IV. BIOCHEMICAL SYSTEMS 341 



8.25%, histidiiK' 2.75%, lysine 13.7%, tyrosine 7.75%, pheiiylahiiiiiie 

 5.75%, tryptophan 4.8()%, cystine 0.18%, filutamic acid 7.17%,, and as- 

 partie acid 2%. Since only 20% of the total snll'nr conld he acconntcd lor 

 as cystine, it is prohahle (hat other siilfnr-containiiiji; amino acids are pres- 

 ent in the protein. According to these analyses, which account for al)out 

 ()G% of the total nitrogen, the yellow enzyme contains approximately" 33 

 molecules of arginine, 13 molecules of histidine, (M) molecules of lysine, 40 

 molecules of proline, 30 molecules of tj'rosiiie, 24 molecules of phenylalanine, 

 17 molecules of tryptophan, 34 molecules of glutamic acid, 12 molecules of 

 aspartic acid, and only 1 to 2 molecules of cystine. 



h. Mechanism of Action of Yellow Enzyme 



After Barron and Harrop^-' ^^ found that methylene blue could catalyti- 

 cally increase the respiration of erythrocytes, Warburg and Christian^^ 

 repeated the experiments, using hexose monophosphate (Robison ester) as 

 a substrate in an extract of horse erythrocytes. From this extract they were 

 later able to separate a thermolabile substance, an enzyme designated as 

 zwischenferment (Robison ester dehydrogenase), and a thermostable "zwi- 

 schenferment-coferment" (triphosphopyridine nucleotide) to form a com- 

 plete, iron-free, respiratory chain capable of reacting with molecular oxy- 

 gen. The isolation of the yellow^ enzyme from bottom yeast^ led to the 

 chemical characterization of the thermolabile substance in blood cells 

 wliich became known as the yellow oxidation enzyme. 



Warburg^^ has stated that in respiring cells the yellow enzyme cannot 

 successfully compete with hemin enzymes which react with oxygen at a 

 much greater rate. In hemin-free cells, e.g., in facultative anaerobic lactic 

 acid bacteria, the respiration is catalyzed by the yellow enzyme which es- 

 tablishes direct contact with molecular oxygen.^^ However, the "turnover 

 number" of the old yellow enzyme (the number of times that a molecule 

 of enzyme is both oxidized and reduced in a minute) under optimum con- 

 ditions at 38° in pure oxygen is only 55.-" At the low oxygen tensions which 

 exist in animal tissues, flavoprotein would hardly be autoxidizable. This 

 would mean that the old yellow enzyme, at best, fulfills a highly special- 

 ized, not yet understood role in normal cell respiration, that it may react 

 with other unknown acceptors, or that it is an artifact-' of preparation. 



The classical method of describing the action of the old yellow enzyme 



^^ C. Oppenheimer and K. G. Stern, Biological O.xidation, p. 192. Nordemann, New 



York, 1939. 

 " E. S. G. Barron and G. A. Harrop, .7. Exptl. Med. 48, 207 (1928). 

 " E. S. G. Barron and G. A. Harrop, J. Biol. Chem. 79, 65 (1928). 

 " O. Warburg and W. Christian, Biochem. Z. 242, 206 (1931). 

 ■• O. Warburg and W. Christian, Biochem. Z. 238, 131 (1931). 

 '•^ O. Warburg, Xalurwissenschaften 22, 441 (1934). 



