Mitochondrial uptake of 45Ca2+ bound to calcium-binding protein isolated from rat liver cytosol.

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Abstract
The mitochondrial uptake of 45Ca2+ bound to 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 3 mM 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 15 s of the start of incubation and was saturated at 5 min. 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 (10 .mu.M), lanthanum chloride (250 .mu.M), and oxidized form of nicotinamide adenine dinucleotide (NAD+; 2.5 mM). 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.