The intracellular acid-extractable (acid-soluble) amino acid pool in mammalian cells: 2 Displacement.

Amino acid displacement is a separate and easily distinguishable phenomenon from discharge of the acid-extractable pool. It is characterized by its extreme rapidity for certain amino acids, with a resetting of the concentrations of different species in the pool at a new level, from which normal discharge kinetics then continue. It is possible to insert a labelled amino acid into the acid-extractable pool by displacement by the same mechanism as labelled amino acid can be ejected from the pool. The amount of displacement which occurs is dependent on the total number of potential displacing molecules available and not simply on their concentration in the medium. Differences occur between amino acids in their ability to displace the pool or to be displaced from the pool. The major determining factor is the strength with which a resident amino acid in the pool is complexed. Displacement for the more weakly complexed amino acids occurs at 0--2 degrees C, and requires no energy supply. The results are discussed in relation to the nature of the pool itself, and the implications of displacement on the uptake of amino acids into the cell.