title: Monensin-mediated antiport of Na+ and H+ across liposome membrane.
abstract: The mechanism of monensin-mediated transport of Na+ and H+ across large unilamellar liposome membrane was investigated. The inside negative membrane potential (delta psi) was generated by the addition of monensin to the liposomes with an outward Na+ gradient. The effects of intravesicular H+ bufferring power and medium pH on the initial rates of delta psi formation, Na+ efflux and H+ influx were examined. The results showed that (i) the initial Na+ flux (JNa) was larger than the initial H+ flux (JH) at any H+ bufferring power, (ii) the JH increased with increasing inner buffer concentration, but the effect of H+ bufferring power on the JNa was small, (iii) the initial rate of delta psi formation increased linearly with the increase in the value of (JNa-JH), and (iv) the JNa increased with increasing H+ concentration. The generation of delta psi was not due to H+ leak from the liposome, since the delta psi was generated even when H+ concentration gradient was inwardly directed. The monensin-mediated transport of Na+ and H+ in this system occurred at the ratio of Na+/H+ greater than 1.0 and the resultant net electric charge efflux is the cause of the inside negative membrane potential. Tetraphenylphosphonium retarded both the delta psi formation and the H+ influx, but did not affect the Na+ efflux, suggesting that the driving force of H+ influx is the inside negative membrane potential generated by Na+ efflux. This idea also well accounts for the observed H+ bufferring power effects on the Na+ efflux, H+ influx and delta psi formation. It was suggested that Na+ was transported in the form of 1:1 complex between protonated monensin and Na+.
authors.affiliation: Department of Chemistry, Niigata College of Pharmacy, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakazato
authors.forename: Y
authors.initials: Y
authors.lastname: Hatano
mesh_terms: D001190:Arylsulfonates; D001692:Biological Transport / Q000187:drug effects; D002021:Buffers; D002352:Carrier Proteins / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008081:Liposomes / Q000378:metabolism*; D008564:Membrane Potentials; D008985:Monensin / Q000494:pharmacology*; D009861:Onium Compounds / Q000494:pharmacology; D009943:Organophosphorus Compounds / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers
chemical_list: D001190:Arylsulfonates; D002021:Buffers; D002352:Carrier Proteins; D008081:Liposomes; D009861:Onium Compounds; D009943:Organophosphorus Compounds; D017923:Sodium-Hydrogen Exchangers; D006859:Hydrogen; D008985:Monensin; D012964:Sodium; C005047:pyranine; C013289:tetraphenylphosphonium