Ca2+ stabilization of respiratory complex I from Escherichia coli

Stability of the membrane-bound and purified H+-translocating NADH:ubiquinone oxidoreductase, Complex I, was studied. The loss of the enzyme activity is strongly increased by alkaline pH and dilution of the sample. Complex I inactivation is prevented specifically by a low concentration of Ca2+ and/or an intracellular stabilization factor (ISF). The action of both, Ca2+ and ISF, on Complex I stability is interdependent. The data are discussed in terms of a release of structural Ca2+-as a reason for Complex I decay and an effect of ISF on the affinity and/or accessibility of Ca2+-binding site.