Regulation of Growth and Polyamine Biosynthesis of the Ciliated ProtozoanTetrahymena thermophila.Effects of L-Arginine Metabolites and Polyamines

Growth of Tetrahymena thermophila in a synthetic nutrient medium with or without the essential amino acid L-arginine was studied in the presence or absence of the arginine metabolites L-citrulline and L-ornithine and the polyamines putrescine, spermidine, and spermine. The effects of the growth conditions on the stimulations of the enzymes of the arginine metabolic and polyamine biosynthetic pathway, arginine deiminase (ADI), citrulline hydrolase (CH), ornithine decarboxylase (ODC), and ornithine-oxo-acid aminotransferase were determined. Tetrahymena cells were unable to grow in the absence of L-arginine and the amino-acid utilization was greatly impaired. None of the metabolites or polyamines was able to substitute for arginine. In the presence of arginine, Tetrahymena cultures grew well and citrulline and ornithine did not alter the growth behaviour in any way. In the presence of putrescine, the lag period was decreased from 3 h to 2 h. Spermidine and spermine acted similar to putrescine but less pronounced. The stimulation of the activity of ADI, the key enzyme of arginine degradation, was absolutely dependent upon the presence of arginine in the medium: in the absence of arginine, the low ADI activity which was present in the cells before inoculation was decreased to zero levels within 30 min. In the presence of arginine, the stimulation of ADI was not altered by citrulline and ornithine but putrescine, spermidine, and spermine decreased ADI-stimulation to half of the control values. The stimulation of CH activity in the presence of arginine was not altered by any added metabolite or polyamine. In the media without arginine, stimulation of CH was greatly reduced, in the presence of ornithine more than in its absence, and even more in the presence of putrescine and spermidine. Stimulation of ODC activity in the presence of arginine was not affected by citrulline and ornithine but in the presence of polyamines it was rapidly decreased to unstimulated levels after an initial ca. 10-fold increase. The "hyperstimulation" of ODC in the absence of free arginine was reduced to normal in the presence of citrulline, the stimulation was decreased even below normal levels in the presence of ornithine and polyamines decreased ODC activity to zero levels. O delta T activity was stimulated more in the presence of arginine than in its absence. In both cases the stimulation was enhanced in the presence of polyamines and only in the absence of arginine--by ornithine.(ABSTRACT TRUNCATED AT 400 WORDS)