PREFACE 



There is no need to emphasize the importance of a field of science 

 which includes among its subjects hemoglobin, the hematin enzymes, 

 porphyrins, and bile pigments. No monograph or handbook has 

 previously been available in the English language; in German we have 

 the valuable handbook of H. Fischer and H. Orth on the chemistry 

 of pyrrole derivatives, but this has been written entirely from the 

 viewpoint of the organic chemist. 



In textbooks of biochemistry and physiology the subject is rarely 

 treated with the care which its importance demands. Perhaps 

 because of the lack of a suitable monograph serious errors are frequent, 

 hypotheses of doubtful value are given as facts, and there is a time lag 

 of many years, sometimes of decades, between the present-day knowl- 

 edge available and that summarized in the textbooks. A factor which 

 may have contributed to this is that errors, inescapable in the rapid 

 development of this field, have, for reasons of prestige, not always 

 been withdrawn as clearly and frankly as would have been in the 

 interest of science. After completion of the script of this book two 

 valuable reviews were published: "Heme-Linked Groups and Mode 

 of Action of Some Hemoproteins," by H. Theorell, and "Distribution, 

 Structure, and Properties of the Tetrapyrroles," by S. Granick and 

 H. Gilder, both reviews in Volume VII (1947) of Advances in 

 Enzymology. 



This book is therefore intended to fill two requirements. It is 

 intended to summarize the present state of our knowledge for the 

 student and for workers in other fields, as well as enable the research 

 worker or anyone wishing to ac(iuire special knowledge in this partic- 

 ular field to gather less laboriously than has hitherto been possible 

 the information needed. For the benefit of the latter particularly, 

 but also for that of the general student, the treatment is as critical 

 as possible and every endeavor has been made to avoid dogmatic 

 statements. 



