The Physiological Role and Catabolism of Arginine in Euglena gracilis

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While arginine was not taken up by Euglena gracilis and accordingly did not support the growth of this protozoan, it was accumulated in both free and peptide forms as a major nitrogen reserve in the cells, when a relative excess of a nitrogen source was supplied. Its content was greatest in the early exponential phase of growth, reaching as high a concentration as 220μmol per 109 cells or 190mM in the cell volume. In the mid-exponential phase it decreased and accumulated again in the late-exponential phase. Addition of ethanol or glucose to a culture of E. gracilis in the late-exponential phase rapidly eliminated the accumulated arginine with renewed cell division, and the further addition of ammonium salts or the amino acids caused renewed accumulation of arginine. The degradation of accumulated arginine involved de novo synthesis of enzymes related to arginine metabolism. Analysis of metabolic products after incubation of labelled arginine in cell-free extracts and assay of related enzymes showed that in Euglena arginine is catabolized to ornithine via citrulline by the so-called arginine dihydrolase pathway, releasing ammonia successively. The enzymes involved in this pathway, arginine deiminase and citrullinase, had different optimum pH values from those of other organisms.
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