How do Ca2+ and 5-aminolevulinic acid-derived oxyradicals promote injury to isolated mitochondria?

The biosynthetic heme precursor 5-aminolevulinic acid (5-ALA) is a generator of oxygen radicals in vitro and possibly in vivo during pathologic situations of 5-ALA overload, for example, acute intermittent porphyria and saturnism. It has been observed that 5-ALA induces, in isolated rat liver mitochondria, permeabilization of the inner mitochondrial membrane (a phenomenon called permeability transition) as verified by the elimination of the transmembrane electrical potential, Ca2+ release, mitochondrial swelling, and increase in state-4 respiratory rate. The damaging process is primarily attributed to ʹOH radicals as elucidated by the protection by catalase, superoxide dismutase, and the Fe(II) chelator o-phenanthroline. Ruthenium red, EGTA, and dithiothretol (DTT) have been observed to prevent the action of 5-ALA-generated oxyradicals, suggesting the participation of both Ca2+ and the oxidation of critical thiol membrane proteins in the process of permeability transition. 5-ALA-induced polymerization of thiol membrane proteins has also been demonstrated by SDS-PAGE electrophoresis of the mitochondrial suspensions, a process similar to that observed in mitochondria treated with tert-butyl hydroperoxide. EGTA addition, in contrast with DTT or antioxidants, restores the previously eliminated electrical potential. Furthermore, EGTA prevents the 5-ALA-mediated polimeryzation of thiol proteins. These observations suggest that Ca2+ participates in a later stage of the permeability transition, after the oxidation of the thiol proteins. The effects of 5-ALA-derived oxyradicals in isolated mitochondria could be used as a tool for more general studies of oxidative stress, such as the mitochondrial injury that follows processes of ischemia and reperfusion or xenobiotic poisoning.