HG CARBON METABOLISM III 



The other acids listed in Table 1 — glutaric, glutaconic, dimethyl- 

 pyruvic, and ethylene oxide a,/J-dicarboxylic acids — have as yet no 

 established physiological or biochemical significance. The first two 

 have been reported only once, from xylose cultures; if their formation 

 can be confirmed, a route from pentose to a-ketoglutaric acid is a pos- 

 sibility. Dimethylpyruvic acid accumulates in cultures poisoned 

 with bisulfite. Suggestions on the biochemical origin of dimethyl- 

 pyruvic acid are reviewed by Birkinshaw (63). 



Ethylene oxide a,/?-dicarboxylic acid (<ra//.s-L-epoxysuccinic acid), 

 chemically related to tartaric acid, is formed from a variety of carbon 

 sources by Aspergillus fumigatus in up to 21 per cent yield (370). 



During the breakdown of citric acid by Aspergillus niger, two other 

 acids, acetonedicarboxylic acid (HOOC— CH 2 — CO— CH 2 — COOH) 

 and malonic acid (HOOC — CH 2 — COOH) appear in the medium, and 

 it has been suggested that they derive from a route of citrate dissimila- 

 tion otherwise unknown (125, 495). 



Conclusions. Some generalizations can be made concerning the 

 physiology and biochemistry of those acids — gluconic, citric, fumaric, 

 succinic, lactic, and oxalic — which are formed in large amounts and 

 which have been studied intensively. Not all the data agree, but in 

 general it can be said: 



1. The acids occur, in small amounts at least, as normal metabolites 

 in organisms other than the fungi. This is also true of acetate, 

 formate, glyoxylate, and malate. 



2. With few exceptions — and these are subject to confirmation — 

 the acids which accumulate in fungus cultures are later utilized for 

 energy and growth unless removed by a trapping agent, e.g., calcium 

 ion, or unless their accumulation poisons the cell. 



3. Most experiments agree that highest acid yields, in terms of the 

 fraction of sugar converted to product, are obtained in media in which 

 carbohydrate is abundant but in which some other factor, usually nu- 

 tritional, restricts growth to a less than maximum amount. 



4. Oxygen is essential for the synthesis of all but lactic acid; this 

 requirement is, of course, obvious from the structure of the acids. 

 Gluconate formation, catalyzed as it probably is by a flavin enzyme, 

 is the most responsive to oxygen pressure. 



5. Carbon dioxide enters by fixation into lactic, citric, fumaric, 

 and succinic acids, and appears to be quantitatively important in 

 providing the carbon of all of these except possibly lactate. 



6. Genetic factors determine whether or not a clonal culture will 

 accumulate acid under given external conditions. 



