Polyamines are small cationic molecules required for cellular proliferation and are detected at higher concentrations in most tumour tissues, compared to normal tissues. Agmatine (AGM), a biogenic amine, is able to arrest proliferation in cell lines by depleting intracellular polyamine levels. It enters mammalian cells via the polyamine transport system. Agmatine is able to induce oxidative stress in mitochondria at low concentrations (10 or 100 lM), while at higher concentrations (e.g. 1-2 mM) it does not affect mitochondrial respiration and is ineffective in inducing any oxidative stress. As this effect is strictly correlated with the mitochondrial permeability transition induction and the triggering of the pro-apoptotic pathway, AGM may be considered as a regulator of this type of cell death. Furthermore, polyamine transport is positively correlated with the rate of cellular proliferation. By increasing the expression of antizyme, a protein that inhibits polyamine biosynthesis and transport, AGM also exhibits a regulatory effect on cell proliferation. Methylglyoxal bis(guanylhydrazone) (MGBG), a competitive inhibitor of S-adenosyl-L-methionine decarboxylase, displaying anticancer activity, is a structural analogue of the natural polyamine spermidine. MGBG has been extensively studied, preclinically as well as clinically, and its anticancer activity has been attributed to the inhibition of polyamine biosynthesis and also to its effect on mitochondrial function. Numerous findings have suggested that MGBG might be used as a chemotherapeutic agent against cancer.

Metabolism of polyamines spermidine and spermine, and their diamine precursor, putrescine, has been a target for antineoplastic therapy since these naturally occurring alkyl amines were found essential for normal mammalian cell growth. Intracellular polyamine concentrations are maintained at a cell type-specific set point through the coordinated and highly regulated interplay between biosynthesis, transport, and catabolism. A correlation between regulation of cell proliferation and polyamine metabolism is described. In particular, polyamine catabolism involves copper-containing amine oxidases and FAD-dependent polyamine oxidases. Several studies showed an important role of these enzymes in several developmental and disease-related processes in both animals and plants through a control on polyamine homeostasis in response to normal cellular signals, drug treatment, environmental and/or cellular stressors. The production of toxic aldehydes and reactive oxygen species, H(2)O(2) in particular, by these oxidases using extracellular and intracellular polyamines as substrates, suggests a mechanism by which the oxidases can be exploited as antineoplastic drug targets. This minireview summarizes recent advances on the physiological roles of polyamine catabolism in animals and plants in an attempt to highlight differences and similarities that may contribute to determine in detail the underlined mechanisms involved. This information could be useful in evaluating the possibility of this metabolic pathway as a target for new antiproliferative therapies in animals and stress tolerance strategies in plants.

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.

Highly purified rat liver mitochondria (RLM) when exposed to tert-butylhydroperoxide undergo matrix swelling, membrane potential collapse, and oxidation of glutathione and pyridine nucleotides, all events attributable to the induction of mitochondrial permeability transition. Instead, RLM, if treated with the same or higher amounts of H 2 O 2 or tyramine, are insensitive or only partially sensitive, respectively, to mitochondrial permeability transition. In addition, the block of respiration by antimycin A added to RLM respiring in state 4 conditions, or the addition of H 2 O 2 , results in O 2 generation, which is blocked by the catalase inhibitors aminotriazole or KCN. In this regard, H 2 O 2 decomposition yields molecular oxygen in a 2:1 stoichiometry, consistent with a catalatic mechanism with a rate constant of 0.0346 s ؊1 . The rate of H 2 O 2 consumption is not influenced by respiratory substrates, succinate or glutamate-malate, nor by N-ethylmaleimide, suggesting that cytochrome c oxidase and the glutathione-glutathione peroxidase system are not significantly involved in this process. Instead, H 2 O 2 consumption is considerably inhibited by KCN or aminotriazole, indicating activity by a hemoprotein. All these observations are compatible with the presence of endogenous heme-containing catalase with an activity of 825 ؎ 15 units, which contributes to mitochondrial protection against endogenous or exogenous H 2 O 2 . Mitochondrial catalase in liver most probably represents regulatory control of bioenergetic metabolism, but it may also be proposed for new therapeutic strategies against liver diseases. The constitutive presence of catalase inside mitochondria is demonstrated by several methodological approaches as follows: biochemical fractionating, proteinase K sensitivity, and immunogold electron microscopy on isolated RLM and whole rat liver tissue.Many human diseases, including cancer and other pathologies associated with aging, such as arteriosclerosis and cataracts, are related to mitochondrial dysfunctions provoked by reactive oxygen species (ROS) 2 (1). In this regard, the so-called free radical theory of aging has been proposed (2). ROS are highly reactive and may be extremely toxic in biological systems, as they attack a variety of molecules, including proteins, polyunsaturated lipids, and nucleic acid (3), causing the cell to die by apoptosis or necrosis. In physiological conditions, 1-2% of molecular oxygen consumption during mitochondrial respiration undergoes incomplete reduction by single electrons to form superoxide anion (O 2 . ) at the level of NADH-ubiquinone reductase (complex I) and ubiquinol-cytochrome c reductase (complex III). These two segments of the respiratory chain generate the superoxide radical by autoxidation of reduced flavin and by transferring an electron from reduced ubisemiquinone to molecular oxygen, respectively (4). Superoxide is rapidly converted to hydrogen peroxide by mitochondrial superoxide dismutase, which then produces the highly reactive hydroxyl radical (OH ⅐ ...

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