Studies on Mammalian Red Cells 79 



of dosage of ^^C, remains reasonably constant at 1-24 ±0-2' 

 A difficulty in interpreting this result arises from the fact that 

 the injected valine was racemic and the exact proportion of 

 the D-isomer which is inverted and becomes available for 

 protein synthesis is therefore unknown. That inversion can 

 take place in the case of leucine has been shown by Ratner, 

 Schoenheimer and Rittenberg (1940) and the same presumably 

 applies to valine. However, the feeding experiments of Rose 

 (1938) show that such inversion may not be extensive. It 

 would thus appear that the average ratio of activities of 

 glycine to that of total available L-valine might be between 

 0-62 and 1-00. 



The finding that the ratio of the activity of the terminal 

 valine to that of non-terminal valine is unity at all points 

 examined is of general interest. It suggests that both types 

 of valine residue are built into peptide chains of haemoglobin 

 at approximately the same time. In other words, it appears 

 that the rate of synthesis of valine into terminal and non- 

 terminal positions is fast compared with the rate of change 

 of labelled valine in the body. A comparison of the rate of 

 incorporation of two different amino-acids is more complex, 

 since it involves in addition to the rates of peptide formation, 

 rates of turnover of free amino-acids in the organism and 

 possible permeability differences. The ratio of the molar 

 activity of glycine to that of valine was more variable than 

 the corresponding ratio for terminal and non-terminal valine. 

 However, the differences were relatively small and irregular 

 and it seems therefore that the rates of turnover of these two 

 amino-acids in the rat are very similar. It is also reasonable 

 to conclude from these results that the rates of incorporation 

 of glycine and valine into haemoglobin are similar. Altogether 

 these findings indicate that haemoglobin is formed by simul- 

 taneous condensation of amino-acids or by a process of rapid 

 successive condensations involving intermediates of relatively 

 short life. 



The results also show that the labelled glycine and valine 

 are diluted in the body to approximately the same extent. 



