Use of the coulombic interactions of the lanthanide series to identify two classes of Ca²⁺ binding sites in mitochondria

The degree of inhibition of respiration-dependent vs respiration-independent Ca²⁺ binding by rat liver mitochondria by different members of the lanthanide family was used to establish the existence of two different classes of Ca²⁺ binding sites. The distinction is based on the differences in cation:site interactions between the two classes of sites and the members of the lanthanide series. Lanthanide inhibition of respiration-dependent Ca²⁺ uptake suggests that the binding site is specific for the calcium ion. Those members of the lanthanide family whose ionic radii are nearer that of Ca²⁺ are the best inhibitors. The inhibition of respiration-independent Ca²⁺ binding is much different, indicating non-specific cation absorption.