214 KoNRAD Block 



failed to serve the specific purpose for which they were 

 designed. 



Time does not permit to discuss the hmited and on the 

 whole inconclusive information which has previously been 

 obtained on the biosynthesis of phenylalanine and tyrosine. 

 The studies of Ehrensvard and his associates (Baddiley, 

 Ehrensvard, Klein, Reco and Saluste, 1950) which resemble 

 ours in many respects and which have greatly aided in the 

 interpretation of our results, will be referred to later. 



Amino-acid synthesis was studied by Dr. Gilvarg under the 

 following experimental conditions. Yeast (S. ccrevisice) was 

 grown from a small inoculum in a medium containing glucose 

 and acetate as carbon sources, citrate buffer, inorganic salts, 

 and 7?z^50-inositol as growth factor. Acetate was the labelled 

 substrate in one series of experiments, while in another 

 [1-^*C] glucose was the source of isotopic carbon (Koshland 

 and Westheimer, 1950). After a suitable period of growth, 

 the cells were harvested. The yeast protein was hydrolysed 

 and the resulting amino-acid mixture was resolved with the 

 aid of ion-exchange resins, according to Stein and Moore 

 (1950). A modification found to be useful for our purpose 

 was a preliminary separation of phenylalanine and tyrosine 

 from the hydrolysate by adsorption on charcoal as described 

 by Partridge (1949). Prior to isotope analysis the amino-acids 

 were crystallized either as such or in the form of suitable 

 derivatives. The identity of the amino-acids was in all cases 

 confirmed by paper chromatography. 



The isotope concentrations of phenylalanine, tyrosine and 

 a few other amino-acids from the proteins of yeast grown with 

 labelled acetate as substrate, are listed in Tables I and II. 

 The mean of the isotope concentrations of the acetate added 

 initially and of the recovered acetate serves as a measure of 

 the average level of isotope in the acetate available to the 

 yeast cell during the growth period. The yeast fatty acids, 

 which are formed by the condensation of acetyl units, contain 

 isotopic carbon in a concentration which is about two-thirds 

 of this average. 



