Mechanism of polyamine spermidine uptake by Xenopus laevis oocytes

In this study, the mechanisms of polyamine spermidine (Spd) uptake were investigated in Xenopus laevis oocytes. Spd uptake followed a sigmoidal kinetics with [S]90/[S]10 = 3 microM and Hill interaction coefficient (n) = 2. The order of magnitude of uptake and efflux was similar (t1/2 = 45 min). The equilibrium potential for Spd, calculated by Nenrst equation, was 90.78 mV. Free energy change for the uptake (delta G) was found to be 2.31 Kcal/mole of Spd. During efflux, Spd was not converted into putrescine or spermine. It seems that there are two types of Spd uptake pathways: Na(+)-dependent and Na(+)-independent since replacement of Na+ from incubation medium did not completely abolish the Spd uptake. The Na(+)-dependent component of Spd uptake was shared neither by system A nor by system ASC amino acids.