THE FLAVOPROTEINS 139 



mediary link between cytochrome c and triphosphopyridine nucleo- 

 tide. It will later receive special attention. 



A glance at Table 1 shows clearly that many of these flavopro- 

 teins are closely associated with either disphospho- or triphosphopyr- 

 idine nucleotide. They therefore constitute a link in the hydrogen 

 transport system. Nor is it surprising that so many of them react 

 with molecular oxygen. In fact, we should expect all of them to be 

 autoxidizable, at least to a small extent, for all of them are pre- 

 sumably dissociable into the prosthetic group and a protein, and 

 the prosthetic group itself is autoxidizable. In some cases, however, 

 the prosthetic group, when attached to the protein, is very probably 

 also autoxidizable. If this were not true, we might expect that those 

 flavoproteins which dissociate to the greatest extent, i.e., those 

 having the largest dissociation constants, would have the greatest 

 rate of autoxidation. There is evidence that those flavoproteins with 

 the greater dissociation constants are more autoxidizable, but it 

 is not conclusive. In making such a comparison we must compare 

 with each other only those flavoproteins having the same prosthetic 

 group and under such conditions that the oxidation of the flavo- 

 protein is the rate-determining step. 



Each of the flavoproteins dissociates to a difi^erent extent into its 

 prosthetic group and its protein moiety. The degree of dissociation 

 of any flavin nucleotide— protein complex is determined by a pro- 

 cedure which involves first splitting the complex into its two con- 

 stituent parts and separating them. By adding increasing amounts of 

 the prosthetic group to a fixed amount of protein and choosing 

 conditions such that the reaction velocity is proportional to the 

 amount of complex formed, it is then possible to determine the dis- 

 sociation constant from the well-known Michaelis-Menten equation, 

 or some modification of it. This equation states that 



(1) ^ '^' 



Vm Kt+(S) 



in which V is the velocity, Vm the maximum velocity (when all pro- 

 tein is in the form of a complex), (S) is the total concentration of the 

 prosthetic group, and K^ is the dissociation constant. When 

 y/Vm = 0.5, then K^ is equal to (S). This equation is valid only 

 when the dissociation constant is so high or the concentration of the 

 protein so low that the amount of the prosthetic group bound to the 

 protein is small as compared with the total amount in the solution. 

 Under these conditions the concentration of the uncombined pros- 



