Zinc Modulation of Cardiac Ryanodine Receptor Gating: Alternate Interpretation of the Interplay between Zinc and Calcium [Letters to the Editor]

Woodier et al. (1) report that the cardiac type 2 ryanodine receptor channel (RyR2) shows two modes (sites) of regulation by zinc. When calcium is activating the channel, zinc increases channel Po around 100 pm (high affinity), whereas in the absence of calcium, zinc must reach 100 nm (low affinity) before channel Po is increased (compare Fig. 1 with Fig. 4, model in Fig. 10). We would like to offer an alternate interpretation of these data. In order to achieve nominally 0 free calcium, 1 mm BAPTA (2,2′-(ethylenedioxy)dianiline-N,N,N′,N′-tetraacetic acid) was added. We believe the primary reason that higher zinc (100 nm) must be added to modulate RyR2 gating in 0 free calcium is that zinc is chelated by BAPTA as well. It is not possible to use BAPTA to selectively chelate calcium without similarly affecting free zinc concentrations because Zn2+ binds to BAPTA (Kd = 7.9 nm) with greater affinity than Ca2+ (Kd = 110 nm) (2–4). Using these values, one can estimate free zinc concentrations under the conditions given in Fig. 4 (with the addition of 1 mm BAPTA). With 1 nm zinc added, the free zinc concentration will be ∼0.1 fm, and with 100 μm zinc added, the free zinc concentration will be ∼1 nm. It is noteworthy that 1 nm free zinc is approximately the same concentration range that modulates gating with calcium present (Fig. 1). Thus, the simplest interpretation of these data is that RyR2 has a single high affinity (1 nm) zinc binding site that...

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