Stimulation of growth and polyamine biosynthesis of the ciliated protozoan Tetrahymena thermophila. Regulation by L-arginine.
Tetrahymena thermophila cells grown in a synthetic nutrient medium for 9 h removed 97% of the free L-arginine but less than 50% of any of the other essential amino acids. The major portion of the arginine was degraded rapidly (76-92%) whereas 5-15% was conserved as intact and only 2.5-10% were incorporated into protein. However, if bovine serum albumin (BSA) was present in the medium as a macromolecular arginine source the incorporation of free arginine into protein was reduced to less than 1% but the degraded fraction was increased. Apparently, the uptake mode of arginine determines its fate: arginine taken up by phagocytosis is bound for protein biosynthesis, arginine taken up by membrane receptors is chanelled to degradation. Media without arginine did not support growth of Tetrahymena. Citrulline and ornithine, the precursors of arginine biosynthesis in yeast and vertebrates, were not able to substitute for arginine. Pronounced morphological changes, e.g. greatly reduced ribosome content, were observed in Tetrahymena cells after 24 h of arginine starvation in otherwise complete medium, but not in cells starved in water, salt solution, or buffer. Thus, arginine is an essential nutrient component for Tetrahymena and the rapid degradation of this compound involving the enzymes arginine deiminase (ADI) and citrulline hydrolase (CH) might be of regulatory importance for the unicellular, as it is the case with acetylcholine and catecholamines in mammalian organisms. Since the product of these enzymes, L-ornithine, is the substrate for the regulatory key enzyme of polyamine biosynthesis, ornithine decarboxylase (ODC), the effects of the presence of absence of arginine on the activities of each particular enzyme of the pathway were studied, including ODC and the enzyme ornithine-oxo-acid aminotransferase (O delta T), which is a competitor of ODC for the common substrate. The arginine-degradative pathway was stimulated by extracellular free but not by peptide-bound arginine and was modulated by extracellular protein which induced phagocytosis; O delta T was stimulated with a time lag. The stimulation of ODC was in a reciprocal relation to the arginine concentration and enhanced by phagocytosis and previous arginine starvation.(ABSTRACT TRUNCATED AT 400 WORDS)