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Current Topics in Biochemical Research   Volumes    Volume 4 
Abstract
Interactions of polyamines with mammalian mitochondria
Antonio Toninello
Pages: 37 - 48
Number of pages: 12
Current Topics in Biochemical Research
Volume 4 

Copyright © 2001 Research Trends. All rights reserved

ABSTRACT

Naturally occurring polyamines are polycationic at physiological pH and are present in the millimolar range in the cytosol. They bind to the mitochondrial membranes at the level of two monocoordinated binding sites (S1and S2) for spermine and spermidine, and one monocoordinated binding site (S2) for putrescine. These sites exhibit low affinity and high binding capacity with a calculated binding energy that is characteristic for weak interactions. Differences in the polyamines` flexibility and hydrophilicity most likely account for the observed variations in their binding parameters.

Polyamine binding sites are probably located in close proximity to each other on the inner membrane, but do not exhibit any appreciable cooperative effect and are filled in different ways depending on the polyamine. Binding parameters are affected by deenergizing agents and polycationic molecules. Both S1 and S2 are involved in the electrophoretic mechanisms of polyamine transport into mitochondria. Flux-voltage and free-energy profiles derived from analyses of spermine transport provide evidence for the presence of a channel exhibiting two asymmetrical energy barriers with an energy trough, at the level of S1, located near the external surface at 1/8 of the length of the channel. Spermine accumulated into the mitochondrial matrix is able to flow out by a ΔpH-dependent mechanism. This process establishes a bi-directional transport of the polyamine in and out of mitochondria, driven by membrane potential and pH gradient, respectively, and suggests that there is a continuous energy-dependent influx-efflux cycling of spermine. Polyamine transport is strongly inhibited by deenergizing agents, valinomycin plus K+, inhibitors of phosphate transport, magnesium ions and non-physiological polycationic molecules.

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