Synthesis of Citrulline and Arginine in Chlorella pyrenoidosa

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Abstract
The distribution of radioactivity in Chlorella during dark 14CO2 fixation was investigated either (a) in normal cells with and without added ammonium chloride, or (b) in nitrogen-starved cells supplied with intermediates of the Krebs-Henseleit urea cycle. In the control experiments almost all the activity was present in compounds of or associated with, the tricarboxylic acid cycle. The amino-acids citrulline and arginine became radioactive only in the presence of ammonia or ornithine where initially they comprised 40–60 per cent. of the total activity, reactions of the Krebs–Henseleit urea cycle being implicated in their formation. No evidence could be found for a complete urea cycle. Unidentified compounds deriving their radioactivity from the C4 carbon of citrulline and/or arginine were detected and formed up to 40 per cent. of the total 14CO2 incorporated after 25 min.

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