INHIBITOR-STABLE METABOLISM 449 



thermore, the intracellular inhibitor concentration is often less than in the 

 medium and a selected concentration, based on nearly complete inhibition 

 of an isolated enzyme, will not necessarily achieve the desired effect in the 

 cell. For this reason, it is advisable to plot a curve of the metabolic rate 

 against the inhibitor concentration to determine the level necessary to 

 induce maximal inhibition, rather than to rely on isolated enzyme studies. 

 Finally, it is unfortunate that this formulation has not been extended to 

 other inhibitors to see if or when it is applicable. There are certainly some 

 forms of inhibitor-stable metabolism to which this simple treatment would 

 not be valid, as will be described below. 



Some of the conditions responsible for an inhibitor-stable fraction of the 

 metabolism will now be discussed. 



I. Presence of branched, polylinear, or parallel chains. When there is more 

 than one pathway for the utilization of a substrate or the formation of 

 a product, inhibition of an enzyme in only one of these paths, even though 

 it is complete, will generally not reduce the total process completely. The 

 total respiration of a cell is usually made up of several i^athways, some of 

 which are branched and interrelated, some of which are separate, often 

 utilizing different substrates, so that the inhibition of any particular 

 enzyme will seldom block every pathway. There are many examples which 

 may be cited. In a tissue possessing an active hexose monox)hosphate shunt, 

 the action of an inhibitor such as iodoacetate (which here fairly selectively 

 blocks the triosephosphate step in the Embden-Meyerhof pathway) will 

 not completely depress glucose utilization or oxygen uptake. In some tis- 

 sues the arsenicals, by blocking the pyruvic oxidase, may essentially com- 

 pletely prevent the utilization of pyruvate, but in others, especially micro- 

 organisms, there are other pathways of pyruvate metabolism not susceptible 

 to the arsenicals. Again, if formation of ATP is being determined, 2,4- 

 dinitrophenol will not always reduce this to zero because the uncoupling 

 action is not exerted on phosphorylation linked with substrate level trans- 

 fers. There is evidence that cytochrome c may be oxidized not only by the 

 cytochrome a and ag systems but by a peroxidase (Chance and Williams, 

 1956), so that an inhibitor acting on cytochrome ag, but relatively less on 

 peroxidase, could not completely abolish the uptake of oxygen. Conditions 

 such as these are perhaps the most important in explaining inhibitor- 

 stable metabolism. 



II. Reactions proceed ahead of tJte step blocked in a chain. If in a monolin- 

 ear chain such as A ^ B -> C ^ D, the last step is blocked, the ear- 

 lier reactions may continue. A complete block at a single enzyme in a se- 

 quence of oxidations will allow oxygen to be taken up in all the steps occur- 

 ring before the block. This may also occur in cyclic systems. When the suc- 

 cinic dehydrogenase in the tricarboxylic acid cycle is inhibited, the cycle 



