Passive potassium ion permeability of Halobacterium halobium cell envelope membranes

Cell envelope vesicles, prepared from Halobacterium halobium, were loaded with 3 M KCl suspended in 3 M NaCl, and the loss of K⁺ was followed at various temperatures. The Arrhenius plot of the K⁺-efflux rates shows a break at 30°C, with higher energy of activation above the break. This temperature dependence is consistent with earlier studies of chain motions in liposomes prepared from isolated lipids. The efflux of K⁺ is more rapid with increasing pH between pH 5 and 7. Since these vesicles do not respire under the experimental conditions it was expected that the K⁺-efflux data would be related to the passive permeability of the membranes to K⁺. The apparent K⁺ permeability at 30°C is 1–2· 10^?10 cm·^?1. This value corresponds to a 5-h half-life for retained K⁺ in the envelope vesicles and to a probably much longer half-life in whole cells. The previously observed ability of Halobacterium to retain K⁺ in the absence of metabolism can thus be explained solely by the permeability characteristics of the membranes.