Magnesium binding to DM-nitrophen and its effect on the photorelease of calcium

The effect of Mg2+ on the process of Ca2+ release from the caged Ca2+ compound DM-nitrophen (NP)was studied in vitro by steady light UV photolysis of NP in the presence of Ca2+ and Mg2+. Ca2+ release during photolysis and its relaxation/recovery after photolysis were monitored with the Ca2+-sensitive dye fura-2. Mg2+ speeds the photorelease of Ca2+ during photolysis and slows the relaxation of Ca2+ to new steady-state levels after photolysis. Within the context of a model describing NP photolysis, we determined the on and off rates of Mg2+ binding to unphotolyzed NP (k(on) = 6.0 x 10(4) M-1 s(-1); k(off) = 1.5 x 10(-1) s(-1)). Furthermore, to fully account for the slow postphotolysis kinetics of Ca2+ in the presence of Mg2+ we were forced to add an additional photoproduct: to the standard model of NP photolysis. The additional photoproduct is calculated to have a Ca2+ affinity of 13.3 mu M and is hypothesized to be produced by the photolysis of free or Mg2+-bound NP; photolysis of Ca2+-bound NP produces the previously documented 3 mM Ca2+ affinity photoproduct.