Adriamycin inactivates cytochrome c oxidase by exclusion of the enzyme from its cardiolipin essential environment

The regulation of ligand binding to the muscarinic acetylcholine receptor in developing chick heart has been studied using the radiolabeled antagonist ³H]quinuclidinyl benzilate (QNB). In assays containing only buffer and a source of receptor protein, the antagonist radioligand bound to a single, high affinity state of the receptor. If Mg²⁺ and EDTA were added, ³H]QNB bound to a single, low affinity state. The guanine nucleotide analog, guanylylimidodiphosphate Gpp(NH)p], reversed the effect of $\text{Mg}{}^{\mathrm{2+}}\text{EDTA}$ so that ³H]QNB again bound only to a single, high affinity state. Sodium could also reverse the effect of $\text{Mg}{}^{\mathrm{2+}}\text{EDTA}$ on antagonist binding but the effects of sodium and Gpp(NH)p on ³H]QNB binding were not additive.