In the presence of taste membranes, several bitter compounds activate transducin and/or gustducin. Trypsin assays to monitor G-protein activation. (A) In the presence of taste membranes (Upper), native transducin was activated by several bitter compounds (note the 32-kDa fragment); with nontaste membranes (Lower), transducin was not activated by any bitter compounds (no 32-kDa fragment). (B) In the presence of taste membranes (Upper), in vitro translated gustducin was activated by denatonium (note the 37-kDa fragment); with nontaste membranes (Lower), gustducin was not activated (no 37-kDa fragment). Den, 5 mM denatonium benzoate; Qne, 1 mM quinine; Str, 5 mM strychnine; Spa, 5 mM sparteine; Nic, 10 mM nicotine; Atr, 5 mM atropine; Qnc, 5 mM quinacrine; Nar, 5 mM naringin; Epi, 5 mM epicatechin; Caf, 5 mM caffeic acid; Rho, 1 μM rhodopsin.