Activation of chicken liver dihydrofolate reductase by tetrathionate

Dihydrofolate reductase from chicken liver (5,6,7,8-tetrahydrofolate:NADP + oxidoreductase, EC 1.5.1.3) is activated approximately six-fold by tetrathionate. A 30-fold excess is required for full activation within a 24 hour period. The activation is accompanied by stoichiometric binding of a sulfur-containing moiety of tetrathionate, presumably thiosulfate, to the single sulfhydryl group of the enzyme. The effect can be completely reversed with β-mercaptoethanol even after several days in the activated state. The activated enzyme is stable for at least a week at 0°.