Partial characterization of cadmium-binding protein from roots of tomato

Cd-binding protein was extracted from tomato roots and purified on QAE-Sephadex A-25 and on Sephadex G-75 in 1 molar KCl buffer. The protein preparation was light brown and contained predominantly Cd and small amounts of Zn and Cu. Polyacrylamide gel electrophoresis at pH 6.9 removed the brown material from protein which now bound mostly Cd and some Cu. The apparent molecular weight was 3,100 daltons in high ionic strength medium (1 molar KCl buffer) and 21,500 daltons at low ionic strength. Ionic strength also affected the apparent molecular weight of the Cd-binding protein in crude root extracts. The protein contained 26% cysteine, 53% glutamic acid/glutamine, and 2.8 gram atoms (Cd+Zn+Cu)/mole. The (Cd+Zn+Cu):cysteine ratio was 1:2.3. Circular dichroism measurements indicated Cd-thiolate coordination. The tomato Cd-binding protein was more similar to phytochelatins than to animal metallothioneins.