644 D. B. COWIE 
least one enzyme in the biosynthetic pathway; (3) inhibition of the activity of at 
least one enzyme in the biosynthetic pathway. 
The mass-law equilibrium type of reaction certainly applies to the competitive 
effects observed among the endogenous and exogenous amino acids which occur 
prior to or at the time of incorporation of these amino acids into the internal pool. 
The inhibition of biosynthetic enzyme activity through a negative feedback me- 
chanism has been shown in a number of 77 vitro examples!°—"*, It is significant that 
in each of these investigations the end-product has been shown to inhibit the earliest 
step leading specifically to its own synthesis”. 
The data obtained from exponentially growing Candida utilis indicates that in 
these cells the earliest steps are naturally maximally inhibited. The steady-state 


Amino acids 
and 
Analogs 


Cold water 1, Cold TCA 
extractable pecoani 
Cell eptab 
fraction 
wall pst TCA Soluble | 
Fig. 8. won flow in C. utilis. 
production of family head amino acids is sufficient to saturate the internal (or con- 
version) pool of amino acids thereby preventing immediate entry of exogenous 
material along the chain of reactions of the biosynthetic pathway. Entry at the first 
step of the linked enzyme system ts possible, however, as long as the correct end-product 
(or any of tts intermediates) 1s available and the site (or system) 1s empty. 
It is significant that at the 1961 Federation Meetings F. LYNEN reporting on the 
biosynthesis of fatty acids showed that an enzyme-substrate complex (molecular 
weight ~ 2 x 10%) was capable of at least five sequential biosynthetic reactions 
which could not be interrupted by the addition of intermediary compounds. 
Carbon flow in Candida utilis 
Fig. 8 summarizes what is believed to be the carbon flow in C. utilis. While both 
the expandable and internal pool appear to have functional differences, no direct 
chemical distinctions have ever been obtained even though a variety of extraction 
methods have been employed. Despite this the amino acids contained in the internal 
pool appear to be closely coupled to final protein formation. Attempts to prevent 
or interfere with their incorporation have always been negative. In fact it has been 
demonstrated that while these cells maintain two mechanisms selecting an amino 
References p. 645 
