154 



METABOLISM AND PHYSIOLOGY 



reveal the presence of isocitrate. Fractions obtained from the super- 

 natant fluid by ammonium sulfate precipitation did not show aconitate 

 hydratase activity. Possible activators such as /?-mercaptoethanol, 

 EDTA, and various metal ions were tried without effect. Extracts 

 were also inactive after preincubation with ferrous ions and cysteine 

 under the conditions described for activation of pig heart aconitate 

 hydratase (20), 



Aconitate Hydratase in Other Photosynthetic Bacteria 



Since aconitate hydratase was not found in C. thiosulfatophilum, 

 several other photosynthetic bacteria were examined for this enzyme. 

 The extraction and assay procedures were the same as used for C. 

 thiosulfatophilum . The extracts were also assayed for isocitrate de- 

 hydrogenase. Both enzymes were present in all the extracts examined 

 (Table 2), For comparison, the aconitate hydratase activity of a pea 

 leaf extract was determined, A value of 21 //moles aconitate utilized 

 per minute per gram of protein was obtained. The activities of the 

 bacterial extracts are either comparable or higher than this value. 

 There is no reason to suspect that enzymes of C, thiosulfatophilum 

 are easily inactivated during the extraction procedure. In a previous 

 study, C. thiosulfatophilum extracts were assayed for thirteen enzymes 

 of the photosynthetic carbon cycle and activity was demonstrated in 

 every case (21), However, the possibility remains that these extracts 

 contain an inhibitor highly specific for aconitate hydratase. According- 

 ly, the effect oi C .thiosulfatophilum extrdLcts on the aconitate hydratase 



TABLE 2 



Aconitate hydratase and isocitrate dehydrogenase activities 

 in extracts of photosynthetic bacteria 



Assayed by TPN reduction in the presence of aconitate and excess isocitrate 

 dehydrogenase. 



