The thioredoxin system is the major cellular reductant system present in the cell, whose role is to maintain cellular redox homeostasis. It does this in part, by regulating the activity of many other enzymes including ribonucleotide reductase, which is essential for DNA synthesis. It also acts as an antioxidant, reducing destructive reactive oxygen species. The thioredoxin system is comprised of thioredoxin (Trx) which reduces target protein disulfide bridges by thiol-disulfide exchange and thioredoxin reductase (TrxR) which reduces Trx back to its active state. Thioredoxin reductase is a common target for many cancer drugs including cisplatin and auranofin. Recently we have shown that the Florida red tide toxin, brevetoxin-2 (PbTx-2) can inhibit mammalian TrxR1. Brevetoxin-2 has α, β-unsaturated aldehyde moiety that was proposed to inhibit the enzyme by forming a Michael adduct. Several compounds which are similar to brevetoxin in size and functionality have a similar effect on TrxR. These compounds include antitumor and antibiotics such as manumycin A, geldanamycin, rifamycin SV and thiostrepton and toxins such as brevetoxin-3, nodularin and microcystin-LR. Manumycin A behaves as a typical TrxR1 inhibitor while other compounds screened activate the reduction of small disulfides such as DTNB (5,5’-dithiobis-(2-nitrobenzoic acid)). Mammalian thioredoxin reductase is a homodimer with two redox center viz. N-terminal dithiol buried in the enzyme and C-terminal selenosulfide located on the flexible C-terminal tail. Modification of the C-terminal tail of TrxR by these test compounds can expose N-terminal redox thiol that could reduce DTNB. The C-terminal Sec, a nucleophile can form a Michael adduct with α, β-unsaturated carbonyl moiety of test compounds. Together with point-specific mutant enzymes (C-terminal tail truncated, dead tail and Cys mutant) and enzyme assays that are specific/dependent on C-terminal Sec were used to decipher the site-specific interaction between these test compounds and TrxR. Inhibition of TrxR at the C-terminal redox center produces a pro-oxidant known as SecTRAP (Selenium Compromised Thioredoxin Reductase-derived Apoptotic Proteins), which uses NADPH to produce superoxide radical anion as observed with manumycin A. Since many cancer drugs target TrxR the present study has the potential to discover new cancer drugs.