Ion Transport by Heart Mitochondria XXI. DIFFERENTIAL EFFECTS OF MERCURIAL REAGENTS ON ADENOSINE TRIPHOSPHATASE ACTIVITY AND ON ADENOSINE TRIPHOSPHATE-DEPENDENT SWELLING AND CONTRACTION

Abstract Three principal effects of mercurial reagents on the ATPase activity of isolated heart mitochondria have been defined. The observed ATPase activity depends on the extent of binding of the mercurial to the membrane and on ion movements across the membrane as follows. 1. Under conditions of low mercurial binding (approximately 20 nmoles per mg of protein or less), ATP-dependent ion uptake and swelling are activated by mercurials and an elevated ATPase activity is observed. For example, p-chloromercuriphenylsulfonate strongly activates ATPase in heart mitochondria suspended in salts of strong acid anions. The observed ATPase activity shows a marked specificity for K+ and is closely associated with the accumulation of ions and osmotic swelling of the mitochondria. Respiration opposes the swelling under these conditions but actually enhances the ATPase activity. The observed effects on mitochondrial swelling and ATPase activity appear to be related to the inability of the phosphate carrier to remove Pi from the matrix in the presence of the mercurial, and to the fact that p-chloromercuriphenylsulfonate induces passive permeability to Cl-. p-Chloromercuriphenylsulfonate does not induce passive permeability to H+ or cations, but increases K+ uptake in the presence of ATP. 2. Less polar mercurials such as p-chloromercuribenzoate react with internal as well as superficial thiols (approximately 40 nmoles per mg total) and these reagents induce passive permeability to both anions and cations. Under these conditions ATPase activity is strongly inhibited. 3. Heart mitochondria which have been swollen osmotically in the presence of mercurials by either active or passive ion uptake can be contracted by an ATP-dependent reaction if the mercurial is removed by excess dithioerythritol. The resulting ion extrusion is accompanied by elevated ATPase activity. A model based on the simultaneous entrance of cations with ATP and the postulates of the chemiosmotic coupling hypothesis is presented to account for the observed effects of mercurials on the ATPase activity of heart mitochondria.