2.B.2 The Monensin (Monensin) Family

Monensin A and B are two polyketide antibiotics synthesized by Streptomyces cinnamonensis. They insert into membranes and exert their toxic effects by transporting monovalent ions (K⁺, Na⁺ and H⁺) across biological membranes in an electroneutral exchange process, thus collapsing individual ion gradients without dissipating the membrane potential. Na⁺ is strongly preferred to K⁺. Thus, Na⁺:H⁺ antiport is the favored reaction. Recently monensin has been shown to also mediate the rapid selective transport of lead (Pb²⁺) (Hamidinia et al., 2002).

Monensin is a lipid-soluble naturally occurring  antimicrobial. Its activity is mediated by its ability to exchange Na⁺ and K⁺ ions across the cell membrane thereby disrupting ionic gradients and altering cellular physiology. It is approved by Food and Drug Administration as a veterinary antibiotic to treat coccidiosis. Besides veterinary applications, monensin exhibits broad spectrum activities against opportunistic pathogens of humans such as bacteria, virus, fungi and parasites in both drug sensitive and resistant strains. This ionophore can selectively kill pathogens with negligible toxic effect on mammalian cells (Rajendran et al. 2018).

The generalized reaction catalyzed by monensins is:

C¹⁺ (in) + C²⁺ C¹⁺ (out) + C²⁺ (in)