Effect of inhibitors on the sigmoidicity of the calcium ion transport kinetics in rat liver mitochondria

The kinetic plot (initial rate of Ca²⁺ transport versus concentration) of mitochondrial Ca²⁺ transport is hyperbolic in a sucrose medium. The plot becomes sigmoidal in the presence of competitive inhibitors of Ca²⁺ binding to low affinity sites of the membrane surface such as Mg²⁺ and K⁺. The plot also becomes sigmoidal in the presence of Ba²⁺. Ba²⁺ is a competitive inhibitor of both Ca²⁺ transport and Ca²⁺ binding to the low affinity sites. The K_i for the inhibition of Ca²⁺ transport by Ba²⁺ increases in the presence of K⁺ and Mg²⁺, which suggests a competition for the low affinity sites between the cations. The plot is still hyperbolic in the presence of La³⁺, which inhibits Ca²⁺ transport competitively. Ruthenium red which is a pure non-competitive inhibitor of mitochondrial Ca²⁺ transport, does not affect the shape of the kinetic plot. These results indicate that the surface potential, which depends on the ions bound to the low affinity sites, determines whether the kinetics of Ca²⁺ uptake in mitochondria is sigmoidal or hyperbolic.