THE METABOLISM OF EGGS, II 71 



. TABLE 13 

 Further evidence for the existence of a classical glycolytic cycle 

 in sea-urchin eggs (Cleland & Rothschild, I952h) 



1. Accumulation of pyruvate and lactate anaerobically and in presence of 



HCN. 



2. Inhibition of O2 uptake by fluoride and reversal by pyruvate. 



3. Accumulation of pyruvate after addition of glucose-6-phosphate. 



4. No accumulation of pyruvate after addition of phosphogluconate. 



5. Active oxidation of hexose diphosphate with pyruvate accumulation. 



6. Stimulation of endogenous O2 uptake by DPN. 



7. Stimulation of fructose oxidation by DPN. 



8. Inhibition of 6 and 7 by fluoride and iodoacetate. 



shunt, or an oxidative pathway, was mainly responsible for carbo- 

 hydrate breakdown in sea-urchin eggs. As these pathways may 

 not be so famihar to some readers as the usual glycolytic one, they 

 are reproduced on p. 72. 



The evidence in favour of the operation of a scheme along these 

 lines in sea-urchin eggs is: (a) egg homogenates actively metabolise 

 phosphogluconate. In addition, Krahl et al. (1955) have shown 

 that Arbacia homogenates contain glucose-6-phosphate and 6- 

 phosphogluconate dehydrogenases, and can form ribose from 

 either substrate. These workers found that the rate of TPN re- 

 duction with glucose-6-phosphate as substrate was about 3 /x,- 

 moles/min./g. eggs, enough to permit an O2 uptake six times that 

 of fertilized and twenty-four times that of unfertilized eggs. On the 

 other hand, the rate of DPN reduction by egg extracts with 

 fructose- 1, 6-diphosphate as substrate was o- 1-0-2 ju,-moles/min./g. 

 eggs. At the same temperature, the Oo uptake of the fertilized eggs 

 would require 0-5 ju,-moles of DPN to be reduced if all the Og up- 

 take were associated with the glycolytic breakdown of carbohydrate. 

 Although all the Oo uptake of fertilized eggs is almost certainly not 

 associated with carbohydrate breakdown, the amount of carbo- 

 hydrate which can pass the aldolase-oxidizing enzyme step appears 

 to account for only 20-40% of the oxygen actually consumed in 

 the eggs of Arbacia punctulata. (b) Low concentrations of iodo- 

 acetate do not inhibit the endogenous Oo uptake of egg homo- 

 genates ; there are differences of opinion on this point. Cleland & 

 Rothschild (1952a) found that iodoacetate did inhibit glycolysis 

 in egg homogenates of Echinus esculentus, and so did Yeas (1954), 

 using egg homogenates of Stro?igylocentrotus purpuratus and 

 Lytechinus pictus. Lindberg & Ernster's experiments were done in 



F 



