Energy Exchange and Enzyme Development During Embryogenesis 543 



found that the chick embryo was deficient 

 in four essential components of the Embden- 

 Meyerhof system: (1) the embryo could not 

 attack glycogen or a number of hexosephos- 

 phates, although glucose could be broken 

 down readily, and it was concluded that 

 phosphorylase, the enzyme esterifying glyco- 

 gen, was absent or present in too low con- 

 centration to be effective; (2) since triose- 

 phosphate could not be broken down, it 

 was concluded that triosephosphate dehy- 

 drogenase was absent; (3) ATP was present, 

 but in quantities too small to be effective; 

 and (4) DPN was absent. It was postulated, 

 therefore, "that in the chick embryo there 

 are two separate routes of carbohydrate 

 breakdown: (1) a non-phosphorylating gkx- 

 colysis mechanism, very active and closely 

 bound to the cell structure, and (2) a phos- 

 phorylating system closely similar to that 

 in muscle, dealing with glycogen and hexose- 

 diphosphate, but of very low activity" be- 

 cause of the deficiencies noted above. 



Novikoff, Potter, and Le Page ('48) ob- 

 tained residts which were quite different 

 from those of the Cambridge workers, for 

 they found that embryo homogenates would 

 glycolyze hexosediphosphate, fructose-6-phos- 

 phate, and glucose-6-phosphate as well as 

 glucose. Moreover, they extracted from em- 

 bryos, three to ten days of age, various 

 hexosephosphates and their breakdown prod- 

 ucts, ATP, and DPN in amounts roughly 

 equivalent to those in adult tissues. They 

 concluded, therefore, that their results did 

 not rule out of consideration a non-phos- 

 phorylating pathway for glycolysis but that 

 postulating one was unnecessary since all 

 enzymes and intermediates in the Embden- 

 Meyerhof scheme were present in adequate 

 concentrations. 



There can be little doubt, after consider- 

 ing the experimental findings reviewed in 

 the foregoing pages, that in the sea urchin 

 egg and in the chick embryo all of the com- 

 ponents necessary for the operation of the 

 scheme shown in Figure 202 are present. 

 Unfortunately, much less is known for most 

 embryos. To suggest that the scheme has 

 general applicability to the other forms 

 commonly vised in embryological studies 

 would be no more than to hazard a guess. 

 Such a conclusion may be correct, but it 

 cannot be justified from the experimental 

 evidence available at present. The recon- 

 struction of a complex and intricate mech- 

 anism for energy release and energy storage 

 from a few biochemical fragments that have 

 come to hand is not unlike the restoration 



of a skull from a few chips of brain case, 

 a jawbone, and perhaps a tooth or two. 

 Museums throughout the world give evi- 

 dence of successes along this line, but the re- 

 cent disclosures concerning the Piltdown man 

 point up the dangers inherent in such a 

 procedure. 



ENZYMES IN ONTOGENESIS 



Synthesis of Respiratory Enzymes. It seems 

 reasonable to expect that increased oxygen 

 consumption during development would be 

 accompanied by a corresponding change in 

 the enzymes through which respiratory proc- 

 esses are mediated. Because of its key posi- 

 tion in respiration, cytochrome oxidase has 

 been most extensively investigated, and the 

 results obtained in the grasshopper (Bodine 

 and Boell, '36a; Allen, '40), the salamander 

 (Boell, '45), and the chick (Albaum and 

 Worley, '42; Albaum, Novikoff, and Ogur, 

 '46; Levy and Young, '48) agree that in- 

 crease in respiration is paralleled by in- 

 crease in the amount of cytochrome oxidase 

 in the embryo.* In the sea urchin, Deutsch 

 and Gustafson ('52) have reported that 

 cytochrome oxidase rises during the first 

 four hours after fertilization, but then it 

 falls, during the next twenty hours, to a 

 level equal to or less than the initial value. 

 The homogenates used in these experiments 

 were prepared by subjecting eggs to a freez- 

 ing mixture and then shaking them vigor- 

 ously during thawing. It is well known that 

 cytochrome oxidase and certain dehydro- 

 genases as well are inactivated by such 

 treatment; hence the differences in enzyme 

 activity found by Deutsch and Gustafson 

 are probably due largely to variations in 



* It is impossible to express the quantity of an 

 enzyme by the usual metrical units employed for 

 other chemical entities. In general, what is meas- 

 ured in enzyme studies is the activity of the enzyme 

 under optimal conditions, so that the rate of reac- 

 tion, during a reasonable period of time, is limited 

 only by the amount of enzjone present in the reac- 

 tion system. If care is taken to insure such condi- 

 tions, reaction rate is found to be proportional to the 

 concentration of the enzyme in the system. Meas- 

 urement of enz5Tne activity in tissue minces or 

 homogenates gives only an indication of the total 

 potential activity of the enzyme. It provides no 

 information on the degree to which the enzyme 

 functions in vivo. This is particularly well illus- 

 trated in developing and diapause grasshopper em- 

 bryos, where it is found that the amounts of cyto- 

 chrome oxidase are identical. But the enzyme func- 

 tions to only a slight extent during diapause, as is 

 shown by the lower rate of respiration and the de- 

 creased sensitivity to cyanide. 



