GENERAL CONSIDERATIONS OF MULTIENZYME SYSTEMS 379 



rather the specific concentration (A)', that is important. The over-all con- 

 centration of an intermediate in a reaction mixture may be low but the 

 level within a compartment may be much higher also. Nevertheless, some 

 of the general conclusions on the effects of compartmentalization brought 

 out by Hearon and Bierman are qualitatively valid. 



Holzer (1959) has pointed out well that it is difficult to determine the 

 behavior of multienzyme systems, especially branched chains, because the 

 intracellular concentrations of the substrates and intermediates are gen- 

 erally not known. The concentration of inorganic phosphate within the 

 cell as calculated from the equilibrium reaction catalyzed by phosphory- 

 lase is much lower than that based on the analysis of total inorganic phos- 

 phate. This must be interpreted to mean that phosphate is very unequally 

 distributed within the cell. Certainly the rates of many intracellular en- 

 zyme reactions must depend on the particular region in which the enzymes 

 occur and on the concentrations of the involved substrates in these regions. 



Structural Disruption of Multienzyme Systems 



When multienzyme systems are spatially organized, either by aggre- 

 gation of enzymes into complexes or localization into specific regions, in- 

 hibition of the metabolic rate may be brought about by the inhibitor dis- 

 turbing in some manner this organization rather than by a direct action 

 on the enzyme. This possibility must always be kept in mind when inter- 

 preting the results of the action of an inhibitor on such complex systems. 

 The types of structural disturbance to which a system is sensitive will 

 depend on the way in which the component enzymes are organized. If the 

 enzymes are held together in a complex, a substance could alter the distance 

 between active sites or actually split the complex by reacting with the en- 

 zyme protein or with some cementing material. Compounds which react 

 avidly with protein groups, such as the sufhydryl reagents, or form hy- 

 drogen bonds readily, such as urea, or are adsorbed at water-protein in- 

 terfaces, such as a detergent, would be most likely to interfere in this man- 

 ner. If the consecutive enzymes interact by rotation, as has been postulated 

 by Chance (1956) for electron transport, a substance could hinder rotation 

 without necessarily splitting apart the enzymes. Restriction of rotation 

 could be achieved sterically by simply reacting with certain protein groups. 

 Finally, if the enzymes are compartmentalized by membranes, a substance 

 could decrease the permeability to the substrate or intermediates, or pos- 

 sibly disrupt the membrane so that organization and the steady state are 

 abolished. In these and other ways, nonenzymic effects (that is, those that 

 would not be observed with the isolated enzyme) may depress sequential 

 reactions in cells and some of the effects observed in mitochondrial suspen- 

 sions may be attributable to changes in the surrounding membrane, lead- 

 ing to loss of coenzymes and disorganization of structure from swelling. 



