The pivotal position of the mitochondrial thiol group exposed by dinitrophenol located by means of ATP energized mitochondrial volume changes requiring gramicidin, showdomycin, and dinitriphenol.

An appropriate concentration of DNP (2, 4-dinitrophenol) is capable of inhibiting the ATP energized mitochondrial volume change induced by gramicidin in the presence of the permeant ions potassium and L-malate. The further addition of the non-mercurial thiol reagent, showdomycin reinstates, an effect of gramicidin which is dependent upon potassium ion. Thus the strategically located nucleophilic mitochondrial thiol group exposed by DNP occupies a pivotal position between two cycles. One of the cycles meshes with the respiratory chain and the other cycle meshes with ATP. This latter cycle contains the site sensitive to gramicidin.