THE PROBLEM OF ENDOGENOUS METABOLISM 443 



Case 3: two enzymes and one substrate (the enzyme responsible for the 

 endogenous rate is different from the enzyme which acts on the added 

 substrate). 



S, + El — ESe ^ E + P 



S + E2 — E^S ^ E + P 



Each rate is given by the characteristic expression and they are additive 

 to give the total rate when the substrate is added. This situation could 

 occur only if the two enzymes are so separated that the substrate from 

 different sources could reach either one or the other. 



Case 4: tivo enzymes and two substrates (the endogenous rate is due to 

 one substrate and its enzyme, the exogenous rate to another substrate and 

 another enzyme). 



S, + El — EiS, -> El + P 



S' +E,^ E,S' -^ E^ + P 



The individual rates, as in case 3, are additive. 



The results will be interpreted differently depending on what is measured 

 in the enzyme reaction. It has been assumed above that a common product 

 is determined. If the two substrates involved are two phosphate esters, 

 one can measure the inorganic phosphate released during hydrolysis, or 

 one can determine the formation of the different dephosphorylated frag- 

 ments. The latter method gives more accurate information but usually 

 cannot be applied. In the determination of respiration it is generally im- 

 possible to fractionate the total oxygen uptake into the various reactions 

 proceeding. 



A rule for these simple systems is seen to emerge from the treatment of 

 the four cases above. The individual rates are additive when two enzymes 

 are involved and are not additive when a single enzyme is responsible for 

 both exogenous and endogenous reactions. In practice it is often difficult 

 to know if one or more enzymes are participating. Reiner (1959, p. 89) 

 has presented methods for determining if one or more than one substrate 

 or enzyme is involved in such cases. This is the only quantitative approach 

 to the problem of endogenous metabolism known to the author. 



Effect of Endogenous Metabolism on Inhibition Kinetics 



The reciprocal plot of Ijv against 1/(S), where S is the exogenous sub- 

 strate, will no longer be linear when endogenous metabolism occurs, and 

 this is true whether the endogenous and exogenous rates are additive or not. 



