Evidence that spermine, spermidine, and putrescine are transported electrophoretically in mitochondria by a specific polyamine uniporter.

We present evidence that polyamine uptake into rat liver mitochondria is mediated by a specific polyamine uniporter. Polyamine transport is not mediated by the ornithine, lysine, or Ca2+ transporters of mitochondria. Polyamine transport is a saturable process, with apparent Km values of 0.13 mM for spermine, 0.26 mM for spermidine, and 1 mM for putrescine. These substrates are mutually competitive inhibitors, indicating a common transport system. Polyamine transport is strictly dependent on membrane potential and insensitive to medium pH, showing that these polycations are transported electrophoretically. Spermine, spermidine, and putrescine are taken up by rat liver mitochondria at rates that increase with increasing valence of the transported species. The activation enthalpies for transport were 24, 32, and 59 kJ/mol for putrescine, spermidine, and spermine, respectively. These values, which amount to about 12 kJ/mol per charge transferred, may be compared to a value of 76 kJ/mol observed for monovalent tetraethylammonium cation. Flux-voltage analysis is consistent with the hypothesis that the mitochondrial polyamine transporter catalyzes transport via a channel mechanism.