Spatially resolved cellular responses to singlet oxygen

Singlet oxygen (O-1(2)) is unique amongst reactive oxygen species formed in cells in that it is an excited state molecule with an inherent upper lifetime of 4 mu s in water. Whether the lifetime Of O-1(2) in cells is shortened by reactions with cellular molecules or reaches the inherent maximum value is still unclear. However, even with the maximum lifetime, the diffusion radius is only similar to 220 nm during three lifetimes (similar to 5% O-1(2) remaining), much shorter than cellular dimensions indicating that the primary reactions Of O-1(2) will be subcellularly localized near the site Of O-1(2) formation. This fact has raised the question of whether spatially resolved cellular responses to O-1(2) occur, i.e. whether responses can be initiated by generation and reaction Of O-1(2) at a particular subcellular location that would not have been produced by O-1(2) generation at other subcellular sites. In this paper, we discuss examples of spatially resolved responses initiated by O-1(2) as a function of distance from the site of generation Of O-1(2). Three levels are recognized, namely, a molecular level where the primary oxidation product directly modifies the behavior of a cell, an organelle level where the initial photo-oxidation products initiate mechanisms that are unique to the organelle and the cellular level where mediators diffuse from their site of formation to the target molecules that initiate the response. These examples indicate that, indeed, spatially resolved responses to O-1(2) occur in cells.