Energy Exchange and Enzyme Development During Embryogenesis 539 



initial changes occur equally well in the 

 absence or presence of oxygen. The so-called 

 processes of anaerobic glycolysis are mediated 

 by what is commonly called the Embden- 

 Meyerhof scheme of phosphorylating gly- 

 colysis, and the end result of the breakdown 

 of glycogen or glucose is pyruvic acid. In the 

 absence of oxygen, pyruvic acid is reduced 

 to lactic acid by hydrogen transferred from 

 reduced diphosphopyridine nucleotide (DPN). 

 During the anaerobic reactions, some energy 

 is released, and this is stored in high-energy 

 phosphate bonds such as those in ATP. 

 Pyruvic acid is broken down to carbon di- 

 oxide and water by a series of oxidative steps 

 involving the tricarboxylic or citric acid 

 cycle. Pyruvate is first oxidatively decarbox- 

 ylated, Ijy processes involving the participa- 

 tion of coenzyme A, and the two-carbon frag- 

 ment remaining combines with oxaloacetate 

 to enter the cycle as citrate. Electrons and 

 hydrogen derived from the degradation of 

 citrate back to oxaloacetate are transferred 

 to DPN or to triphosphopyridine nucleotide 

 (TPN) and thence to oxygen via the War- 

 burg-Keilin system of the cytochromes and 

 cytochrome oxidase. During the operation 

 of the citric acid cycle, more high-energy 

 phosphate bonds are generated. 



It has been shown during the past few 

 years that the citric acid cycle also plays a 

 role in the oxidation of fatty acids and amino 

 acids. Fatty acids are first broken down by 

 )3-oxidation to two-carbon fragments, and 

 these, throvigh coenzyme A, may enter the 

 citric acid cycle. Many amino acids, after de- 

 amination, also enter the cycle — in some cases 

 through coenzyme A, in others more directly. 

 These processes and the relationships of the 

 Embden-Meyerhof scheme, the citric acid 

 cycle, and the Warburg-Keilin system are 

 shown diagrammatically in Figure 202. 



A conservative estimate shows that some 

 20 or 30 different enzymes are required to 

 operate the complete scheme, and, in addi- 

 tion, several coenzymes, "factors," and other 

 chemical agents are needed. It does not fol- 

 low that the scheme outlined above operates 

 in every living cell, but it has been shown 

 to be widely applicable, not only in the case 

 of various vertebrate tissues but also in 

 yeasts and other microorganisms, and one is 

 perhaps not far wrong to conclude that the 

 basic theme, with minor variations, probably 

 applies to most aerobic organisms. 



Our problem now is to determine to what 

 extent this complex array of biochemical 

 machinery is present and operative in the 

 embryo. It will be apparent in what follows 



that many of the enzymic mechanisms which 

 operate in cellular respiration in the adult 

 organism are present, and functional, during 

 early development, and the egg, far from 

 being a mass of protoplasm with simple 

 enzyme equipment, is provided with or soon 

 synthesizes an impressive battery of enzymes. 

 Respiratory Mechanisms in Echinoderm 

 Eggs. The sea urchin egg and embryo have 



Oxygen 



t 



Cytochrome oxidase 



t 



Cytochrome a 



t 

 Cytochrome c 



\ 

 Cytochrome b 



t 

 Flavoproteins 



t 

 DPN and TPN 

 t 

 Citric acid 

 cycle 

 t 

 Coenzyme A 

 / 

 Pyruvic 

 acid 



Embden- 

 Meyerhof 

 scheme 



Carbohydrates 



Fatty acids 



Amino acids 



Fig. 202. Schematic representation of mechanisms 

 concerned with oxidation of foodstuffs to carbon 

 dioxide and water. 



been more thoroughly investigated, with re- 

 spect to their oxidative enzyme equipment, 

 than those of any other form. The systematic 

 investigation of respiratory mechanisms dur- 

 ing echinoderm development had its be- 

 ginnings in the thorough studies of Rimn- 

 strom more than twenty years ago. Follow- 

 ing Warburg ('10), Runnstrom ('30) showed 

 that cyanide strongly depressed respiration 

 of the fertilized sea urchin egg but that the 

 unfertilized egg was completely resistant to 

 this inhibitor. In addition, he found that 

 carbon monoxide, also effective as a depres- 

 sant of respiration of fertilized eggs, stimu- 

 lated oxygen consumption before fertilization. 

 Next, Runnstrom ('30, '32) investigated 

 the ability of eggs to oxidize dimethyl-p- 

 phenylenediamine, the substrate used to test 

 for Atmungsferment (cytochrome oxidase), 

 and found that the potential activity of the 



