The diversity of Na(+)-independent uptake systems for polyamines in rat intestinal brush-border membrane vesicles

Na(+)-independent uptake rate and binding to the membrane surface of polyamines (spermine, spermidine and putrescine) have been characterized using rat small intestinal brush-border membrane vesicles. The uptake of spermine and spermidine was saturable (Km = 30.4 microM and 148.1 microM, respectively), however, putrescine uptake was not saturable up to 8 mM. In contrast, the values of binding to the membrane surface of all polyamines were not saturable in the present studies. In Dixon plot analysis, spermine competitively inhibited the uptake rate of spermidine with a Ki value of 33.8 microM, while the putrescine inhibitory effect on the spermidine uptake rate was non-competitive (Ki = 3.28 mM). These uptake systems were not affected by the valinomycin-induced K(+)-diffusion potential (inside negative). These results suggested that there were two different Na(+)-independent uptake systems for spermine and spermidine, as well as for putrescine, on this membrane. However, they were not the same as the electric potential-dependent uptake system for monocationic compounds. Furthermore, this uptake system for spermine and spermidine might not be a carrier protein, because the intravesicular spermine exhibited no trans-stimulation effect on the uptake of spermidine.