FUNCTION 



When rats were maintained on a ribofla vine-deficient diet, the 

 heart and liver were found to contain less flavine adenine dinucleotide 

 than the corresponding organs from normal rats.^^ The dinucleotide 

 was synthesised from ribofiavine in vitro by human red blood cells 

 when incubated at 30 to 34° C. with a solution of ribofla vine, or in 

 vivo by red blood cells on ingestion of a suspension of ribofiavine.^® 



Riboflavine monophosphate and riboflavine adenine dinucleotide 

 account for practically all the riboflavine present in rat kidney, and 

 the dinucleotide accounts for 70 to 90 % of the total riboflavine in all 

 tissues. 2^° 



Flavine Mononucleotide : Cytochrome c Reductase 



The first flavine enzyme to be recognised was the enzyme system 

 that oxidised hexose-monophosphoric acid to phosphohexonic acid. 

 It was first isolated by O. Warburg and W. Christian ^^ and is now 

 known as " old " yellow enzyme. They postulated that the oxidation 

 was effected by the following chain-reaction : 



(i) Hexose-monophosphate + triphosphop5n:idine nucleotide-pro- 

 tein -> dihydro-triphosphopyridine nucleotide-protein -f phos- 

 phohexonic acid ; 



(2) Dihydro-triphosphopyridine nucleotide-protein -f flavoprotein 

 -> triphosphopyridine nucleotide-protein -|- dihydro-flavo- 

 protein ; 



(3) Dihydro-fiavoprotein + O2 -> flavoprotein -f- H2O2. 



H. TheorelP^ pointed out, however, that the rate of oxidation of 

 this dihydro-flavoprotein at the oxygen tension of the organism was 

 too small for dehydrogenation to occur by this route, and he suggested 

 that cytochrome c was responsible for the reoxidation, though the rate 

 of reaction of the " old " yellow enzyme with cytochrome c was also 

 too small to account adequately for the facts. Support was given to 

 the suggestion, however, by the observation of E. S. G. Barron ^3 that 

 the oxidation of hexose-monophosphate by certain bacteria was 

 completely inhibited by hydrogen cyanide, which is known to inhibit 

 the action of cytochrome c. Final proof of Theorell's hypothesis was 

 afforded by Haas et al.,^^ who isolated from yeast a new flavoprotein, 

 which reacted very rapidly with both oxidised cytochrome c and 

 reduced triphosphopyridine nucleotide. The prosthetic group of the 

 new enzyme, which they called cytochrome c reductase, was shown to 

 be alloxazine mononucleotide. 



The new enzyme, unlike the " old " yellow enzyme, reacted specific- 

 ally with cytochrome c, but the prosthetic groups of cytochrome c 

 reductase, the " old " yellow enzyme and amino acid oxidase proved 



13 193 



