Determination of —SH and —SS— groups in proteins. I. A reassessment of the use of methylmercuric iodide

Amperometric titration with methylmercuric iodide was found to be unsatisfactory for the quantitative determination of —SH and —SS— values in several soluble proteins. It is concluded that the "high reactivity" of the mercurial has previously been overemphasized. Consistent values can, however, be obtained by permitting protein samples to react with an excess of the mercurial, for about 100 h, followed by the polarographic estimation of the remaining reagent. Contrary to general belief, this procedure was found to be more precise than amperometric titration, and, if appropriate precautions are taken, it is applicable to considerably smaller amounts (<0.5 μmole —SH or —SS—) of either soluble or insoluble protein. The necessary experimental procedure is described and results are reported for several proteins. For the compounds studied, methylmercuric iodide did not react at non-sulfhydryl sites, and did not additionally bind to the mercaptides formed; there was no indication of disulfide bond cleavage, despite the long reaction times used.