The present invention is based on the therapeutic potential of a reduced form of thionein. Accordingly, the invention features modified metallothionein or thionein proteins, for example, where at least one sulfur atom is substituted with selenium (e.g., a cysteine substituted with selenocysteine), and fragments thereof. The invention also features methods for screening for candidate compounds that (i) decrease binding of metal (e.g., zinc) to metallothionein or thionein and (ii) do not change the oxidation state of metallothionein, thionein, or another protein. Also featured are methods for generating modified thionein proteins with reduced metal affinity and methods for treating patients with a disease associated with oxidative stress.