FIGURE 4.

Adenine nucleotide sensitivity of the Kir6.2 mutant R192C+UR1. (A, B, and C) Inward currents measured in the absence of Mg²⁺ were suppressed by increasing concentrations of ATP, ADP, and AMP with the IC₅₀ near 150 μM, 2 mM, and 18 mM, respectively. On average, ADP was 13-fold less potent and AMP 120-fold less potent than ATP at blocking the channel. Compared with wild-type channels, the ATP, AMP, and ADP sensitivity of the mutant decreased by ∼10-fold. (D) Plot of channel ATP (•), ADP (○), and AMP (▴) sensitivity for Kir6.2+SUR1 in the absence of Mg²⁺. Fit of the data points yielded k = 150 μM for ATP (n = 3), k = 2.1 mM (n = 3) for ADP, and k = 18 mM for AMP (n = 3). The estimated Hill coefficients were close to unity for ATP and ADP data points. The AMP data were fitted using a Hill coefficient of 1.