2.7.2. Ionophores for Na+
Monensin A is an ionophore for monovalent cations like Li+, Na+, K+, Ag+ and also has crucial role as an Na+/H+ antiporter. It exhibits antibiotic, antimalarial and inhibits protein transport. It is a polyether isolated from Streptomyces cinnamonensis.
Gramicidin A is also a monovalent (Na+ & K+) ionophore obtained from the soil bacterial species Bacillus brevis, used to induce hemolysis in lower concentrations than bacteria cell death. It is active against Gram positive and some Gram negative bacteria.
2.7.3. Ionophores for K+
Valinomycin is natural neutral ionophore obtained from Streptomyces tsusimaensis and Streptomyces fulvissimus. It is a dodecadepsipeptide highly selective for K+ ion over Na+ within the membrane. The value of stability constant K with these ions are 106 and 10 respectively. It consists twelve alternate amino acid residues and esters that creates its affinity for metal ions and responsible for salvation in polar solvents. Recently Valinomycin is suggested as a potent agent against SARS corona virus, which is responsible for severe acute respiratory syndromes.
Nigericin is also a monensin like antibiotic produced by Streptomyces hygroscopicus but commercially it is obtained during fermentation of geldanamycin, as a byproduct. It is also named as Antibiotic K178/X-464, Helixin C, Azalomycin M or Polyetherin A. Both salinomycin and gramicidin are the antibiotics that also work as K+ ionophores.