548 Energy Exchange and Enzyme Development During EmbryogeneSis 



fact that various enzymes, even in the same 

 organ, appear at different times and develop 

 at independent rates indicates very strongly 

 that functional maturation in a particular 

 organ is associated with the development of 

 particular enzymes (Boell, '45, '48; Boell and 

 Shen, '50; Dumm and Levy, '49; Moog, '52; 

 Gustafson and Hasselberg, '51). 



It should be emphasized, of course, that 

 several enzymes directly concerned with the 

 same activity would more than likely develop 

 somewhat in parallel. In the nervous system, 

 for example, it would be reasonable to as- 

 sume that the mechanism for acetylcholine 

 synthesis, i.e., the cholinacetylase system, 

 and that for hydrolysis would develop to- 

 gether. No studies on the development of 

 cholinacetylase have so far been made. How- 

 ever, in the synthesis of acetylcholine energy 

 from ATP is required, and it is of interest to 

 note, therefore, that there seems to be a fair 

 degree of parallelism in the development of 

 ATPase and cholinesterase in the central 

 nervous system. This may be seen in the rat 

 brain by a comparison of data for ATPase and 

 cholinesterase provided by Potter, Schneider, 

 and Liebl ('45) and Metzler and Humm 

 ('51), respectively. It is also shown in the 

 chick brain by comparison of Moog's ('47) 

 curve for ATPase and Nachmansohn's ('39) 

 for cholinesterase (Moog, '52). Similar ex- 

 amples are represented by dipeptidase and 

 aminopeptidase in the chick embryo (Levy 

 and Palmer, '43) or by dipeptidase and tri- 

 peptidase in Amblystoma mexicanum (L0V- 

 trup, '53c). 



There is a substantial body of data which 

 shows that enzyme development and func- 

 tional differentiation during development go 

 hand in hand. But it should by no means be 

 concluded that the development of an enzyme 

 is in any way causally linked with the differ- 

 entiation process with which it appears to be 

 associated. Statements may nevertheless be 

 found in the literature of embryology to the 

 effect that specific enzymes, or the reactions 

 catalyzed by them, control such processes 

 as determination, morphogenesis, or differen- 

 tiation. So far this has not been proven for 

 any enzyme. (See the discussions by Holter, 

 '49, p. 73, and Weiss, '53, pp. 173-4, on this 

 question.) 



CONCLUSION 



In this review an attempt has been made 

 to give an account of some of the features 

 of energy exchange during development. It 

 is clear that a great deal is known about the 



over-all energy metabolism of the embryo. 

 Furthermore, the enzymes involved and their 

 relationships to metabolic processes are gradu- 

 ally being revealed. But the crucial problem 

 of how energy liberated is used by the em- 

 bryo for "developmental work" has not been 

 penetrated. It is of little comfort to note that 

 other biological disciplines are plagued by 

 the same problem — the translation of energy 

 released into work done. In spite of the tre- 

 mendous advances that have been made in 

 the biochemistry and physiology of muscle, 

 there still exists a gap between the system 

 concerned with the synthesis and breakdown 

 of high-energy bonds and the change in 

 muscle proteins from the extended to the 

 contracted state. And on the question of how 

 selective secretion is accomplished against 

 a gradient, even less is known. 



The major problem in embryology, as in 

 physiology, is to provide a bridge between 

 events at a molecular level and the struc- 

 tural components in which they find expres- 

 sion. The common aim of students of develop- 

 ment, no matter toward what aspect their 

 special interests or energies direct them, is 

 to describe as fully as possible the complex 

 and intricately coupled reactions and inter- 

 actions — involved at all levels, from molecular 

 to organismic — in the transformation of the 

 egg into the differentiated individual. Chem- 

 ical studies of development contribute to 

 this end by providing one of the dimensions 

 of description. 



REFERENCES 



Albaum, H. G., Novikoff , A. B., and Ogur, M. 1 946 

 The development of the cytochrome oxidase and 

 succinoxidase systems in the chick embryo. J 

 Biol. Chem., 165:\2S-\iQ. 



, and Worley, L. G. 1942 The develop 



ment of cytochrome oxidase in the chick embryo, 

 J. Biol. Chem.. 144:697-700. 



Allen, T. H. 1940 Enzymes in ontogenesis. XL 

 Cytochrome oxidase in relation to respiratory ac 

 tivity and growth of the grasshopper egg. J. Cell, 

 & Comp. Physiol., i6:U9-163. 



Amberson, W. R. 1928 The influence of oxygen 

 tension upon the respiration of unicellular or- 

 ganisms. Biol. Bull., 55.-79-81. 



, and Armstrong, P. B. 1933 The respira- 

 tory metabolism of Fundulus heteroclitus during 

 embryonic development. J. Cell. & Comp. Phys- 

 iol., 2.-381-397. 



Andresen, N., Holter, H., and Zeuthen, E. 1944 

 The respiration of syncytia formed by abnormal 

 development of Ciona eggs. Compt. Rend. Lab. 

 Carlsberg., ser. chim., 25.-67-85. 



Atlas, M. 1938 The rate of oxygen consumption 

 of frogs during embryonic development and 

 growth. Physiol. ZooL, ^^.-278-291. 



