CONCEPTS AND TERMINOLOGY 321 



dria, cells, or tissues are unstable and tend to lose various substances from 

 the cells and pick up others from the medium. These substances may not 

 be involved directly in the multienzyme system but may influence its 

 behavior; the common inorganic ions may be placed in this category. 

 (3) The concentrations of the component enzymes may change. Inactivation 

 of enzymes in metabolic preparations usually occurs and leakage of enzymes 

 or coenzymes from cellular or subcellular structures is well known. Fur- 

 thermore, the enzyme concentration may fluctuate with the concentration 

 of its substrate in the manner of adaptive enzymes, (-i) Cell populations are 

 seldom static and are generally either increasing by growth or decreasing 

 by various processes of degeneration or death. Such processes would have 

 direct and indirect effects on any multienzyme system being studied. For 

 practical purposes, therefore, one must usually be satisfied with a system 

 approaching steady state kinetics. Such an approximation can often be 

 accomplished by restricting the experimental time interval to a period 

 during which negligible changes have occurred in the rates and concen- 

 trations. It would be more correct to speak of such systems as in a pseudo- 

 steady state. In such situations there is a continuous succession of steady 

 states with time, the transitions between them being smooth and continuous. 

 The more important multienzyme systems to be discussed may be clas- 

 sified in the following manner. 



I. Monolinear chains A-^B^-...^P 



II. Branched chains 

 A. Convergent 



B. Divergent 



III. Polylinear chains (shunts) 



IV. Distributive systems 



V. Cyclic systems 



