216 - KoNRAD Block 



cent of the protein carbon from acetate and from the Cg units 

 formed from glucose. (2) The isotope concentrations in 

 different protein constituents vary over a wide range from 

 maximal values in lysine, which approach those of the higher 

 fatty acids, to very low levels in serine and glycine and insig- 

 nificant concentrations in phenylalanine and tyrosine. 

 Evidently carbon from glucose and from acetate is used in 

 amino-acid synthesis in widely varying proportions. For the 

 purposes of our discussion the isotope concentrations in the 

 dicarboxylic amino-acids and in alanine are pertinent because 

 they reflect the ^^C and ^*C levels in the corresponding a-keto 

 acids, which are members of the citric acid cycle. The 

 extensive labelling of glutamate, aspartate and alanine is 

 taken to indicate that this cycle is operating under the 

 conditions of our experiment, i.e. in growing yeast.* This 

 has previously been demonstrated for resting yeast (White 

 and Werkman, 1947; Weinhouse, Millington and Lewis, 1948). 

 By the same token the failure of acetate to provide carbon 

 atoms for phenylalanine and tyrosine excludes as precursors 

 not only acetate itself but also pyruvate and the intermediates 

 of the tricarboxylic acid cycle. This restriction applies to 

 both the benzene rings and to the side chains of the aromatic 

 amino-acids. The amino-acids which have the same carbon 

 chains as the cvcle intermediates do not all have the same 

 isotope concentrations. The values are highest in glutamic 

 acid, considerably lower in aspartic acid and still lower in 

 alanine. Most likely, these differences are the result of the 

 following events. The condensation of acetate with oxalo- 

 acetate produces highly active a-ketoglutarate and hence 

 glutamate with a high isotope concentration. The dicar- 

 boxylic acids and oxaloacetate formed in the course of the 

 citric acid cycle should contain isotopic carbon at the same 

 level, but are actually diluted by oxaloacetate resulting from 

 the carboxylation of unlabelled pyruvate, which is continually 

 produced from glucose. The isotope concentration in aspartic 



♦This conclusion is supported by some preliminary studies of the isotope 

 distribution in these amino-acids. 



