Abstract

Escherichia coli has two Na^+/H^+ antiporters which can be used to lower intracellular pH transiently elevated upon exposure to alkaline stress and to extrude intracellular Na^+. Previous studies on the propertied of the Na^+/H^+ antiporter were done, but later studies showed that E. coli has two antiporters(Pinner E., Kolter Y., Padan E., Schuldiner S. (1993) J. Biol. Chem 268, 1729~1734). The properties of each antiporter were studied in this report. Both antiporters were specific only to Na^+ and Li^+, and showed hyperbolic kinetics. K_M values of NhaA were 0.8 mM and 2,2mM for Li^+ and Na^+, respectively. K_M values of NhaB were 2.8mM and 12mM for Li^+ and Na^+, respectively. The pH effects can be summarized as follows: 1) both antiporters do not show activity at very acidic pH values; 2) NhaB seems to work in a neutral pH range; 3) NhaA seems to show activity at alkaline pH. The effect of pH on the kinetics of the antiporters was studied. V_max of NhaB remained maximum in the pH range 7.0~8.2. V_max of NhaA increases steeply in the pH range 6.5~7.8 and remained maximum thereafter up to pH 8.6. When the pH was increased, the K_M decreased sharply, especially for NhaA.

• Keywords: E. coli; homeostasis; Na^+/H^+ antiporter; pH