NITROGEN ATOMS IN THE SUNFLOWER 909 



paratively larger than that (12 per cent) found by us in the course of 

 12 days. When comparing these figures we must envisage the possibihty 

 that the replacement rate in buckwheat may be different from that in 

 sunflowers. Furthermore, the rate of replacement is not necessarily 

 proportional with time. Some types of the protein molecules present 

 in the leaves may be renew^ed at a much faster rate than others. 



In a recent paper of Vickery, Pucher, Schoenheimer and Ritten- 

 BERG (1940), the ^^N content of the nitrogen isolated from 7 different 

 nitrogen compounds was compared. Free NH3 nitrogen was found to 

 be easiest replaceable, while the volatile bases contained the less replace- 

 able nitrogen. 



Summary 



Bj'' making use of heavy nitrogen (i^N) as an indicator it was found that in 

 "old" leaves of the sunflower, which did not develop further during the experiment, 

 12 per cent of the protein molecules present were renewed within 12 days. In 

 growing leaves the replacement of old protein molecules by new ones was found 

 to take place at an enhanced rate. 



References 



1. G. Hevesy, K. Linderstr0m-Lang and C. Olsen, Nature 137, 66 (1936) 

 and Nature, Land. 139, 149 (1937). 



2. D. RiTTENBERG, A. S. Keston, F. Rosebury and R. Schoenheimer, /. 

 Biol. Chem. 127, 291 (1939). 



3. H. B. Vickery, G. W. Pucher, R. Schoenheimer and D. Rittenberg, 

 J. Biol. Chem. 129, 791 (1939). 



4. H. B. Vickery, G. W. Pucher, R. Schoenheimer and D. Rittenberg, 

 in print. 



Comment on paper 93 



Professor Urey supplied us most generously with appreciable amounts of 

 amimonium chloride containing 2.5 per cent heavy nitrogen enabling us to find 

 out whether the protein molecules of the leaves of the sunflower are renewed or 

 not. In the non-growing leaves we found a 1 per cent renewal per day, in growing 

 ones a more rapid renewal rate. Rittenberg et al. (1939) arrived simultaniously 

 at a similar result. Professor Schoenheimer most kindly put at our disposal 

 his mass spectrograph to determine the ^^N content of our samples. The deter- 

 mination was carried out by Keston. 



Reference 



D. Rittenberg, A. S. Keston, F. Rosebury and R. Schoenheimer (1939), 

 J. Biol Chem. 127, 291. 



