Stimulatory effect of lithium ion on proline transport by whole cells of Escherichia coli

Abstract

The effect of monovalent cations on proline transport in whole cells of E. coli K-12. was examined. Li+ added to the uptake medium stimulated proline transport severalfold and K+ and Na+ were slightly effective; Rb+, Cs+ and NH4+ were completely without effect. The stimulatory effect of Li+ on proline transport was not due to an increase in osmolarity of the uptake medium, and 5 mM p-chloromercuribenzene sulfonic acid completely blocked Li+ effect without affecting the basal rate of proline transport. The Arrhenius plots for Li+-stimulated transport showed a clear transition point at 35.degree. C plus 20.degree. C which was also detectable in the basal transport. Li+ stimulated proline transport synergistically in the presence of glucose and succinate as a C source. The addition of 2.5 mM KCN or 0.5 mM arsenate did not inhibit this synergistic effect, although the presence of these inhibitors inhibited completely the stimulation of proline transport induced by the addition of C source. Carbonylcyanide m-chlorophenylhydrazone and 2,4-dinitrophenol blocked both the basal and Li+-stimulated proline transport. When membrane potential of E. coli cells was measured by the dibenzyldimethylammonium uptake method, the incubation of Li+ with the cells did not affect the preexisting membrane potential. Li+ probably stimulates proline transport by intact E. coli cells in a manner somehow affecting membrane component(s) different from the transport carrier of proline. Li+ may or may not be directly involved in the mechanisms of energy coupline of proline transport.