Interaction of lanthanides with muscle microsomes

The interaction of gadolinium, a lanthanide with calcium-blocking action, with isolated muscle microsomes has been studied. Two classes of binding sites were present, 80 and in excess of 300 nmol/mg in number, respectively. Divalent cation, including Ca, Zn, and Cd, blocked the higher affinity site. Antibiotic ionophores X537A and A23187 enhanced binding, with positive cooperativity. ATP enhanced binding at low concentration (10-20 muM) of nucleotide, and without ATP-hydrolysis. The data suggest a dissociation in the intact membrane between the binding and ATP-hydrolytic portion of the transport site.