Polyamine uptake, binding and release in rat brain

The uptake, binding and release of the polyamines, spermidine and spermine, and of their diamine precursor, putrescine, were examined in synaptosomal preparations from rat hippocampus. The specific and relatively high-affinity uptake by synaptosomes was found only with putrescine (V max = 21.6 pmol/mg protein per h; K m =28.6 nM) and not with the other polyamines. In contrast, specific binding to membranes was found for spermidine (B max = 28.6 pmol/mg protein; K d = 42.9 nM) and for spermine (B max = 156.3 pmol/mg protein; K d = 83.3 nM), but not for putrescine. High potassium concentrations (35 mM) both induced the release of accumulated polyamines from synaptosomes and inhibited their binding. Specific polyamine binding evidently occurs selectively on the inner but not on the outer surface of synaptosomal membranes.