1 02 CARBON METABOLISM I 



be mentioned. First, it is not always taken into account that the 

 typical preparation contains many enzymes in addition to the one 

 under study. Thus, the appearance of isomaltose during the hy- 

 drolysis of starch by amylase was once taken as evidence for the 

 structure of the starch molecule; it is now known that it merely de- 

 notes the presence of a contaminating transglycosidase (17). 



A second common difficulty in enzymatic work is the tendency to 

 read too much into experiments with intact cells. Thus, the appear- 

 ance of unsaturated fatty acids in the medium during the metabolism 

 of saturated fats by intact cells is certainly prima-facie evidence for 

 the existence inside the cells of a fatty acid dehydrogenase; it is not, 

 however, a demonstration of the enzyme and should not be described 

 as such. Ex hypothesi, an intracellular enzyme can be demonstrated 

 unequivocally only in a cell-free preparation, although properly de- 

 signed experiments with whole cells may establish a high probability 

 that the enzyme is present. 



The variability of fungi is a real problem which arises in connection 

 with virtually all phases of metabolism. However, the amount of 

 variation should not be exaggerated and should not be used as a 

 crutch for a failing hypothesis. Complete loss of metabolic capacities 

 is, of course, well known to be inducible by mutagenic agents and 

 presumably, therefore, may occur as a result of spontaneous mutation. 

 Most such mutants, however, are non-viable except under carefully 

 adjusted conditions of culture. It appears that quantitative changes 

 are more common than qualitative; an organism may change in the 

 amount of a given substance formed but still form some. Exceptions 

 will occur and must be taken into account, but the progress of a cumu- 

 lative science requires that the material foundation be assumed con- 

 stant unless the evidence clearly proves that it is not. 



Variability may result from changes in the growth rate, such as 

 those described among geographical isolates and cultural variants of 

 Fomes annosus (10), rather than from a loss of metabolic capacities. 

 If two strains have the same metabolism but different growth rates, 

 analysis at one arbitrary time for a metabolite may give an exaggerated 

 picture of the differences between them. Thus, if a given compound 

 is first formed and then utilized after exhaustion of the principal 

 carbon source, a rapidly growing strain may complete the process of 

 reutilization at a time when a more slowly growing strain is still in 

 the phase of rapid formation of the compound; it would appear from 

 analysis at this single time that the rapidly growing strain was deficient 

 in the synthesis. To circumvent this difficulty it is necessary to study 

 cultures at the same physiological age — judged by the amount of 



