Stability of metallothionein in gastric juice

Metallothionein (MT), is presumably the major Cd-binding component of human food. It is not or only partially destroyed by cooking. To study whether MT is stable in gastric juice MT was incubated at various pH values with pepsin and human gastric juice in vitro. Above pH 3.5 nearly all Cd remained bound to the protein and Cd-MT was resistant towards proteolysis. At pH values of 2.5 and 1.7 the protein was digested to 80% and 100%, respectively. At pH 2.5 two Cd-containing peptides with 25-30 amino acids similar to the alpha-domain of the protein were detected. At pH 1.7 these fragments were further degraded to smaller peptides. The results indicate that the pH-dependent proteolytic degradation of Cd-MT depends on the metal content of the protein. Furthermore, the identical results obtained with pepsin and human gastric juice suggest that Cd-MT proteolysis occurs in vivo and will affect Cd resorption.