Mitochondrial Uptake of ⁴⁵Ca²⁺ Bound to Calcium-Binding Protein Isolated from Rat Liver Cytosol

Abstract

The mitochondrial uptake of ⁴⁵Ca²⁺ bound to calcium-binding protein newly isolated from rat liver cytosol was investigated. The binding of ⁴⁵Ca²⁺ to calcium-binding protein increased linearly with increasing amount of the protein. ⁴⁵Ca²⁺ bound to the binding protein was taken up by rat liver mitochondria and microsomes in the presence of 3mM adenosine 5'-triphosphate (ATP) in the incubation medium, while the uptake was slight in the absence of ATP. The mitochondrial uptake of ⁴⁵Ca²⁺ bound to the binding protein started within 15s of the start of incubation and was saturated at 5min. The amount of ⁴⁵Ca²⁺ taken up by the mitochondria from ⁴⁵Ca²⁺-binding protein increased linearly with increasing concentration of the protein-bound ⁴⁵Ca²⁺. The mitochondrial uptake of ⁴⁵Ca²⁺ from the binding protein was markedly inhibited by the presence of mitochondrial calcium uptake inhibitors, ruthenium red (10μM), lanthanum chloride (250μM), and oxidized form of nicotinamide adenine dinucleotide (NAD⁺ ; 2.5mM). The present results suggest that the cytosolic calcium-binding protein binds calcium ion in rat liver cytosol and the metal is subsequently transported into the organelles. This protein may play a role in the regulation of the cytosolic calcium ion level.