Abstract
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.
Highlights
From the Dipartimento de Chimica Biologica,Centro Studio de FisiologiaMitocondriale del ConsiglioNazionale delle Richerche, Uniuersita’di Padoua, Italy and the $Department of Pharmacology, MedicalCollege of Ohio, Toledo, Ohio 43699-0008
We present evidence that polyamine uptake into rat bidirectionally across the inner membrane both in rat liver liver mitochondriais mediated by aspecific polyamine [27] and in hearmt itochondria [28].The influx, which occurs uniporter
Parent K, values of 0.13 mM for spermine, 0.26 mM To further characterize the polyamine uptake mechanism, for spermidine, and 1 mM for putrescine. These sub- we studied its dependence on A*,’ pH,temperature, and strates are mutually competitive inhibitors, indicating a common transportsystem.Polyaminetransport is strictly dependent on membrane potential and insensitive to mediumpH, showing that these polycations are transportedelectrophoretically.Spermine, spermidine, and putrescineare taken up by ratliver mitochondriaat rates that increasewith increasing valence substrate concentration
Summary
Yotes [15,16,17,18] and in eukaryotes during cellular responses to proliferative stimuli, such as growth factors and hormones [13, 19, 20] and following inhibition of de novo polyamine synthesis [21,22,23,24]. We have previously shown that spermine is transported. Three features are noteworthy: (i) all of the cations exhibited nonohmic conductance; (ii) monovalent tetraethylammonium (TEA’) is transported a t a considerably lower rate than the polyamines; (iii) the rates of polyamine uptake increase with the valence of the. At high values of A*, the equation forinflux, J,of a cation of valence, z, can be writtenas follows [35], defrayed in part by the payment of page charges.
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