Agmatine is transported into liver mitochondria by a specific electrophoretic mechanism

Abstract

Agmatine, a divalent diamine with two positive charges at physiological pH, is transported into the matrix of liver mitochondria by an energy-dependent mechanism the driving force of which is DeltaPsi (electrical membrane potential). Although this process showed strict electrophoretic behaviour, qualitatively similar to that of polyamines, agmatine is most probably transported by a specific uniporter. Shared transport with polyamines by means of their transporter is excluded, as divalent putrescine and cadaverine are ineffective in inhibiting agmatine uptake. Indeed, the use of the electroneutral transporter of basic amino acids can also be discarded as ornithine, arginine and lysine are completely ineffective at inducing the inhibition of agmatine uptake. The involvement of the monoamine transporter or the existence of a leak pathway are also unlikely. Flux-voltage analysis and the determination of activation enthalpy, which is dependent upon the valence of agmatine, are consistent with the hypothesis that the mitochondrial agmatine transporter is a channel or a single-binding centre-gated pore. The transport of agmatine was non-competitively inhibited by propargylamines, in particular clorgilyne, that are known to be inhibitors of MAO (monoamine oxidase). However, agmatine is normally transported in mitoplasts, thus excluding the involvement of MAO in this process. The I2 imidazoline receptor, which binds agmatine to the mitochondrial membrane, can also be excluded as a possible transporter since its inhibitor, idazoxan, was ineffective at inducing the inhibition of agmatine uptake. Scatchard analysis of membrane binding revealed two types of binding site, S1 and S2, both with mono-co-ordination, and exhibiting high-capacity and low-affinity binding for agmatine compared with polyamines. Agmatine transport in liver mitochondria may be of physiological importance as an indirect regulatory system of cytochrome c oxidase activity and as an inducer mechanism of mitochondrial-mediated apoptosis.

Further characterization of agmatine binding to mitochondrial membranes: involvement of imidazoline I2 receptor

“…The present study, performed in the aim to further characterize the above mechanism, confirms, as previously proposed (Salvi et al 2006), the presence of two binding sites on mitochondrial membranes, but it also evidences the involvement of the imidazoline receptor I 2 in this mechanism.…”

“…Previous studies, conducted in our lab, demonstrated that the net amount of transported agmatine, in the presence of idazoxan, is not altered, while its initial binding to mitochondrial membranes is significantly modified (Salvi et al 2006). The analyses of Figs.…”

“…As above mentioned, a previous study demonstrates the presence of a specific agmatine transport system on mitochondrial membranes (Salvi et al 2006). This system is characterized by an electrophoretic behaviour with an apparent exponential force/flux relationship suggesting the involvement of a channel.…”

“…Agmatine that can be considered as another polyamine is also transported by a similar electrophoretic mechanism, having several identical properties, but it utilizes a specific transporter identifiable as a ''single-binding centre-gated pore'' (Salvi et al 2006). …”

“…In addition, agmatine is transported into the mitochondrial matrix by a specific energy-dependent mechanism whose driving force is the membrane potential (DW) (Salvi et al 2006). This transport system is characterized by different features depending on the organs .…”