THE FLAVOPROTEINS 137 



cytochrome c reductase (9). In Table I the characteristics of the 

 known and identified flavoproteins are presented in outhne form. 



Inasmuch as three excellent reviews of the respiratory enzymes 

 by Kalckar (20), Green (21), and Oppenheimer and Stern (22) have 

 recently appeared which collectively deal at length with the flavo- 

 proteins, I shall consider only the highlights that characterize them 

 and treat them by comparison with one another. Later I shall con- 

 sider in some detail that particular flavoprotein with which I am 

 most familiar and which was isolated by my two collaborators, Haas 

 and Horecker— cytochrome c reductase. I should like to add that 

 Mr. Haas is responsible for much of the experimental work reported 

 by me here. 



The Old Yellow Enzyme— Kher Barron and Harrop (23) found 

 that methylene blue could bring about the respiration of erythro- 

 cytes, Warburg and Christian repeated these experiments, using an 

 extract of horse erythrocytes, with hexose monophosphate as the 

 substrate. They were able to separate from this extract three factors 

 necessary in the respiration process. From these three separate com- 

 ponents, the old yellow enzyme, triphosphopyridine nucleotide, and 

 Zwischenferment were later isolated. The old yellow enzyme was 

 thus the first flavoprotein to be discovered. The prosthetic group of 

 the old yellow enzyme is alloxazine mononucleotide or riboflavin 

 phosphate. 



An interesting observation that has never been explained came 

 out of these first experiments of Warburg and Christian. Whereas it 

 was necessary to add methylene blue to a fresh extract of horse 

 erythrocytes to bring about respiration, the addition of the dye was 

 not necessary when the extract had been dried and subsequently 

 dissolved. In the latter case the old yellow enzyme presumably 

 reacted directly with oxygen. If so, then why did not the same 

 reaction also take place with the fresh extract? 



The d- Amino Acid Oxidase— In 1934 Krebs (10) identified both 

 the d- and the Z-amino acid oxidases in an extract obtained from 

 kidney cortex. He found that whereas the Z-amino acid oxidase is 

 inhibited by cyanide, the d-amino acid oxidase is not. Furthermore, 

 when the extract is dried, the Z-amino acid oxidase is destroyed, 

 whereas the cZ-amino acid oxidase remains active. 



Beginning with Krebs' findings, Warburg and Christian (11) 

 isolated the cZ-amino acid oxidase and found that its prosthetic 

 group was a dinucleotide made up of riboflavin phosphate and 

 adenylic acid. The cZ-amino acid oxidase reacts with most of the 



