Abstract
The uptake characteristics of polyamines have been investigated using rat intestinal brush-border membrane vesicles (BBMV). The uptake of these polyamines into BBMV was high and the order of uptake activity was spermine > spermidine > putrescine, and the medium pH affected remarkably the uptake of polyamines (pH 7.5 > pH 5.5). We have also examined the binding behavior of polyamines to liposome, and the similar phenomenon such as uptake activity and pH-dependency was observed. An inward Na+ gradient did not stimulate the initial uptake of all polyamines. The uptake rate of spermine and spermidine were saturable (Km = 30.4 and 148.1 μM, respectively), but putrescine was not saturable up to 8 mM. Spermine competitively inhibited the uptake rate of spermidine (Ki = 33.8 NM), while putrescine inhibited spermidine non-competitively (Ki = 3.28 mM). However, intravesicular spermine exhibited no trans-stimulation effect on the uptake of spermine and spermidine. Therefore, the polyamines were considered to be taken up via a passive diffusion mechanism rather than the carrier-mediated systems. The interaction between polyamine and inner-layer acidic phospholipids of BBMV might play an important role as a driving force of the polyamine uptake mechanism. Furthermore, the uptake mechanism of polyamine was different from cationic compounds because valinomycin-induced potassium diffusion potential (inside negative) did not affect the uptake of polyamines.
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