Antagonism by L-glutamine of toxicity and growth inhibition caused by other amino acids in suspension cultures of Nicotiana silvestris

The toxicity of L-amino acids toward exponentially dividing cells of Nicotiana silvestris in suspension culture was monitored by following growth rates throughout a span of 8 days. Except for L-glutamine, all 19 protein amino acids inhibited cell growth. Inhibition progressed with an initial stage of slowed exponential growth. Cells in this stage were receptive to complete recovery under some conditions. Otherwise an irreversible stage of total growth inhibition and progressive cell deterioration followed. Electron microscopy showed that amino acids triggered a state of cell shrinkage which eventually degenerated to total cellular disorganization. An apparent exocytotic deposition of condensed and blackened cellular debris occurred between the cell wall and plasmalemma. L-Glutamine was not only an effective agent for prevention of amino acid toxicity, but enhanced the final growth yield. L-Glutamine also was able to completely reverse inhibitor effects in cells which had been in the slowed exponential phase for up to 3 days. We propose that any amino acid inhibition which can be completely antagonized by L-glutamine be called 'general amino acid inhibition'. 'Specific amino acid inhibition', resulting from particular pathway imbalances caused by certain exogenous amino acids, can be recognized and studied in the presence of L-glutamine which abolishes the complicating effects of general amino acid inhibition. It is hypothesized that the L-glutamine:amino-acid ratio established in vivo might influence susceptibility to apoptosis, a eukaryotic process of programmed cell death which has been characterized most extensively in animals.