Ratiometric BRET imaging of ARSeNL purified protein using the Spectral Instruments (SI) Ami HT whole-animal imaging system. (A) The dynamic range is not affected by the use of standard GFP (510/20 nm) and RFP (650/20 nm) emission filters in the SI Ami HT imaging system (KD = 1.1 ± 0.2 mM, n = 3). Data were fitted to the Hill equation. (B,C) The mScarlet/NanoLuc BRET ratio (purple) is stable over time in solutions containing 0 mM ATP (B) or 10 mM ATP (C) and 1 nM ARSeNL protein (n = 4), while the total luminescence intensity (blue) decays. (D,E) Whole-animal imaging of unshaved adult Balb/c mice. ARSeNL, pre-equilibrated with or without ATP, was placed subcutaneously in the left and right hindlimb areas, respectively, and the ATP-dependent ratio difference was preserved when imaged through tissue and fur (mean ± sd, n = 3, p < 0.001).