G. ROBERT GREENBERG 



reversible and that the compound is a product, ahhough, of 

 course, the effect could be due to an inhibition. If one observes 

 the utilization of a labeled substrate with and without addition 

 of a suspected product, the results may not be different. Thus, 

 depending on the equilibrium and rate of the reaction (A* ;:± B), 

 the labeled substrate A* may be converted quite rapidly into B 

 in short experiments even in the presence of added B. Only 

 after B dilutes the specific activity of A does an apparent dimin- 

 ished utilization of A occur, and this will also depend on the 

 relative concentrations of A and C. 



CONCLUSIONS 



Biosynthetic pathways tend to be relatively slow reactions 

 in comparison with energy-yielding reactions. The enzymes 

 that are involved frequently are present in small quantity. 

 Nevertheless the pathways may manifest their importance as 

 controlling mechanisms, as, for example, the rate of syn- 

 thesis of DPN. The resolution of the intimate mechanisms of 

 these pathways and of their involvement with other metabolic 

 systems must depend on all the available tools. First a cell-free 

 system carrying out the total synthesis is necessary. Then 

 logical intermediates must be sought wherever possible. With 

 the intermediate a fractionation of the system may be obtained. 

 In many cases a fractionation of the enzyme or the use of special 

 devices may bring about the accumulation of the intermediate. 



Finally much of what we have said may be negated by the 

 observation that in these studies the element of intuition and 

 guess plays the largest and most successful role. 



References 



1. Buchanan, J. M., B. Levenberg, J. G. Flaks, and J. A. Gladner, in 

 W. D. McElroy and H. B. Glass, eds., Amino Acid Metabolism, p. 743. 

 Johns Hopkins Press, Bahimore, 1955. 



2. Buchanan, J. M., and M. P. Schulman, J. Biol. Chem., 196, 513 (1952). 



3. Carter, C. E., and L. H. Cohen, J. Am. Chem. Soc, 77, 499 (1955). 



4. Chance, B., Harvey Lectures. Ser. 49, 145 (1953-54). 



558 



