. Ciba Foundation conferences on istopes in biochemistry;. Radioactive tracers in biochemistry; Isotopes -- Physiological effect. Phenylalanine and Tyrosine Synthesis 217 acid will be lower to an extent which is determined by the relative rates of the cycle reactions and of CO2 fixation. Some labelled pyruvate will be formed by decarboxylation of oxaloacetate, but its quantity is probably small compared to that of non-isotopic pyruvate originating directly from glucose. The isotope concentrations in alanine will therefore be reduced still further. These relationships are illustrated in Fig. 1.
. Ciba Foundation conferences on istopes in biochemistry;. Radioactive tracers in biochemistry; Isotopes -- Physiological effect. Phenylalanine and Tyrosine Synthesis 217 acid will be lower to an extent which is determined by the relative rates of the cycle reactions and of CO2 fixation. Some labelled pyruvate will be formed by decarboxylation of oxaloacetate, but its quantity is probably small compared to that of non-isotopic pyruvate originating directly from glucose. The isotope concentrations in alanine will therefore be reduced still further. These relationships are illustrated in Fig. 1. PYRUVI^C ACID °CH. GLUCOSE = ALANINE °CH, ol ^ =^ HCNH, COOH ASPARTIC ACID tOCH "«-KETOGLUTARICAClD GLUTAMIC ACID ,CH,COOH. CH-COOH I ^ CH, I ^ HCNH, I ^ COOH HO-C-COOH ^CHo *COOH CITRIC ACID COOH *COOH ACETIC ACID Carbon atoms derived from glucose marked ° Carbon atoms derived from acetate marked Fig. 1. Probable pathway of incorporation of acetic acid into benzenoid amino acids. The non-participation of acetate, pyruvate and inter- mediates of the citric acid cycle in the synthesis of phenylala- nine and tyrosine limits the possible carbon sources for these amino-acids to glucose itself, or to intermediates of glycolysis. For the identification of the precursors, [1-^^C] glucose seemed a suitable and relatively accessible substrate. The yeast growth experiment carried out with this labelled glucose was in all respects identical with earlier ones, except that the acetic acid in the medium was labelled by ^^C only (Table III). In this experiment the isotope distribution in pheny- lalanine and tyrosine was of primary Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Ciba Foundation. Philadelphia, Blakiston
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