Cellular Metabolism 



85 



cipal use. But in this case it was not clear 

 that the injected material was actually 

 carried into the embryo at an enhanced 

 rate. 



There are no unequivocal cases, either, to 

 show that differentiated enzymes are in- 

 hibited by lack of necessary cofactors. In a 

 very thorough study, Novikoff et al. ('48) 

 demonstrated that even at three days, the 

 earliest stage investigated, the chick embryo 

 contains ATP, DPN and phosphocreatine in 

 amounts falling within the same range as in 

 various mature rat tissues. They also found 

 that numerous intermediates of the phos- 

 phorylating glycolytic system are present in 

 substantial quantities, indicating that this 

 system proceeds with about equal intensity 

 in embryonic and mature tissvies. The find- 

 ing of Potter and DuBois ('42) that cyto- 

 chrome c is present only in traces in the six- 

 day chick embryo is, on the other hand, an 

 example of the difficulty of accepting nega- 

 tive evidence in this field. For we know that 

 during the first three days the intact embryo 

 takes up oxygen freely (Philips, '40), that it 

 gives a strong Nadi reaction that is inhibited 

 by azide (Moog, '43), and that homogenates 

 contain abundant cytochrome oxidase (Al- 

 baum et al., '46); the latter authors make 

 the rather equivocal statement that in young 

 embryos "autoxidation" of added substrate 

 in the absence of added cytochrome c repre- 

 sents a large fraction of the observed oxygen 

 uptake. 



But perhaps it is naive to assume that 

 the embryo differentiates or accumulates 

 some elements of its biochemical machinery 

 in anticipation of others. The lack of evi- 

 dence demonstrating this type of control may 

 in itself be evidence that the various factors 

 are produced only as they are needed, and 

 in correlation with other necessary factors. 

 This view is in harmony with the previously 

 mentioned parallelism between enzyme ac- 

 cumulation and function. 



Recent studies on the formation of the so- 

 called adaptive enzymes may provide a more 

 subtle approach to the problem of enzyme 

 regulation by material supply. The im- 

 portant discussion by Spiegelman ('48) sug- 

 gests a mechanism by which substrate avail- 

 ability may direct the actual formation of 

 enzymes, and thus control the processes of 

 growth and morphogenesis dependent on the 

 enzymes formed. In essence the hypothesis 

 of long-term regulation of enzyme pat- 

 terns in cells, whether embryonic or adult, 

 by adaptive formation of enzymes, rests upon 

 three postulates, which we will consider 



here. Each of these postulates, it should be 

 noted, is a demonstrated fact in itself; the 

 extent to which these three processes actually 

 cooperate in regulating cell metabolism, how- 

 ever, remains to be studied in detail. 



1. The amount of enzyme normally pres- 

 ent in a cell represents a balance between 

 synthesis and destruction. 



2. Enzymes tend to be stabilized (i.e., de- 

 struction is retarded?) when combined with 

 suitable substrate (cf. Bayliss, '19). Accord- 

 ing to more recent evidence, substances other 

 than normal substrate can promote the for- 

 mation of adaptive enzymes (Spiegelman, 

 '48). 



3. Most, perhaps all, of the protein of a 

 living cell is potential material for the 

 formation of enzymes. Thus synthesis of 

 special enzymes must involve competitive 

 interactions. In yeast, under conditions of 

 starvation, even an enzyme regarded as con- 

 stitutive (glycozymase) can be depleted dur- 

 ing the formation of a clearly adaptive en- 

 zyme, galactozymase (Spiegelman and Dunn, 

 '47). 



A plausible, though not yet demonstrated, 

 integration of these three facts into a single 

 regulatory mechanism is shown in Figure 

 9. This figure illustrates how, in the normal 

 steady state, with raw materials supplied 

 and breakdown products removed, the rela- 

 tive amount of activity of enzymes A and B 

 will depend on availability of substrate, 

 among other factors. With low supply of 

 substrate for B, for example, the total amount 

 of B would diminish; but increase of sub- 

 strate for B would raise the activity only as 

 fast as new B could be formed. Presence of 

 a stabilizing (inhibiting) substance (S') 

 would, however, save the enzyme from de- 

 struction and allow immediate increase of 

 activity of B if normal substrate were added 

 in excess, the excess substrate then out- 

 competing the inhibitor. 



Although enzyme patterns have been al- 

 tered by supplying excess substrate in yeasts 

 and bacteria, it should be noted that such 

 treatment does not necessarily affect enzyme 

 concentration. For example, the substrate 

 might be present in excess to begin with, or 

 an enzyme might be held in large quan- 

 tities as a form of storage protein. Some seeds 

 contain tremendous quantities of urease, 

 which falls sharply during development 

 (Williams, '50). Assuming that the urease- 

 synthesizing system is a good competitor, 

 one might interpret the observation to mean 

 that the seed uses urease as a means of stor- 

 ing protein out of reach of other enzyme 



