82 



Cellular Structure and Activity 



basis of known molar concentrations. In 

 the case of enzyme activity, neither molar 

 concentrations nor causes of activity varia- 

 tions are usually known. In this section we 

 shall consider enzyme activity* in terms of 

 variations in molar concentrations and in 

 activities. The diagram in Figure 8 may help 

 to illustrate some of the variety of factors 

 that might be operative in controlling the 

 rate of an over-all reaction catalyzed by a 

 given enzyme system. 



Supply of Substrate and Cofactors. A gen- 

 eral illustration of this type of control in- 

 volving the cofactor ATP has been developed 

 with special acuity by Potter ('44). The way 

 in which ATP and DPN aid in regulating 

 phosphorylations and fermentations should 

 be kept in mind here, for the general rea- 

 soning used in these cases probably applies 

 to other substances both in the entire cell 

 and in localized areas within the cell. In- 

 teresting modifications in patterns of activity 

 must arise when any single substance or 

 cofactor serving several enzyme systems is in 

 short supply. Under such conditions com- 

 petition for the limiting factor might even 

 result in the complete exclusion of one re- 

 action in favor of another. 



Exogenous substrate supply may be called 

 a controlling factor of the second order, since 

 it appears to be itself under the control of 

 selective mechanisms in the outer layers of 

 the cell. Phosphate and sugar, for example, 

 appear to enter yeast cells only during active 

 metabolism (Spiegelman and Kamen, '47), 

 and even simple elements such as potassium 

 have selective exchange systems related to 

 the production of specific organic acids (Roth- 

 stein and Meier, '48). The clearest examples 

 of active regulation of movement of material 

 in a living system are contained in the 

 studies on sugar resorption in the kidneys 

 (Pitts and Alexander, '44); and these find- 

 ings and interpretations are useful in con- 

 sidering the penetration of foodstuffs into 

 cellular systems. The more orthodox aspects 

 of permeability do not seem to shed much 

 light on the problems of transfer of material 

 in living cells (cf. material in the mono- 

 graph by Davson and Danielli, '43). 



An interesting aspect of svirface control is 

 the finding of Rothstein and Meier ('48) that 

 numerous esterases and other enzymes local- 

 ized on the outer layers of yeast cells will 

 break down various tested intermediates of 

 the glycolytic and oxidative cycles, and re- 

 lease the hydrolytic products quantitatively 

 into the medium. Thus they may guard the 



* See preceding footnote. 



orderly train of events going on within the 

 cell by preventing the entrance of unplanned- 

 for intermediates. We might, for exam- 

 ple, infer that glucose, in order to be properly 

 channeled into cell metabolism, must origi- 

 nally enter into the metabolic reactions of 

 the cell by being phosphorylated in the ini- 

 tial stages; previously formed glucose phos- 

 phate appears to be excluded. 



The Structural Orientation of Enzymes. The 

 tendency of many enzymes to be associated 

 with insoluble particulates after cellular dis- 

 ruption has already been pointed out. At 

 the present time a tentative understanding 

 of many significant intracellular phenom- 

 ena can be arrived at by assuming that such 

 associations exist within the cell, and that 

 they control activity both by keeping en- 

 zyme and substrate apart or bringing them 

 together, and also by bringing enzyme into 

 contact with activators or inhibitors. Such 

 geometrical arrangements are probably at 

 the basis of very rapid changes in activity — 

 for example, the rise in respiration in muscle 

 and nerve that can be detected within milli- 

 seconds after the onset of electrical activity. 

 This rise is probably due to the "switching 

 in" of enzymes previously immobilized, to 

 act on substrates previously held in reserve. 

 That the system switched in may be at least 

 in part different from the resting system 

 is indicated by studies like those of Stan- 

 nard ('39), who showed that azide does not 

 affect resting muscle respiration, but abol- 

 ishes the increase of respiration upon acti- 

 vation. 



In comparing enzyme reactions in tissue 

 under different physiological states, time is 

 an important criterion in determining 

 whether observed increases are due to al- 

 terations in activity of pre-existing enzyme 

 molecules, or to synthesis of new molecules. 

 A short time change is of more diagnostic 

 value because it most likely indicates en- 

 zyme activation or inhibition. A change oc- 

 curring over a long time, on the other hand, 

 might indicate either enzyme activation or 

 enzyme formation. 



There are numerous cases suggesting, but 

 not proving, that enzyme localizations may 

 serve as isolating mechanisms. All the stud- 

 ies showing enzyme binding to cytoplasmic 

 particles might be cited here, subject, of 

 course, to confirmatory studies showing evi- 

 dence of controlled change during activity. 

 Chantrenne's ('43) examination of the effects 

 of moving the cytochrome oxidase-rich gran- 

 ules of intact Amphiuma liver cells by cen- 

 trifugation is especially interesting. In these 



