Metabolism of Urea & Ornithine Cycle Intermediates by Nitrogen-Starved Cells of Chlorella vulgaris

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Abstract
Nitrogen-starved cells of C. vulgaris carry out several interconversions of ornithine cycle intermediates which can be explained by reactions of the classical Krebs-Henseleit cycle. For example, when C14 -ornithine is supplied to the cells, citrulline and arginine are labeled. The cells also accumulate citrulline and arginine after introduction to unlabeled ornithine. The organism apparently departs from the classical pathway in that it degrades arginine via the arginine desimidase catalyzed reaction rather than by arginase. Free amino acid changes in Chlorella after feeding unlabeled urea indicated that the usual urease-catalyzed hydrolysis of urea might not occur. This indication was strengthened by the observed distribution of C14 in amino acids after feeding C14 urea, and by negative tests for urease activity in extracts of the organism. A close interrelationship between glutamate, proline, and ornithine was found in this organism which agrees with known pathways in animals and microorganisms.
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