RIBOFLAVINE 



The former acted as reductant, whilst the latter comprised the 

 component capable of being excited by oxidation. Some of these 

 excited molecules emitted radiations without being destroyed, whilst 

 others were destroyed by the energy absorbed. It is this latter 

 phenomenon that is responsible for the degradation of riboflavine on 

 exposure to light. The authors conclude : " Thus, in luminous bac- 

 teria, light emission presumably occurs when fiavoprotein, reduced by 

 hydrogen from suitable substrates (e.g. glucose) via the dehydrogenase- 

 coenzyme system, is oxidised directly by oxygen ". 



Oxidation of Flavoproteins 



The fiavoprotein dinucleotides can be oxidised by molecular 

 oxygen, but in the organism they are, like the mononucleotide, 

 oxidised by cytochrome c and this explains why the oxidation of 

 D -amino acids and of pyruvic acid by bacteria possessing cytochromes 

 is completely inhibited by hydrogen cyanide.^'* ®^ When cytochrome 

 is absent, as in some bacteria and protozoa, oxidation may be effected 

 by molecular oxygen. The effect of cyanide on oxygen consumption 

 and luminescence, respectively, indicates that most of the hydrogen 

 proceeds stepwise, by electron transfer through the cytochrome- 

 haeme system, to oxygen. If cl^lorophyll were substituted for the 

 related haeme molecule, the same system of catalysts operating in the 

 reverse direction would lead to photosynthesis. In luminescence two 

 hydrogen atoms were oxidised for each quantum emitted, whilst in 

 photosynthesis single hydrogen atoms were made available. 



To bring about this transfer of hydrogen in these enzymic reactions, 

 the riboflavine within the coenzyme undergoes alternate reduction and 

 oxidation. The reduction product is dihydro- or leuco -riboflavine 

 and, as already stated, it readily undergoes re-oxidation on shaking 

 with air. 



According to R. Kuhn and R. Strobele ^^ the mechanism of the 

 reduction is not just simple addition of hydrogen across the centre 

 ring, but proceeds via three intermediate compounds, the bronze- 

 green verdoflavine, the grass-green chloroflavine and the carmine-red 

 rhodofiavine. These changes are represented by the formulae on the 

 opposite page (where R = ribityl-). 



Other Functions of Riboflavine : Riboflavine in the Eye 



The occurrence of fiavines in the retinae of fishes has long been 

 known and the possible importance of riboflavine for vision in dim 

 light was confirmed by observations made by P. Karrer and 

 H. Fritzsche '° on the fluorescence curves of riboflavine and its 



198 



