15N investigation into the effect of a pollutant on the nitrogen metabolism of Tetrahymena pyriformis as a model for environmental medical research

A pilot study was performed to examine the potential of stable isotope techniques for monitoring the impact of a harmful substance on the cellular nitrogen metabolism in the ciliate species Tetrahymena pyriformis. After identical cultivation periods of control cells and toluene-exposed cells in a defined culture medium enriched with [guanidino-N-15(2)]L-arginine, a number of nitrogen-containing pools were analyzed: 1) quantity and N-15 abundance of ammonia as the end product of nitrogen metabolism in the system; 2) pattern and N-15 abundances of the protein-bound amino acids in the cells; 3) pattern and N-15 abundances of free amino acids in the cells; and 4) pattern and N-15 abundances of the amino acids in the culture medium. In addition to N-15 emission spectrometry, a new gas chromatography/combustion interface-isotope ratio mass spectrometry/mass spectrometry analytical system was used. The production and 15N content of ammonia were higher in the toluene-exposed system by 30% and 43%, respectively indicating higher deamination rates and greater arginine consumption. The toluene-exposed cells exhibited increased N-15 abundances of protein-bound amino acids in alanine, aspartic acid, glutamic acid, and tyrosine. Furthermore, structural analyses revealed the presence of N-omega-acetylarginine and pyrrolidonecarboxylic acid-compounds that had not previously been detected in Tetrahymena pyriformis. Differences in the N-15-enrichment of free amino acids were also evident. This new effect-monitoring system designed to investigate the impact of a pollutant on protein metabolism by using a stable isotope-labeled cell culture is a powerful tool for environmental medical research.