140 PHYSIOLOGY OF THE FUNGI 



monosaccharides, is likewise an enzymatically catalyzed chain of meta- 

 bolic processes. It may be assumed that the chemical composition of the 

 fungus will be about the same, irrespective of the carbon source utilized. 

 Therefore, at some place along the path of synthesis the initial carbon 

 sources are converted into the same compounds. It is probable that the 

 original compounds are converted into the same intermediate compounds 

 before synthesis. Thus, galactose is apparently transformed by Sac- 

 charomyces fragilis into galactose- 1-phosphate, which is then converted 

 into glucose- 1-phosphate (Caputto et at., 1949). These intermediate 

 compounds then enter the various metabolic reaction chains which lead 

 to the production of materials which make up the fungus. We may 

 suppose that the first steps in utilization are those which transform a 

 carbon source into key intermediates. 



The intermediate metabolic products should also serve as a source of 

 carbon for the fungus in question. If a fungus transforms compound A 

 into compound B, then compound B should serve as a source of carbon. 

 Nonutilization of compound B indicates that this compound is not part of 

 the metabolic pathway. This simple hypothesis neglects two important 

 considerations : compound B may not enter the fungus cells with the same 

 facility as compound A, or compound B may be toxic in the concentra- 

 tions present. As an example of this approach, the work of Steinberg 

 (1942) may be consulted. Since Aspergillus niger made only a trace of 

 growth on D-gluconic acid, it seems probable that the first step in the 

 utilization of glucose by this fungus is not the oxidation of the aldehyde 

 group. The isolated enzymes from a fungus may also be studied to 

 determine the reactions catalyzed, or the effect of specific enzyme 

 inhibitors on the intact fungus may be studied. 



In some instances intermediates of sugar dissimilation are excreted into 

 the medium and may be isolated. Thus, the production of acetaldehyde 

 may be demonstrated by adding bisulfite to the medium. Acetaldehyde 

 forms an insoluble addition product with this reagent. The excretion 

 of intermediate metabolites may be due to slowness of the next step in the 

 metabolic process. These products are usually utilized in the course of 

 time. Among such intermediates which have been identified are acet- 

 aldehyde, ethyl alcohol, and pyruvic acid. 



On the basis of the evidence now available we may not assume that all 

 fungi utilize a sugar or other carbon source in exactly the same way, or 

 that a fungus has only one metabolic pathway for the utilization of a 

 sugar. Nord and Mull (1945) consider that species of Fusarium dis- 

 similate carbohydrates by oxidation, by splitting the carbon chain, and 

 by a phosphorylation mechanism. The relative importance of these 

 three methods of attack depends upon the fungus involved and upon the 

 environmental conditions. Identity of a metabolic product formed by 



