Cell‐free extracts of Tetrahymena pyriformis S synthesize citrulline from ornithine, and arginine from citrulline. The extracts also show arginase activity. Urease activity at various pH values has maxima at both pH 5.6 and 7.4. This points to two distinct enzymes. The presence of urea in ciliate cultures during early stages of growth, and its absence during the stationary phase of cultures, correlate with the shift in pH of the culture media and with the activity of the “alkaline” urease at various pH values.
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Gerald R. Seaman (1954) studied this question.
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