The compartmentalised nature of the mechanisms governing superoxide formation and scavenging in cells exposed to arsenite.

In this study, respiration-proficient (RP) and -deficient (RD) cells were exposed to 2.5 or 10 μM arsenite to generate superoxide (O) respectively in the mitochondrial respiratory chain or via NADPH oxidase activation. These treatments, while causing similar, although mitochondrial permeability transition-dependent (RP-cells) or independent (RD-cells), delayed apoptosis, surprisingly generated identical kinetics and levels of dihydrorhodamine oxidation, indicative of O formation. These similarities were attributable to the involvement of a common upstream event resulting in activation of the two O-generating systems, and to intrinsic features of the cells. Both mechanisms required an initial and sequential mobilization of Ca from the inositol-1,4,5-trisphosphate receptor and the ryanodine receptor (RyR), with however different implications. The close contacts existing between the RyR and the mitochondria created optimal conditions for the Ca clearance, and the ensuing formation of O in RP-cell mitochondria. Exposure to low concentrations of l-ascorbic acid (AA) transported by high affinity mechanisms in cells and mitochondria, suppressed O formation. Much more Ca, and hence more arsenite, was necessary to promote NADPH oxidase activation in RD-cells, as a consequence of the cytosolic dilution and mitochondrial clearance of Ca. For the same reasons, an exposure to high concentrations of AA was required to suppress O formation under these conditions.