1. Rapid labelling occurs when myxamoebae of Dictyostelium discoideum strain AX2 are incubated with [1,4-14C]
putrescine. Labelling is energy-dependent. 2. The label enters a pool from which rapid exchange with extracellular
putrescine does not occur, and labelling is believed to represent uptake into the cells. 3. The concentration-dependence of
putrescine uptake indicates that a number of systems are involved, at least one of which is saturable, with a Km of 9.1 micro M-
putrescine. At high
putrescine concentrations the overall uptake process is non-saturable. 4. Significant metabolism of
putrescine and loss of intracellular
putrescine to the medium only occurred when cells were incubated with millimolar concentrations of extracellular
putrescine. 5.
Putrescine uptake was inhibited by
diamines,
polyamines, bivalent
metal ions and omega-aminocarboxylic
acids. 6. The ability to take up
putrescine at low concentrations decreased during
starvation of myxamoebae. 7. The results are interpreted in terms of a model for
putrescine uptake involving adsorptive pinocytosis at low concentrations and fluid-phase pinocytosis at high concentrations.