Interaction of Organic Mercurial Compounds with Dihydrofolate Reductase from Ehrlich Ascites Carcinoma Cells

Stimulation of the Ehrlich ascites cell enzyme by organic mercurial compounds occurs as a result of the formation of an isolable complex between organic mercurial and enzyme, presumably through a mercaptide linkage. Whereas an excess of a thiol such as 2-mercaptoethanol will reverse this stimulation, with certain buffers (phosphate, citrate) the presence of a small amount of the thiol (10^-3 m) was necessary to produce activation. These apparently contradictory results have been found to be the result of both an inhibitory and stimulatory effect of mercurials on this enzyme activity; the former effect may be abolished by treatment with 10^-3 m mercaptoethanol, 5 × 10^-3 m EDTA, or passage of the enzyme-p-mercuribenzoate complex through a Sephadex G-25 column. The p-mercuribenzoate enzyme complex obtained after gel filtration was not stable at 23 °, but inactivation was prevented by either reduced nicotinamide-adenine dinucleotide phosphate or dihydrofolate. That an alteration in tertiary structure occurs upon mercurial binding, which may explain the stimulation, is suggested by: (1) the observed increase in sensitivity of the complex to denaturants and heat, (2) the change in substrate specificity, and (3) the observation that an activated enzyme, with similar substrate specificity to that of the mercurated enzyme, can be formed in the presence of thiourea or guanidine-HC1.