The antibacterial peptide seminal plasmin alters permeability of the inner membrane of E. coli

Seminal plasmin (SPLN) a 47-residue peptide, isolated from bovine seminal plasma, exhibits antibacterial activity against Gram-positive and Gram-negative bacteria. Although SPLN strongly inhibits the transcription of various natural and synthetic templates by E. coli RNA polymerase in vitro, it also associates with model membranes of phosphatidylcholine and phosphatidic acid. We have undertaken experiments to ascertain whether SPLN permeabilizes the bacterial inner membrane and thereby exerts its antibacterial activity, as in the case of recently isolated antibacterial peptides from mammalian sources. Our results show that SPLN affects the permeability properties of the bacterial inner membrane which is reflected by increased uptake of ortho-nitrophenylgalactoside (ONPG), which can normally be translocated only by protein transporters. SPLN has also been shown to act on the outer membrane, since divalent cations inhibit antibacterial activity.