Increase in the susceptibility of hemoglobin to proteases upon treatment with p-mercuribenzoate.

Native human oxyhemoglobin, which has a rigid conformation resistant to proteases such as trypsin and subtilisin, could be hydrolyzed by these proteases at pH 7.0 after treatment with p-chloromercuribenzoate. The digestion curve of hemoglobin as a function of concentration of the mercurial was essentially parallel to the titration curve of hemoglobin with the mercurial, indicating that a relationship exists between susceptibility to proteases and modification of thiol groups of the protein. On the other hand, when myoglobin was used as a substrate, the degree of proteolysis did not increase after treatment with the mercurial. Circular dichroism measurements and gel-filtration experiments showed that the observed increase in susceptibility of hemoglobin to proteases was not due to a conformational change involving unfolding of alpha-helical structure, but was due to the dissociation of the tetrameric hemoglobin molecule into dimer and monomer after treatment with the mercurial.