Kinetic studies of temperature changes, oxygen utilization, pH, and potassium concentration changes in a differential calorimeter: The enthalpy of potassium accumulation by rat liver mitochondria

Abstract

This paper describes the application of a differential calorimeter equipped with oxygen, potassium, and pH electrodes to the study of valinomycin-stimulated potassium transport by rat liver mitochondria. For the aerobic, succinate-supported accumulation of potassium by mitochondria, the enthalpy of substrate oxidation was −36.2 ± 1.1 kcal per atom of oxygen. There were 3.35 ± 0.2 moles of potassium accumulated and 0.24 ± 0.04 moles of protons released per atom of oxygen utilized during such experiments. For uncoupler-stimulated, anaerobic potassium release, Δ H was −0.03 ± 0.5 kcal per mole of potassium released. There were 0.14 ± 0.01 moles of protons taken up per mole of potassium extruded during the anaerobic discharge of potassium by mitochondria. An energy balance using experimentally determined stoichiometries was constructed for the potassium movement, and it was calculated that the potassium movements were thermally neutral, i.e., Δ H = −0.6 ± 0.75 kcal per mole potassium, for inward movements of potassium. The enthalpy measurements demonstrate that the endogenous and transported potassium are in free solution in the inner mitochondrial water space. The implications of this measurement for three theories of cation transport by mitochondria are discussed.