VI. ESTIMATION 



167 



It is apparent that no one procedure can bo recommended for digestion 

 of the conjiiiiatos in all types of natural materials. In order to determine 

 whether complete hydrolysis has been obtained, it may be necessary to 

 employ different enzyme systems and to vary the amounts of enzyme. 



2. Microbiological Procedures 



Although a number of organisms have been found which will respond to 

 pteroylglutamic acid, only two organisms, L. casei and S. faecalis, have 



TABLE VIII 



Comparison of PGA Assays Made by Assays avith Chicks and with Lactobacillus 



casei before and after Enzyme Treatment" 



" Courtesy J. Biol. Chem. (Bird et aU^). 

 EnzjTiie method: 1. Acetone desiccated hog kidney, pH 4.2-4.5. 



2. Extract of unheated almonds, pH 6.0-7.0. 



3. Extract of commercial almond meal, pH 6.0-7.0. 



4. Extract of autolyzed hog kidney, pH 4.5. 

 .Kn aqueous extract of plasmolyzed yeast. 



Difco Laboratories, Detroit. 



been used for the assay of this vitamin. L. casei has the advantage in that 

 it is the more sensitive of the two organisms. It gives half -maximum growth 

 with approximately 0.05 m7 of pteroylglutamic acid per milliliter of me- 

 dium, whereas S. faecalis reciuires 0.3 m7. L. casei does not give ma.ximum 

 growth with purines and thymine, whereas *S. faecalis does. Thus L. casei 

 is better suited to the assay of low-potency materials and those products 

 which may contain thymine. Approximately five thousand times as much 

 thymine is re(|uired to produce the same response as PGA both for S. faecalis 

 and L. cnsei. L. casei is a slower gi'owiug organism than S. faecalis, and the 

 assays with the former organism are usually carried out 72 hours and the 



