Figure 4.

Specificity of inositol phosphate binding to the purified InsP₃ receptor. (A) Displacement of 10 nM [³H]1,4,5-InsP₃ by unlabeled 1,4,5-InsP₃ (•) and 1,3,4,5-InsP₄ (▪). This experiment shows that 1,4,5-InsP₃ is 30× more effective than 1,3,4,5-InsP₄ at the 50-nM site. One of two similar experiments is shown. (B) Displacement of 6 μM [³H]1,4,5-InsP₃ by unlabeled 2,4,5-InsP₃ (▴), 1,4,5-InsP₃ (•), and 1,3,4,5-InsP₄ (▪). The two points at each concentration represent two experiments on two different preparations. The points at 30 μM 1,3,4,5-InsP₄ were displaced slightly to the right to show that one of the data points falls in the same location as the data points for 30 μM 1,4,5-InsP₃. This experiment shows that 2,4,5-InsP₃ is ineffective and 1,3,4,5-InsP₄ is more effective than 1,4,5-InsP₃ at displacing 6 μM [³H]1,4,5-InsP₃ from the 10-μM site.