INTRODUCTION 101 



It is not always appreciated that a carbon balance implies an oxidation- 

 reduction balance, and that some error must have been made if the 

 oxidized products cannot be accounted for by the reduced products. 



The use of resting cells in metabolism studies is another valuable 

 technique, but its limitations are not always recognized. The essential 

 feature of the method is that cells are grown in a complete medium, 

 washed, and put in contact with, usually, a single compound. This 

 compound may be a putative precursor of a fungal metabolite which 

 is of interest, or one may be concerned with the dissimilation of the 

 compound itself. In a short space it is impossible to describe all the 

 pitfalls of this method, but some of the more important may be sum- 

 marized: 



1. The previous history of the cells may influence the results ob- 

 tained, at least quantitatively (1, 14, 19). 



2. The thick, inhomogeneous mat of a surface culture is probably 

 always less desirable than the more nearly homogeneous mycelium 

 from a shaken or aerated culture. 



3. The purpose of the method is defeated if a source of nitrogen is 

 included; generally the cells contain and liberate some nitrogenous 

 compounds. 



4. The endogenous metabolism must be taken into account or even, 

 if possible, manipulated, e.g., by starvation. Thus, it is possible that 

 an added compound may simply protect an endogenously formed 

 metabolite from utilization (7) and appear, therefore, to be a precursor 

 of it. 



5. Probably the technique is of more value in the study of the dis- 

 similation of a compound than it is in the study of synthetic pathways. 

 A compound, e.g., acetate, has so many metabolic channels open to it 

 that its effect on a synthesis is difficult to interpret. Similar problems 

 arise in studies of dissimilation, but in general the interpretation is 

 clearer, provided the products account for the bulk of the compound 

 disappearing. It has been observed (16) that the weight of "resting 

 cells" may increase during incubation with sucrose; presumably this 

 is a manifestation of oxidative assimilation (Chapter 7). 



The use of the resting cell technique is discussed by Foster (11), who 

 offers the particularly valuable suggestion that in precursor studies a 

 known low level of carbohydrate — just sufficient to allow minimal syn- 

 thesis of the product — should be supplied. 



The details of particular enzymatic methods are beyond our scope 

 here; a compendium of such methods has recently appeared (9). Two 

 common problems which arise repeatedly in the fungi may, however, 



