56 OXIDATION-REDUCTION POTENTIALS 



As would be expected from its participation in oxidation-reduction reactions, 

 riboflavin, the prosthetic group of the yellow enzymes, is subject to reversible 

 oxidation-reduction reactions. The potential of the half reduced pigment at pH 7*0 

 having been given as — 0-22 v. Reduction of the pigment, however, occurs in two 

 stages, and in acid solution an intermediate red form, a semi-quinone can be observed, 

 whilst in neutral solution the intermediate form is green (Michaelis, Schubert and 

 Smythe, Kuhn and Waguer-Jauregg, 1934 ; Klemperer, Bessey and Hastings, 1937). 

 Haas has found that when the flavin is combined with its carrier protein the inter- 

 mediate red form is seen even in neutral solution. 



The electrode potential of the fla^^n is not affected by phosphorylation but com- 

 bination with the carrier protein does affect the potential (Kuhn and Boulanger, 

 1936) ; Eq of the flavinphosphates is 0-187 v. and of the flavoprotein 0*373 v. 



Among the many flavoproteins described is diaphorase from heart muscle, 

 a flavin adenine dinucleotide. (Straub, 1939 ; Euler and Hasse, 1938 ; Dewan 

 and Green, 1938). 



AMINO-ACID OXIDASE 



d-Amino-acid oxidase has been isolated from animal liver, kidney, etc., and is a 

 flavoprotein containing adenine-flavine dinucleotide as prothetic group. It oxidises 

 only d-amino acids and since natural amino acids are of 1-configuration its significance 

 is doubtful, but a relatively feeble 1-amino-acid oxidase has been found. d-Aniino- 

 acid oxidase oxidises most d-amino acids with the notable exception of glutamic acid, 

 but 1-amino-acid oxidase attacks only mono-amino-monocarboxylic acids and not 

 glycine or 1-amino-acids containing the hydroxyl group. The oxidation proceeds as 

 follows : — 



R.CH (C00H).NH2 -> R.C (COOH) : NH + 2H 

 and the imino-acid is hydrolysed spontaneously to the keto acid and ammonia. 



R.C (COOH) : NH + H^O -^ R.CO.COOH + NH3 



XANTHINE OXIDASE 



Xanthine oxidase preparations contain adenine flavine dinucleotide so that it is 

 probably a flavo-protein but other prosthetic groups may be present. The oxidase 

 occurs in milk and catalyses the oxidation of hy|Doxanthine to xanthine, and of 

 xanthine to uric acid. 



— CH = N— + H2O -> — CHOH.NH > — CO.NH— -f H^Oa 



In both cases a hydrate is an intermediate and hydrogen peroxide is formed. 

 The system itself does not give rise to an electrode potential although the peroxide 

 formed does so in the presence of oxygen. If methylene blue is added it is reduced 

 and the characteristic potential of the dye is observed. With alloxan the potential 

 characteristic of the alloxan : dialuric system is established. 



ALDEHYDE DEHYDROGENASE 



In animal tissues the dismutation 



Acetaldehyde -^ Ethyl alcohol + acetic acid 



occurs independently of the xanthine oxidase system. This was originally thought to 

 be caused by a " mutase " enzyme activating a Cannizaro reaction, but it is now shown 



