This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Biological role of metallothioneins is to sequester harmful metal ions. These bind which of the following ions? 
Select and explain. 
A. Cd++, Cu++ & Zn++, B. Al+++, Hg++ & NH4+, C. Pt++, As+++ & P04, D. Fe+++, Na+ & K+
A: Cd++, Cu++ & Zn++
Answer: A: Cd++, Cu++ & Zn++ 
Explanation: Metallothioneins are metal binding proteins of relatively low molecular mass with a high content of cysteine residues that have a paicular affinity for cadmium, as well as for zinc and copper. Zinc metallothioneins can detoxify free radicals. Cadmium induced metallothioneins is able to bind cadmium intracellularly and in this way protects the organism against the toxicity of this metal. Ref: Mineral Components in Foods edited by Piotr Szefer page 152