Change in the calcium-binding capacity of sea urchin egg homogenate caused by treatment with ATP and magnesium ions

The dissociation constant of the calcium and calcium-binding substance complex in sea urchin egg homogenate increased during incubation of the homogenate with CaCl₂ at concentrations greater than 0.5 mM to the same value as that found in the fertilized egg homogenate. The dissociation constant of the calcium-binding substance in the unfertilized egg homogenate (pre-incubated with CaCl₂), as well as the homogenate of the fertilized egg, was found to decrease to the value found in the unfertilized egg homogenate on treatment with 5 mM ATP and 5 mM MgCl₂. It was found that a calcium-binding substance of the 'unfertilized egg type' was easily transformed to a 'fertilized egg type' by calcium ions and the fertilized egg type substance reversibly became an unfertilized egg type on treatment with ATP and Mg ions. This substance is therefore thought to play an important role in the regulation of the intracellular concentration of calcium.