Alloxan effects on mitochondria in vitro, studied with regard to inhibition of mitochondrial aconitase.

The recently observed inhibition by alloxan of mitochondrial aconitase was studied with regard to oxygen consumption, accumulated Ca2+ and volume of isolated mitochondria from mouse islets (not used for respiratory experiments), liver and kidneys. Altered respiration, efflux of Ca2+ and volume increase were induced in a manner dependent on the concentration of alloxan, the energetic state of the mitochondria, the addition of Ca2+ and reductant, and the type and concentration of substrate used. State 3 and, less pronounced, state 4 respiration were inhibited, accumulated Ca2+ was released, and the mitochondrial volume was increased at a minimum concentration of alloxan of 10-100 microM when pyruvate, citrate or cis-aconitate were substrates. The alloxan effects were less marked or absent when other substrates were used. The efflux of Ca2+ induced by alloxan in mitochondria respiring on pyruvate, citrate or cis-aconitate was inhibited or reversed by isocitrate. Potentiation of the alloxan effects was seen on increasing the concentration of alloxan, decreasing the concentration of substrate, addition of low concentrations of Ca2+ or reductant, and addition of alloxan before the substrate (non-energized state). The data support the finding of inhibition by alloxan of mitochondrial aconitase.