The histochemistry of thiols and disulphides. IV. Protective fixation by organomercurial-formalin mixtures.

Formation of mercaptides as the result of adding organomercuric salts to neutral formalin used for fixation was found to protect protein thiols from autoxidation, provided the tissues were washed in distilled and not tap water. Such blocking, in contrast to that given by HgCl2, could be reversed quantitatively by mercaptoethanol made strongly acid to keep it from reducing disulphides. However, some cleavage of disulphides by the mercurials themselves caused slight artifactual thiol staining in a limited number of sites. Three of the nine compounds tested are sufficiently soluble to penetrate tissues with reasonable speed, stable enough to preclude more than incidental mercurial deposits and currently available commercially. Of them, the diuretic mercurial Mersalyl is at present the protecting agent of choice since methyl- and ethylmercuric chlorides are too toxic to recommend for routine use.