Competitive binding of calcium, magnesium and zinc to Streptococcus sanguis and purified S. sanguis cell walls.

Calcium bound by oral bacteria may have important effects on plaque cohesion and mineral dynamics. Equilibrium dialysis was used here to study the effect of magnesium and zinc competition on calcium-binding by Streptococcus sanguis 7863. Experiments were carried out using 45Ca in 0.1-5.0 mmol/l Ca2+ carrier. Increasing [Mg] and [Zn] decreased the apparent Ca-binding affinity, allowing calculation of the affinities for Mg2+ and Zn2+. Dissociation constants were found to be 0.94 +/- 0.23, 1.32 +/- 0.39 and 1.26 +/- 0.41 mmol/l (whole cells) and 1.87 +/- 0.49, 3.05 +/- 1.24 and 4.08 +/- 1.29 mmol/l (cell walls), for Ca, Mg and Zn respectively. Binding capacities for Ca (and, by inference, Mg and Zn) were 27.7 +/- 2.8 mumol/g wet weight (whole cells) and 54.2 +/- 1.6 mumol/g wet weight (cell walls). These data suggest that Mg and Zn act as true competitive inhibitors of calcium binding.