Biophys J, December 1999, p. 3384-3393, Vol. 77, No. 6

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

Department of Molecular and Cell Biology, University of California, Berkeley, California 94720 USA

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 × 10⁴ M¹ s¹; k_off = 1.5 × 10¹ s¹). 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 µM 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.

Biophys J, December 1999, p. 3384-3393, Vol. 77, No. 6
© 1999 by the Biophysical Society   0006-3495/99/12/3384/10  $2.00

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