AbstractA previous report from this laboratory (Rothenberg et al., 1983a) demonstrated the presence of an Na+/H+exchanger in human epidermoid carcinoma A431 cells. We now characterize surface‐derived membrane vesicles from this cell line which contain a functional Na+/H+exchanger. The Na+/H+exchanger in A431 vesicles shares a number of characteristics in common with previously described Na+/H+exchangers including the following: (1) Na+uptake is stimulated by an outward‐directed pH gradient and inhibited by an inward‐directed pH gradient. (2) Na+uptake is inhibited by amiloride and its analogs and their relative effectiveness is similar in vesicles and A431 cells. (3) The Na+/H+exchanger uses Na+or Li+as a substrate but not K+or Cs+. (4) H+efflux is stimulated by an inward‐directed Na+gradient and inhibited by the amiloride analog 5‐N‐dimethylamiloride. The Na+/H+exchanger in these membrane vesicles is activated allosterically by low intravesicular pH. The apparent pKa of the activating site is 6.4–6.6, characteristic of the NA+/H+exchanger before activatio
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