224 RESPIRATION 



acids; malic acid appears usually in smaller quantities (Chapter 6). 

 Although their occurrence is suggestive of the cycle, it is in no sense 

 proof. Nor can it be argued from the mere fact that intact cells 

 oxidize these organic acids that the cycle is present. 



Alpha-ketoglutaric acid is formed in detectable amounts by Penicil- 

 Uwn chrysogenum from lactic acid (136), by Aspergillus oryzae from 

 glucose (252), and by A. niger from endogenous reserves (311). Its role 

 in glutamic acid synthesis is considered later (Chapter 8). 



Individual reactions of the cycle have been studied especially in 

 seven organisms: Blastocladiella emersonii, Ashbya gossypii, Neuro- 

 spora crassa, Aspergillus niger, Penicillium chrysogenum, Streptomyces 

 coelicolor, and Zygorhynchus moelleri. For some of these more or 

 less data is also available from isotope distribution experiments. 



Most of the known enzymes of the cycle have been demonstrated in 

 Blastocladiella emersonii, as has also the formation of citrate (43, 46, 

 47). The cycle, however, appears to be at best weakly functional in 

 this organism, and development of the resistant sporangia is accom- 

 panied by loss of most of the enzyme activities of the cycle (43). Can- 

 tino and Horenstein (45) have suggested that in B. emersonii a reduc- 

 tive succinate-ketoglutarate-isocitrate (S.K.I.) cycle is operative; the 

 most striking feature of this proposal is that light, which accelerates 

 growth, is visualized as providing the energy for the carboxylation of 

 succinate and a-ketoglutarate. 



In Ashbya gossypii acetate oxidation is accelerated by dicarboxylic 

 acids, citrate is synthesized from acetate and oxalacetate, and citrate 

 accumulation is increased by malonate (195). 



Several of the characteristic enzyme activities of the cycle have been 

 demonstrated in extracts of Neurospora crassa (82, 213, 258, 275). 

 Incorporation of carbon-14 from several substrates into aspartic and 

 glutamic acids indicates that in this fungus the citric acid cycle plays 

 a major role in the synthesis of these amino acids (248). 



Aspergillus niger is of particular interest in that some strains ac- 

 cumulate citric acid (Chapter 6) and in that tracer studies have been 

 made. Enzymatic analyses are the most complete for any fungus, and 

 show that all of the known enzymes of the citric acid cycle are present 

 in extracts made from mycelium (190, 239, 240, 241). The enzyme 

 systems acting upon tricarboxylic acids are rather complex: there is 

 evidence for an aconitase of the usual type, an aconitic hydrase con- 

 verting aconitate only to citrate (73, 210), and two isocitric dehydro- 

 genases (241). Excellent agreement between the enzymatic content of 

 cells and the accumulation of citric acid affords further evidence for 



