ethyl esters of succinate, malate, citrate, and butyrate; glucose penta- 

 acetatej glucosamine; and hippuric acid all increased consumption of 

 oxygen. Apparently, esters are able to penetrate the cell more easily. 

 It would appear that the TCA cycle is operative in oyster spermatozoa, 

 since most of the intermediates of the TCA cycle are able to stimulate 

 respiration. Malonate, mainly considered to be an inhibitor of succinate 

 dehydrogenase, was found to cause a significant increase in uptake of 

 oxygen. There is evidence in other animals (Glass 1951) that malonate 

 may serve as an uncoupling agent for oxidative phosphorylation. If this 

 effect of malonate also occurs with oyster spermatozoa,' a reduction in 

 the efficiency of oxidative phosphorylation would account for the increase 

 ed consumption of oxygen. A small amount of information about the TCA 

 cycle in a few other aquatic animals is also available; this information 

 is summarized below. 



The demonstration of the TCA cycle in sea-urchin eggs ( Arbacia ) has 

 been confined to the study of their ability to oxidize pyru vat e, oxalac e- 

 tate, tt-ketoglutarate, and succinate and to the study of the occurrence 

 of oxidative phosphorylation (Krahl 1°3>0). 



Goldinger and Barron (l°li6) determined the rate of disappearance 

 of preformed pyruvate from intact unfertilized and fertilized eggs of 

 Arbacia . Fertilized eggs were over six times as active as were unferti- 

 lized eggs, Krahl et al. (19U2) observed a similar pattern for the rate 

 of disappearance of added pyruvate to cytolysates of unfertilized and 

 fertilized eggs except that the breakdown of the egg structure in prepar- 

 ing the cytolysates raised the rate for unfertilized eggs closer to that 

 of the level of fertilized eggs. The fate of pyruvate in these studies 

 was not determined. Changes in succinic dehydrogenase activity of sea- 

 urchin eggs in relation to the process of fertilization also were studied 

 (Hida 1957). 



Evidence presented by Cleland and Rothschild (1952) for the occur- 

 rence of the enzymes of the TCA cycle in the sea-urchin egg ( Echinus 

 esculentus ) includes : (l) significant stimulation of uptake of oxygen 

 by as addition of citrate, a-ketoglutarate, succinate, fumarate, malate, 

 and glutamate; (2) inhibition of endogenous uptake of oxygen by malonate 

 and fluoroacetate; and (3) complete oxidation of added pyruvate. Uptake 

 of oxygen of sea-urchin spermatozoa has been measured (Wicklund 195U)« 



Succinoxidase activity has been demonstrated in mitochondrial pre- 

 parations of lobster leg and claw nerves (Foster 1956), 



Baldwin (1938), in a study of the respiratory metabolism of Helix 

 pomatia, has found the hepatopancreas to resemble liver tissue of verte- 

 brates c losely. Succinate, lactate, a -glycerophosphate, and^ alanine in- 

 creased the rate of respiration. Succinic acid frcji the marxne snail was 

 isolated and identified (Ackermann 1955). 



Even though the frog is not strictly an aquatic animal, metabolic 

 studies made on the frog are of interest here. 4 J^^,** J^f id 

 obtained positive assays for succinic, malic, citric J ^ c *f^),and 

 a-ketoglutaric dehydrogenases and fumarase. A comparxson of the activity 



16 



