Cellular Metabolism 



81 



carry an inactive reserve of enzymes to be 

 used only in periods of unusual stress. This 

 is self-evident in tissues like muscle, in which 

 there is a clear-cut difference between rest- 

 ing and activity metabolism; but it is prob- 

 ably true for many other tissues also. Cells 

 of embryonic tissues, however, may well 

 operate with virtually no safety factors in 

 enzyme concentration. Such cells presumably 

 are never "resting" in the sense that adult 

 muscle fibers or gland cells rest, and accord- 

 ingly enzyme systems might be expected to 

 function continuously at near maximum 

 activity. 



A second element of uncertainty in the 

 homogenate technique is the influence on 

 activity of association of enzyme with struc- 

 tural components of the cell. Although grind- 

 ing tissue may destroy all the coarser ele- 

 ments of cell structure, it is well known, as 

 we have pointed out before, that different 

 enzymes show varying affinities for particu- 

 late matter in breis. Since enzyme activity 

 must certainly depend in large part on pre- 

 cise steric configurations, such as may be 

 involved in the antigen-antibody situation 

 (cf. Pauling, '48), it would not be surprising 

 if a given number of enzyme molecules 

 bound onto particles would exhibit a differ- 

 ent activity from the same number of 

 molecules in free solvition (Mazia and Blvim- 

 enthal, '50). 



That such variations do occur is indicated 

 by several studies in which different homog- 

 enization media were used. In the case of 

 muscle apyrase, for example, a water ho- 

 mogenate has high activity but is insensitive 

 to Ca**, whereas extraction with strong po- 

 tassium chloride produces a soluble enzyme 

 preparation of low activity which is strongly 

 activated by Ca""* (Steinbach, '49). Similar 

 effects have been noted with apyrase in the 

 granules of chick embryo breis (Steinbach 

 and Moog, '45). In neither case is there any 

 assurance that only one enzyme is being 

 dealt with, but the results are consistent with 

 the view that association of enzymes with 

 insoluble material alters their activity. The 

 same interpretation can be placed on the 

 report of Tyler ('50) that respiration is in- 

 hibited by dinitrophenol in tissue breis pre- 

 pared as water homogenates, but is activated 

 by dinitrophenol when the brei is prepared 

 by homogenizing in Ringer's solution with 

 glucose added. 



The same enzyme might possibly vary in 

 its structural relations from tissue to tissue. 

 For this reason the differences in enzyme 

 patterns previously cited may only partly 



reflect differences in actual concentration of 

 enzymes. 



Enzyme patterns have also been studied by 

 the methods of chemical genetics. Here en- 

 zyme activity is measured in terms of ac- 

 tivities of intact cells, the assumption being 

 made (but rarely tested) that other factors 

 are not limiting. An excellent summary of 

 these studies has been published by Tatum 

 ('49). 



DIFFERENCES IN ENZYME ACTIVITY 



Given a fixed number of enzyme molecules 

 in a cell, their in vivo activity will depend 



SUPPLY 



ENZYME UNIT 



SUPPLY 



REGULATED BY OTHER INTRACELLULAR 

 SYSTEMS OR BY EXCHANGE WITH ENVIRONMENT 



Fig. 8. Diagram to illustrate some possible meth- 

 ods by which unit enzyme systems might be con- 

 trolled. The enzyme unit (its protoplasmic analog 

 might be a mitochondrion) is pictured as connected 

 to the rest of the living system by (1) substrate 

 supply, (2) waste or product removal and (3) dif- 

 fusible co-factor supply. Control of activity might 

 be accomplished by control of rate of flow of any of 

 these factors. 



In addition, the state of the enzyme within the 

 unit could be altered by (1) reversible association 

 with inhibitors or (2) reversible change of activity 

 by association or dissociation of enzymes into struc- 

 tures. From our present knowledge of enzymatic 

 content of particulate units it can be deduced that 

 all these factors, and possibly more, might enter 

 into the delicate control of cellular metabolism. 



on the quantity of the substrate available, 

 on the adequacy of the cofactor supply, and 

 on geometrical relations tending to inhibit or 

 promote the reaction in which the enzyme 

 participates. The term "activity," applied to 

 enzyme systems, bears an obviovis relation- 

 ship to the same term used in orthodox chem- 

 istry, i.e., a fudge-factor introduced to ac- 

 count for the differences between observed 

 rates of actions and those calculated on the 



