IONOPHORIC ACTION OF TRANS-ISOHUMULONE ON LACTOBACILLUS-BREVIS

Compounds (e.g. trans-isohumulone) derived from flowers of the hop plant (Humulus lupulus L.) protect beer from spoilage. Cells of Lactobacillus brevis IFO 3960 did not die when they were exposed to 40 mum-trans-isohumulone for up to 120 h. At higher concentrations (80, 120 mum) death occurred after a lag period of about 30 h. trans-Isohumulone dissipated the transmembrane pH gradient of non-growing cells and reduced their ability to accumulate L-[U-C-14]leucine. The membrane potential was dissipated to a smaller extent. Addition of trans-isohumulone to celts that had accumulated L[U-C-14]leucine, under conditions in which no synthesis of protein took place, caused slow leakage of radiolabelled leucine. trans-Isohumulone did not inhibit the activity of the proton-translocating membrane ATPase. Potentiometric experiments with resting cell suspensions suggested that trans-isohumulone acted as an ionophore of the mobile-carrier type, causing electroneutral exchange of H+ for divalent cations such as Mn2+. A second monovalent cation (e.g. K+) was essential for protonophoric activity.