Robert A. Steinberg 361 



carbon dioxide and ammonia, or be reworked to carbohydrate or other 

 compounds. This interpretation would be in better agreement with the 

 surmise of Gregory and Sen that breakdown of protein follows a 

 different course than synthesis. 



Another objection to a simple regenerative amino acid-protein cycle 

 in plants is based on the phenomenon of "nitrogen balance" in animals. 

 A daily loss of amino nitrogen must be compensated in the diet, since 

 it persists even during extreme starvation. The "wear and tear" protein 

 metabolism is accompanied by loss of amino nitrogen as urea or uric 

 acid, even in the starving animal. It seems logical to the writer, there- 

 fore, to assume that a similar cycle of nitrogen and carbon loss occurs 

 in the plant. While traces of urea and uric acid have been identified 

 in plants, respiratory carbon dioxide probably accounts for the carbon 

 while the nitrogen is reutilized to form new amino acids. Vickery and 

 others (56) have remarked on the close similarity between "nitrogen 

 balance" in animals and the rapid intake by tobacco proteins of radio- 

 nitrogen, though Vickery (55) categorically later denied the existence 

 of "nitrogen balance" in plants. It is true that technically "nitrogen 

 balance" as such does not exist in plants since nitrogen is retained and 

 re-used. However, in the larger phenomenon of which it is only one 

 facet, it must be assumed that loss of carbon and nitrogen from proto- 

 plasmic protein is inherent, automatic, and irreversible in all cells. It 

 is truly cyclic. 



Other data bearing on the cyclic interpretation of protein metabolism 

 consist in the fluctuations of protein and carbohydrate in plants. Protein 

 varies little in comparison to carbohydrate, if storage products of both 

 are omitted from consideration. Brief starvation experiments readily 

 demonstrate this difference. Protoplasmic protein can be depleted or 

 altered to such an extent as to prevent resumption of growth only 

 through starvation of long duration. This is not the case, within limits, 

 with carbohydrate. Increases in carbohydrate above minimal have little 

 influence on protein formation, whereas increases in nitrogen (particu- 

 larly as ammonium salts) do and may lead to marked depletion of car- 

 bohydrate. It might also be mentioned that carbohydrate supplied 

 externally to the green plant is utilized efficiently, whereas amino acids 

 so supplied are usually harmful even in high dilution (49). Finally 



