The mechanism of phosphate permeation in purified bean mitochondria

The permeability properties and mechanism of Pi transport wereinvestigated in purified bean mitochondria.

1. Purified bean mitochondria are impermeable to small moleculesand ions. However, Pi, arsenate, acetate and formate can enterthe osmotically active space of bean mitochondria.
2. Nigericinor the association of valinomycin and FCCP cause mitochondrialswelling in isoosmotic potassium phosphate.
3. The SH-blockingreagents mersalyl, pHMB and NEM inhibit variousmitochondrialfunctions dependent on the translocation of Piand arsenateacross the membrane. These include the respirationstimulatedby ADP, Ca²⁺+Pi, and K⁺+valinomycin +Pi; the swellingin ammoniumphosphate medium and, in the presence of nigericin,in potassiumphosphate medium; the energy-linked yalinomycin-inducedswellingand the subsequent CICCP-induced shrinking. The uncoupler-stimulatedrespiration, as well as the other processes when acetate issubstituted for Pi, are not influenced by SH reagents.
4. Mersalyland pHMB cause complete inhibition at about 20 nmoles/mgprotein,whereas, NEM is effective at about 1 µmole/mgprotein.The inhibition by mersalyl and pHMB, but not that byNEM, issigmoidal and reversed by 2-mercaptoethanol. Non-inhibitoryamounts of mersalyl protect the Pi transport from irreversibleinhibition by NEM.
5. We concluded that a carrier-mediated transportsystem for Piis present in bean mitochondria, and that someof its propertiesare similar to the Pi carrier of animal mitochondria.

(Received June 5, 1975; )