Metal-binding characteristics of the parotid salivary protein gustin

Metal binding characteristics of the parotid salivary protein gustin have been examined. When purified to apparent homogeneity, gustin contains 1 gatom Zn/mol which is tightly bound (Kd at pH 7.2, 4.5--10(-11) M). This tightly bound zinc can be removed with strong chelators such as diethyldithiocarbamic acid and 1,10-o-phenanthroline at pH 4.5, but not with EDTA or Chelex 100. Removal of the metal ion causes no appreciable conformational change in the protein. The apoprotein can be reconstituted by dialysis against Zn2+-containing buffer, a process favored by pH greater than 6.0. Only cobalt is able to bind to the apoprotein at this strong binding site. Cobalt binding is appreciably weaker than that of zinc (Kd at pH 7.2, 1.3--10(-7) M) and is maximal at pH 7.0. The weaker binding of cobalt is also illustrated by the loss of 37% of bound cobalt after 96 h of dialysis at pH 7.2, conditions under which the zinc content of gustin does not change. A second gatom Zn/mol may be loosely bound to gustin, but is easily removed by dialysis against metal ion-free buffer. Other metal ions such as copper, nickel, iron or manganese, but not cadmium or mercury, bind loosely to this second zinc site and are removed with ease. Zinc appears to be involved in the formation of the complex between gustin and glycoproteins which are present in human parotid saliva in vivo.