Discussion 223 



cycle and the respiratory chain have been condensed to three equations 

 for the sake of simphcity. Equation 12 represents the reaction of pyru- 

 vate which is exchangeable between the mitochondrial and the cyto- 

 plasmic spaces with the DPN that is enclosed within the mitochondrial 

 membrane (DIN). The DPNH so formed (DIH) is also restricted to the 

 mitochondrial space. Such DPNH reacts with oxygen (OXY) and a 

 low energy of an intermediate in oxidative phosphorylation (X-I) to 

 form oxidized DPN and a common high-energy intermediate in oxida- 

 tive phosphorylation (XSI). This intermediate can then interact with 

 ADP and phosphate in equation 14 to form ATP in store II (2TP). 



In the last functional block, the utilization of ATP is represented by 

 equations 15-17. The last two equations represent the interaction of 

 ATP in store I with phosphate utilization enzymes (PUE) to give an 

 enzyme intermediate PPP which decomposes in a first-order reaction 

 to give ADP and phosphate, which readily activates respiration. 



Equation 15 is a most important one since it represents a mechanism 

 by which ATP can be freed from the mitochondrial store (2TP) and 

 made available to the cytoplasmic store (ITP). Experimentally this is 

 observed to be caused by the addition of uncoupling agent such as 

 dibromophenol and dicoumarol. There may be a physiological sub- 

 stance of unknown nature which may be responsible for this. However, 

 we have simply indicated that there may be added a concentration of 

 dibromophenol (DBP) between zero and 10~^ m to activate this inter- 

 change. 



The reaction velocity constants used in this mechanism are rather 

 arbitrarily chosen. In fact, the deciding factor in choosing the reaction 

 velocity constant has often been that the product of the steady state 

 concentrations of the reactions involved and the reaction velocity 

 constant give a metabolic flux between one and ten per second. The 

 latter figure is reasonable in view of our observations of the turnover 

 number of the respiratory enzymes in the intact cell. Thus, inconsist- 

 encies between initial concentrations and reaction velocity constants of 

 this mechanism and those of the actual cell may occur. Nevertheless, 

 the kinetic and steady state behaviour of the system appears to be a 

 reasonably good representation of that of the intact cell. As more 

 accurate data on reaction rates and concentrations become available 

 the computer data can, of course, readily be amended. 



The programming of the digital computer (Univac I) is quite outside 

 the scope of this talk and will be reported elsewhere by Garfinkel, 

 Higgins and Rutledge who have developed the equation-solving process. 

 In brief, the computer recognizes the names of the chemicals, the nature 

 of their interactions, the reaction velocity constants, the initial con- 

 centration and maximal concentrations, and the stoichiometric ratios. 



Fig. IB shows a graph of the kinetics of some of the chemicals in the 

 system of Table I. This graph is computed from tabular data made 

 available by the digital computer and is typewritten on a high speed 

 printer in the form illustrated. The ordinates are concentrations and 

 the abcissae are computer time intervals (typewritten at bottom). 

 Only the main features of the graph wiU be indicated here. 



