Comparative effects of four uncouplers on the enthalpy change for mitochondrial oxidation of succinate by ferricyanide.

The enthalpy changes for phosphorylating and nonphosphorylating mitochondrial oxidation of succinate by ferricyanide were studied by heatburst microcalorimetry. The enthalpy change observed with a nonphosphorylating system involving sonically disrupted mitochondria agreed with the calculated enthalpy change. The enthalpy changes for phosphorylating and uncoupled systems involving intact mitochondria, however, were all more negative than calculated. The discrepancies between observed and calculated values for systems involving intact mitochondria appear to be caused by something other than neutralization of protons and it is suggested that they may be caused by the type of experimental system used. The observed enthalpy changes for gramicidin-uncoupled and oleic acid-uncoupled oxidation of succinate were lower and higher, respectively, than for a dinitrophenol-uncoupled system. The observed enthalpy change for a dicoumaroluncoupled system, however, was the same as a dinitrophenoluncoupled system. In all cases, the uncoupling of oxidative phosphorylation by use of dinitrophenol, dicoumarol, gramicidin, or oleic acid caused the observed enthalpy change to become more negative. These increases in enthalpy change were equivalent to the enthalpy change for the hydrolysis of ATP to ADP and Pi when dinitrophenol, dicoumarol, or gramicidin were used as uncouplers, but was greater than this when oleic acid was used.