The Action of Phenylmercuric Nitrate

In Papers I and II (1, 2) it was shown that basic phenylmercuric nitrate depressed a number of enzymes (cytochrome oxidase, succinoxidase, catalase, and several dehydrogenases), that it reacted with sulfhydryl groups, and that the depression of yeast respiration by the basic nitrate could be prevented by the presence of sulfhydryl-containing compounds, such as cysteine or homocysteine. These findings suggested that the depression might involve interaction of the mercurial with essential -SH groups in the enzymes. However, it has been reported that certain of the above enzymes, namely cytochrome oxidase and catalase, do not depend upon -SH groups for their activity (3). If the basic phenylmercuric nitrate behaves like certain of the simple RHgX compounds which react specifically and reversibly with -SH groups, it should be possible to reverse the depression by adding a relatively large proportion of competing -SH groups, as noted by Barron and Singer (3). The present paper demonstrates that the basic phenylmercuric nitrate depression of cytochrome oxidase, like that of yeast respiration, can be prevented by the prior addition of sulfhydryl-containing compounds, but the depression of neither cytochrome oxidase nor yeast respiration can be reversed by subsequent addition of sulfhydryl-containing compounds. A series of amino acids containing various reactive groups did not protect either cytochrome oxidase or yeast respiration against the action of phenylmercuric nitrate with the exception of an effect of histidine upon the depression of cytochrome oxidase.