138 A SYMPOSIUM ON RESPIRATORY ENZYMES 



ff-amino acids, oxidizing them to the alpha-keto acids and ammonia; 

 d-glutamic acid is a notable exception. Its reactivity with oxygen is 

 greater than that of the other flavoproteins. 



The New Yellow Enzyme.— The new yellow enzyme, isolated by 

 Haas (12) immediately after the first isolation of the c?-amino acid 

 oxidase, was found to have associated with it, as its prosthetic group, 

 alloxazine adenine dinucleotide. It reacts with diphosphopyridine 

 nucleotide as does the old yellow enzyme; its reactivity toward 

 oxygen is considerably less than that of the old yellow enzyme, but 

 it is very active toward methylene blue. 



The Straub Yelloio Efizyme.—This enzyme (13) was isolated from 

 heart muscle and in all probability is the same enzyme that Haas 

 isolated from yeast. 



The Crossed Yellow Enzy7ne— Warburg and Christian (14) "syn- 

 thesized" a new enzyme by having the protein moiety of the old 

 yellow enzyme combine with alloxazine adenine dinucleotide rather 

 than with the mononucleotide in combination with which it was 

 isolated from yeast. The properties of this crossed enzyme are 

 similar to those of the old yellow enzyme. 



Xanthine Oxidase.— This enzyme, isolated in a purified state by 

 Ball (15), catalyzes the oxidation of the purines, particularly 

 xanthine, the aldehydes, and diphosphopyridine nucleotide (24), by 

 oxygen. Its prosthetic group is alloxazine adenine dinucleotide. Since 

 its exact nature has not yet been elucidated, it is possible that this 

 enzyme contains components not yet accounted for. As defined by 

 its activity toward aldehydes, it was once known as "Schardinger's 

 Enzyme." It was first identified in milk by Morgan, Stewart, and 

 Hopkins (25). 



Fumaric Dehydrogenase.— This enzyme catalyzes the reduction of 

 fumaric acid by one of several leuco dyes, the products of the 

 reaction being succinic acid and the oxidized or colored dye. The 

 activity of the enzyme is measured by the rate at which the color 

 appears. It was discovered by Fischer and Eysenbach (16) in 1937, 

 and in 1939 Fischer, Roedig, and Ranch (17) purified it further by 

 electrophoresis. No physiological reducing agent has been found 

 with which this enzyme is active. 



Aldehyde Oxidase.— hike xanthine oxidase, this enzyme catalyzes 

 the oxidation of aldehydes but diflFers from the xanthine oxidase in 

 that it does not catalyze the oxidation of xanthine. It was isolated 

 in 1939 from liver by Gordon, Green, and Subrahmanyan (18). 



Cytochrome c Reductase— This flavoprotein acts as the inter- 



