Physiology 459 



in the presence of sufficient malonate, which inhibits succinic dehydro- 

 genase, the cycle stops with the accumulation of succinate. Arsenite checks 

 the cycle by inhibiting the oxidation of malate to oxalacetate. In addition, 

 the ability of a species to use components of the cycle may be tested by 



CARBOHYDRATE 



FATTY 

 ACIDS 



ASPARTAT 



PYRUVATE 



NH3 ^ACETYL 

 OXALACETATE 



C^ * C3 

 (acetate, 



PYRU VATE ^ 



•NH3 



ALANINE 



FUMARATE 



cis-ACONITATE 



iso-CITRAT£ 



OXALO- 

 SUCCINATE 



cx-KETOGLUTARATE 



Fig. 8. 2. The tricarboxylic acid cycle. 



GLUTAM- 

 ATE 



measuring oxygen consumption with each as a substrate or by detennin- 

 ing possible stimulation of growth. 



Little work has been done on the tricarboxylic acid cycle in protozoan 

 metabolism. There is no reason a priori, for suspecting that all aerobic 

 Protozoa must complete the oxidation of metabolites through a typical 

 tricarboxylic acid cycle; there may be some species which do not. There 

 apparently are such reactions as Co + Co condensations which skip the Ce 

 acids of the typical cycle. Rhizopiis nigricans can carry out this condensa- 



