Affinity of heavy metal ions to intracellular Ca super(2+)-binding proteins

Parvalbumin, troponin C and vitamin D dependent Ca super(2+)-binding proteins (CaBP type I and II) share the property of calmodulin to interact with some heavy metal ions. In flow dialysis and in spot tests the affinities of Cd super(2+) and Pb super(2+) to these proteins were comparable to those of Ca super(2+). The relative affinities were for calmodulin: Pb super(2+) > Ca super(2+) > Cd super(2+), for troponin C: Ca super(2+) > Cd super(2+) > Pb super(2+), for CaBP I: Ca super(2+) similar to Pb super(2+), for CaBP II; Ca super(2+) > Pb super(2+) > Cd super(2+), and for parvalbumin: Cd super(2+) similar to Ca super(2+) > Pb super(2+). Upon gel filtration of the supernatant of a pig mucosal homogenate, binding for both Pb super(2+) and Ca super(2+) appeared in the MW range of 10,000, together with CaBP II. The authors conclude that the investigated proteins bind heavy metal ions, in particular Pb super(2+) and Cd super(2+), similar to calmodulin. Their role in transport, storage and toxicity remains to be assessed.