156 METABOLISM AND PHYSIOLOGY 



strate aconitate hydratase in C. thiosulfatophilum can be attributed to 

 the existence of inhibitors in the extract. 



Other Enzymes of the Citric Acid Cycle 



Succinic dehydrogenase, fumarate hydratase, and malate dehydro- 

 genase are present in C. thiosulfatophilum (Table 4), Results obtained 

 with another green bacterium, Chloropseudomonas ethylicum, are in- 

 cluded for comparison. It may be noted that the very active fumarate 

 hydratase activity of C. thiosulfatophilum was measured by an assay 

 procedure very similar to one of the assays used for aconitate hydra- 

 tase, C, thiosulfatophilum was not examined for o-ketoglutarate de- 

 hydrogenase. 



DISCUSSION 



The purple photosynthetic bacteria can photometabolize acetate by 

 several alternative pathways. In some, acetate assimilation appears to 

 be predominantly through a glyoxylate cycle (22,23), Others, including 

 R. rubrum, lack isocitratase (22) and utilize acetate by an unknown 

 pathway (24), In addition, acetate may be oxidized via an anaerobic 

 citric acid cycle (6), condensed with pyruvate to yield citramalate 

 (25,26) or reduced to a storage compound, poly y5-hydroxybutyric 

 acid (27). As pointed out by Gest^^ al. (6), the direction of metabolism 

 "is dictated by the nutritional history of the cells and the conditions of 

 incubation." 



Regardless of the initial pathway of incorporation, the eventual 

 utilization of acetate for cellular growth in bacteria which do not 

 contain a glyoxylate cycle is C02-dependent (27,28,29), even though 

 growth on acetate may result in a net production of CO2, This neces- 

 sitates an enzymic pathway for the oxidation of acetate to CO2, In R. 

 rubrum this can take place via a light- dependent anaerobic citric 

 acid cycle (6,30,31). Among the green bacteria, Chloropseudomonas 

 ethylicum can oxidize acetate to CO2. During growth on acetate there 

 is a net production of CO2 amounting to 15 to 20 per cent of the acetate 

 utilized. In contrast, C. limicola can quantitatively assimilate acetate 

 but only with simultaneous assimilation of CO2. Experiments with 

 acetate-Cl4 disclosed that only a very small fraction of the acetate 

 taken up by these cells reappeared as labeled CO2 (5). Since acetate 

 assimilation by C, limicola is CO2 dependent, the mechanism of 

 assimilation may be similar to that in R. rubrum, except that whereas 

 in the latter CO2 and reducing power are produced endogenously 

 from acetate, these must come from an exogenous source in the case 

 of C. limicola. Hence while C. limicola possesses an enzymic 

 mechanism for acetate assimilation, it does not appear to contain a 

 mechanism for acetate oxidation. 



