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Kinetic Properties of DM-Nitrophen Binding to Calcium and Magnesium

Guido C. Faas ^*, Kinga Karacs ^*, Julio L. Vergara  and Istvan Mody ^* 

^* Department of Neurology and Department of Physiology, University of California, Los Angeles, California

Correspondence: Address reprint requests to Istvan Mody, Dept. of Neurology, University of California, Los Angeles, Reed Building 710, Los Angeles, CA 90095. Tel.: 310-206-4481; Fax: 310-825-0033; E-mail: mody{at}ucla.edu.

Caged-Ca²⁺ compounds such as nitrophenyl-EGTA (NP-EGTA) and DM-nitrophen (DMn) are extremely useful in biological research, but their use in live cells is hampered by cytoplasmic [Mg²⁺]. We determined the properties of Ca²⁺ release from NP-EGTA and DMn by using Oregon green BAPTA-5N to measure changes in [Ca²⁺] after ultraviolet flash photolysis in vitro, with or without Mg²⁺ present. A large fraction (65%) of NP-EGTA, which has a negligible Mg²⁺ affinity, uncages with a time constant of 10.3 ms, resulting in relatively slow increases in [Ca²⁺]. Uncaging of DMn is considerably faster, but DMn has a significant affinity for Mg²⁺ to complicate the uncaging process. With experimentally determined values for the Ca²⁺ and Mg²⁺ binding/unbinding rates of DMn and NP-EGTA, we built a mathematical model to assess the utility of NP-EGTA and DMn in rapid Ca²⁺-uncaging experiments in the presence of Mg²⁺. We discuss the advantages and disadvantages of using each compound under different conditions. To determine the kinetics of Ca²⁺ binding to biologically relevant Ca²⁺ buffers, such as Ca²⁺-binding proteins, the use of DMn is preferable even in the presence of Mg²⁺.

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