542 Energy Exchange and Enzyme Development During Embryogenesis 



responsible for the suggestion that a portion 

 of the total respiratory exchange does not 

 go through the usual Warburg-Keilin system 

 but proceeds instead through a system of 

 "non-ferrous" catalysts. Although, as was 

 mentioned previously, so-called cyanide in- 

 sensitivity is sometimes the result of inad- 

 equate care to insure that cyanide concentra- 

 tion remains constant during an experiment, 

 it is nevertheless true that a very substantial 

 portion of the total respiration of the un- 

 fertilized sea urchin egg and of the grass- 

 hopper embryo in diapause cannot be 

 depressed by cyanide or carbon monoxide. 

 Both Lindahl ('40) and Robbie ('46) have 

 suggested that residual respiration in the 

 presence of cyanide may be of a special 

 kind that does not normally occur. That 

 inhibitors may induce extraordinary respir- 

 atory processes has been shown through 

 the use of fluoride on the chick embryo, of 

 dinitrophenol on the grasshopper embryo, 

 and of azide on Paramecium (Boell, '46), 

 Notwithstanding, there remains the pos- 

 sibility that a small but definite fraction of 

 respiratory exchange is mediated by a system 

 of enzymes different from those that operate 

 in the citric acid cycle and the Warbiurg- 

 Keilin system.* The nature of such a pos- 

 sible pathway is unknown, nor is it known 

 whether it operates normally as a collateral 

 to the main system of respiratory catalysts 

 or whether it becomes functional only when 

 the Warbvirg-Keilin system has been in- 

 hibited. 



The Phospho gluconic Acid Shunt. Lindberg 

 has adduced evidence to support the view 

 that respiration in the sea urchin egg, dur- 

 ing the period of exponential rise, proceeds 

 by a pathway other than the Embden-Meyer- 

 hof scheme (Lindberg, '43; Lindberg and 

 Ernster, '48). He has suggested that glucose 

 breakdown is an oxidative process which 

 bypasses the usual glycolytic cycle through 

 what has been termed the phosphogluconic 

 acid shunt or the Warburg-Dickens scheme. 

 In this, glucose-6-phosphate is oxidized to 

 6-phosphogluconic acid, and then, through 

 several intermediate steps, to pyruvic acid. 

 The reaction is catalyzed by a specific 

 enzyme, glucose-6-phosphate dehydrogenase, 

 and TPN is an obligatory cofactor (see 



* One need but recall the reported absence of cyto- 

 chrome c in sea urchin eggs, the early chick em- 

 bryo, and the cerebral cortex, or the observation of 

 Sanborn and Williams ('50) that cytochrome x 

 (which combines certain properties of cytochromes 

 b and c and succinic dehydrogenase) is destroyed 

 during pupation in Cecropia. 



Fruton and Simmonds, '53, p. 459 ff.). Evi- 

 dence for the operation of the scheme in 

 the sea urchin egg stems from the observa- 

 tion that the oxygen consumption of egg 

 homogenates can be stimulated by glucose 

 or hexosemonophosphate, but hexosediphos- 

 phate has a negligible effect. Furthermore, 

 Lindberg and Ernster have shown that 

 iodoacetate, which blocks the Embden-Mey- 

 erhof system by inhibiting phosphoglycer- 

 aldehyde dehydrogenase, does not affect 

 respiration. Additional support of rather in- 

 direct nature has been provided by Hultin 

 ('53) and rests upon the demonstration that 

 carbon dioxide fixation by sea urchin eggs, 

 as measured by the uptake of C^*02, is 

 maximal during the period when respiration 

 rises exponentially. The pertinence of this 

 observation will be apparent when it is 

 recalled that the mechanism of carbon di- 

 oxide fixation by pyruvic acid, in the for- 

 mation of malic acid, is coupled through 

 TPN with the oxidation of glucose-6-phos- 

 phate to 6-phosphogluconic acid (see Fruton 

 and Simmonds, '53, p. 474 ff.). Additional 

 evidence, of still more peripheral nature, 

 has been furnished by Horstadius and Gus- 

 tafson ('47, cited by Horstadius, '49) that 

 phosphogluconic acid exerts a marked an- 

 imalizing influence on sea lu-chin eggs. 

 Finally, although Cleland and Rothschild 

 ('52b) presented cogent arguments for the 

 operation of the Embden-Meyerhof scheme 

 in the sea urchin egg, they showed also 

 that phosphogluconic acid increased the 

 oxygen consumption of egg homogenates and 

 have concluded that the enzymes for the 

 phosphogluconic shunt are present. Perhaps 

 a fraction of respiration proceeds normally 

 through this pathway; on the other hand, 

 as suggested by Hultin ('53), the pathway 

 may have functional significance only when 

 the enzymes concerned with the glycolytic 

 cycle have been inhibited by sulfhydryl 

 reagents. The phosphogluconic acid shunt 

 may thus be analogous to the cyanide-in- 

 sensitive fraction of respiration. 



Non-phosphorylating Glycolysis. Much at- 

 tention has been given to the elucidation of 

 the mechanism of carbohydrate breakdown 

 in the early chick embryo, and the claim 

 has been made that glycolysis does not pro- 

 ceed through the usual steps of the Embden- 

 Meyerhof scheme, i.e., through a series of 

 phosphorylated intermediates, but occurs 

 through a non-phosphorylating mechanism 

 (see Needham, '42, pp. 610-615, for a re- 

 view of the literature and summary of the 

 evidence). Needham and his co-workers 



