Characterization of the Na + /H + exchanger in human epidermoid carcinoma A431 vesicles

A 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 activation by mitogens.