Magnesium Binding to DM-Nitrophen and Its Effect on the Photorelease of Calcium

The effect of Mg 2+ on the process of Ca 2+ release from the caged Ca 2+ compound DM-nitrophen (NP) was studied in vitro by steady light UV photolysis of NP in the presence of Ca 2+ and Mg 2+. Ca 2+ release during photolysis and its relaxation/recovery after photolysis were monitored with the Ca 2+-sensitive dye fura-2. Mg 2+ speeds the photorelease of Ca 2+ during photolysis and slows the relaxation of Ca 2+ to new steady-state levels after photolysis. Within the context of a model describing NP photolysis, we determined the on and off rates of Mg 2+ binding to unphotolyzed NP ( k on = 6.0 × 10 4 M −1 s −1; k off = 1.5 × 10 −1 s −1). Furthermore, to fully account for the slow postphotolysis kinetics of Ca 2+ in the presence of Mg 2+ we were forced to add an additional photoproduct to the standard model of NP photolysis. The additional photoproduct is calculated to have a Ca 2+ affinity of 13.3 μM and is hypothesized to be produced by the photolysis of free or Mg 2+-bound NP; photolysis of Ca 2+-bound NP produces the previously documented 3 mM Ca 2+ affinity photoproduct.