a few rare congenital dis- 

 eases (porphyrias, cf., e.g., Loffler, 1771) the formation and excretion 

 of free porphyrin never approaches the magnitude even of normal 

 metabolism of the prosthetic group of hemoglobin. 



In spite of an enormous amount of work by many investigators 

 many important problems of porphyrin metabolism remain unsolved. 

 The distinction between porphyrins with different side chains (proto, 

 uro, copro) and with different arrangements of the side chains (types 

 I and III of the fundamental etio series) has somewhat facilitated the 

 unravelling of the complicated picture. The most important result 

 of these studies has been the recognition that, in most instances at 

 least, porphyrins must be considered intermediates in hemoglobin 

 synthesis rather than products of hemoglobin breakdown. 



3.2. Endogenous Porphyrin Metabolism 



3.2.1. Introduction. The endogenous metabolism of porphyrins is 

 extraordinarily complicated and very little is known about it. This 

 is due partly to the fact that so small amounts are involved, partly 

 to the fact that porphyrins are formed in the intestine by bacterial 

 synthesis and by the action of bacteria on hemoglobin and other 

 hematin compounds of the food, and that they are probably par- 

 tially reabsorbed from the intestine together with traces of por- 



