[H-3] 9-METHYL-7-BROMOEUDISTOMIN-D, A CAFFEINE-LIKE POWERFUL CA2+ RELEASER, BINDS TO CAFFEINE-BINDING SITES DISTINCT FROM THE RYANODINE RECEPTORS IN BRAIN MICROSOMES

[H-3]9-Methyl-7-bromoeudistomin D ([H-3]MBED), the most powerful Ca2+ releaser from sarcoplasmic reticulum, specifically bound to the brain microsomes. Caffeine competitively inhibited [H-3]MBED binding, [H-3]MBED binding was markedly blocked by procaine, whereas that was enhanced by adenosine-5'-(beta,gamma-methylene)triphosphate. The B-max value was 170 times more than that of [H-3]ryanodine binding. The profile of sucrose-density gradient centrifugation of solubilized microsomes indicated that [H-3]MBED binding protein was different from [H-3]ryanodine binding protein, These results suggest that there are MBED/caffeine-binding sites in brain that are distinct from the ryanodine receptor and that MEED becomes an essential molecular probe for characterizing caffeine-binding protein in the central nervous system.