Middle Molecule Substances in the Hemofiltrate of Dialysis Patients

IV. Phosphate-Dependent Action of Middle-Molecule Fractions on Mitochondrial Respiration

Abstract

Six species of ‘middle molecules (MMs) ’ were separated from crude fractions of both hydrophobic and hydrophilic MMs obtained from the hemofiltrate of a previously reported dialysis patient. With phosphate buffer and Tris buffer as the incubation media, their action on the level of both oxygen consumption and oxidative phosphorylation in rat liver mitochondria was investigated polarographically with a Clark oxygen electrode. hydrophobic MMs species and some of acidic and neutral hydrophilic MMs species were found to have no effect on mitochondrial respiration. On the other hand, a weakly acidic species, B-MMs, stimulated the respiration of state 4 mitochondria in phosphate medium, but not in Tris medium. They also caused an approximately 3 to 4-fold release of the state 3 respiration in oligomycin inhibited mitochondria. These facts indicate that the B-MMs play a role of phosphate dependent uncoupler of mitochondrial function. Based on the effect of a phosphate-transport inhibitor, N-ethylmaleimide, we confirmed the uncoupling action of B-MMs is associated with the phosphate-transport process via a phosphate carrier. This action suggests a close relationship between MMs and the hyperphosphatemia seen during the uremic state of the patient.